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  • Articles (OceanRep)  (163)
  • ASLO (Association for the Sciences of Limnology and Oceanography)  (163)
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  • 1
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    Increased appendicularian zooplankton alter carbon cycling under warmer more acidified ocean conditions (2017)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 62 (4). pp. 1541-1551.
    Details
    Publication Date: 2021-03-30
    Description: Anthropogenic atmospheric loading of CO2 raises concerns about combined effects of increasing ocean temperature and acidification, on biological processes. In particular, the response of appendicularian zooplankton to climate change may have significant ecosystem implications as they can alter biogeochemical cycling compared to classical copepod dominated food webs. However, the response of appendicularians to multiple climate drivers and effect on carbon cycling are still not well understood. Here, we investigated how gelatinous zooplankton (appendicularians) affect carbon cycling of marine food webs under conditions predicted by future climate scenarios. Appendicularians performed well in warmer conditions and benefited from low pH levels, which in turn altered the direction of carbon flow. Increased appendicularians removed particles from the water column that might otherwise nourish copepods by increasing carbon transport to depth from continuous discarding of filtration houses and fecal pellets. This helps to remove CO2 from the atmosphere, and may also have fisheries implications.
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  • 2
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    Differential effects of nitrate, ammonium, and urea as N sources for microbial communities in the North Pacific Ocean (2017)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 62 (6). pp. 2550-2574.
    Details
    Publication Date: 2021-04-23
    Description: Nitrogen (N) is the major limiting nutrient for phytoplankton growth and productivity in large parts of the world's oceans. Differential preferences for specific N substrates may be important in controlling phytoplankton community composition. To date, there is limited information on how specific N substrates influence the composition of naturally occurring microbial communities. We investigated the effect of nitrate ( math formula), ammonium ( math formula), and urea on microbial and phytoplankton community composition (cell abundances and 16S rRNA gene profiling) and functioning (photosynthetic activity, carbon fixation rates) in the oligotrophic waters of the North Pacific Ocean. All N substrates tested significantly stimulated phytoplankton growth and productivity. Urea resulted in the greatest (〉300%) increases in chlorophyll a (〈0.06 μg L−1 and ∼0.19 μg L−1 in the control and urea addition, respectively) and productivity (〈0.4 μmol C L−1 d−1 and ∼1.4 μmol C L−1 d−1 in the control and urea addition, respectively) at two experimental stations, largely due to increased abundances of Prochlorococcus (Cyanobacteria). Two abundant clades of Prochlorococcus, High Light I and II, demonstrated similar responses to urea, suggesting this substrate is likely an important N source for natural Prochlorococcus populations. In contrast, the heterotrophic community composition changed most in response to math formula. Finally, the time and magnitude of response to N amendments varied with geographic location, likely due to differences in microbial community composition and their nutrient status. Our results provide support for the hypothesis that changes in N supply would likely favor specific populations of phytoplankton in different oceanic regions and thus, affect both biogeochemical cycles and ecological processes.
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  • 3
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    Influence of carbon availability on dentrification in the central Baltic Sea (2003)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 37 (6). pp. 1146-1163.
    Details
    Publication Date: 2018-03-20
    Description: Denitrification was investigated in the Baltic proper at two stations with different conditions in the deep water. The Gotland Deep was examined as an example of a basin with anoxic, H2S‐containing deep water and station T was taken as an example of low‐oxygen (〈0.2 ml liter−1), sulfide‐free deep water. Denitrification was measured by the acetylene blockage method; in addition, N2O reduction was followed in samples without acetylene. To shed light on the factors limiting denitrification, we compared in situ rates to denitrification after adding nitrate or electron donors. Denitrification was restricted to the layer of the oxic‐anoxic interface in the Gotland Deep and to the water layer near the sediment of station T. For both stations it could be shown that denitrification was not limited by nitrate availability. A lack of available organic C seemed to limit denitrification rates and growth of denitrifiers. As a result of C limitation in the water column, denitrification was restricted to energy‐rich interfaces. In the low‐oxygen water away from energy‐rich interfaces, the less C‐demanding nitrification‐denitrification coupling (NH4+ → N2O → N2) seemed to be favored. Denitrification in the water of the central Baltic seems to be subjected to strong variability due to changing C supply during the course of the year. However, limitation by C availability can be assumed for most of the year and should be taken into account in calculating the N budget of the Baltic.
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  • 4
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    Dissolved methane in the Beaufort Sea and the Arctic Ocean, 1992-2009; sources and atmospheric flux (2016)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 61 (S1). S300-S323.
    Details
    Publication Date: 2019-09-23
    Description: Methane concentration and isotopic composition was measured in ice-covered and ice-free waters of the Arctic Ocean during 11 surveys spanning the years of 1992–1995 and 2009. During ice-free periods, methane flux from the Beaufort shelf varies from 0.14 mg CH4 m−2 d−1 to 0.43 mg CH4 m−2 d−1. Maximum fluxes from localized areas of high methane concentration are up to 1.52 mg CH4 m−2 d−1. Seasonal buildup of methane under ice can produce short-term fluxes of methane from the Beaufort shelf that varies from 0.28 mg CH4 m−2 d−1 to 1.01 mg CH4 m−2 d−1. Scaled-up estimates of minimum methane flux from the Beaufort Sea and pan-Arctic shelf for both ice-free and ice-covered periods range from 0.02 Tg CH4 yr−1 and 0.30 Tg CH4 yr−1, respectively to maximum fluxes of 0.18 Tg CH4 yr−1 and 2.2 Tg CH4 yr−1, respectively. A methane flux of 0.36 Tg CH4 yr−1 from the deep Arctic Ocean was estimated using data from 1993 to 1994. The flux can be as much as 2.35 Tg CH4 yr−1 estimated from maximum methane concentrations and wind speeds of 12 m/s, representing only 0.42% of the annual atmospheric methane budget of ∼ 560 Tg CH4 yr−1. There were no significant changes in methane fluxes during the time period of this study. Microbial methane sources predominate with minor influxes from thermogenic methane offshore Prudhoe Bay and the Mackenzie River delta and may include methane from gas hydrate. Methane oxidation is locally important on the shelf and is a methane sink in the deep Arctic Ocean.
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  • 5
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    Interactions between changing pCO2, N2 fixation, and Fe limitation in the marine unicellular cyanobacterium Crocosphaera (2008)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 53 (6). pp. 2472-2484.
    Details
    Publication Date: 2018-03-09
    Description: We examined the physiological responses of steady-state iron (Fe)-replete and Fe-limited cultures of the biogeochemically critical marine unicellular diazotrophic cyanobacterium Crocosphaera at glacial (19 Pa; 190 ppm), current (39 Pa; 380 ppm), and projected year 2100 (76 Pa; 750 ppm) CO2 levels. Rates of N2 and CO2 fixation and growth increased in step with increasing partial pressure of CO2 (pCO2), but only under Fe-replete conditions. N2 and carbon fixation rates at 75 Pa CO2 were 1.4-1.8-fold and 1.2-2.0-fold higher, respectively, relative to those at present day and glacial pCO2 levels. In Fe-replete cultures, cellular Fe and molybdenum quotas varied threefold and were linearly related to N2 fixation rates and to external pCO2. However, N2 fixation and trace metal quotas were decoupled from pCO2 in Fe-limited Crocosphaera. Higher CO2 and Fe concentrations both resulted in increased cellular pigment contents and affected photosynthesis vs. irradiance parameters. If these results also apply to natural Crocosphaera populations, anthropogenic CO2 enrichment could substantially increase global oceanic N2 and CO2 fixation, but this effect may be tempered by Fe availability. Possible biogeochemical consequences may include elevated inputs of new nitrogen to the ocean and increased potential for Fe and/or phosphorus limitation in the future high-CO2 ocean, and feedbacks to atmospheric pCO2 in both the near future and over glacial to interglacial timescales.
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  • 6
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    Insights into the ecology of the Black Sea through the qualitative loop analysis of the community structure (2018)
    ASLO (Association for the Sciences of Limnology and Oceanography) | Wiley
    In:  Limnology and Oceanography, 63 (2). pp. 968-984.
    Details
    Publication Date: 2021-06-23
    Description: Overfishing, excess nutrient load, and invasion of Mnemiopsis leidyi acted on the Black Sea from 1960s to 1990s. Under the effect of these drivers, the ecosystem underwent several transformations that culminated in the shift from a planktonic food chain to a network with most of the energy diverted to jellyfish. The interplay between multiple stressors and the intricate web of trophic interactions make it difficult to understand which causative mechanisms linked the sources of change to the observed variations. To study such interplay, we focused on the structure of the trophic interactions and applied loop analysis to qualitatively predict the response of variables to stressors. Significant variations in biomass trends were identified with statistical analysis and considered as benchmark to validate loop analysis predictions. The results of the comparisons were used to select the most influential trophic interactions that explain the changes observed between 1960 and 1990. The models were applied to test (1) the importance of various environmental drivers and (2) the mechanisms that produced the observed changes. The results suggested that the changes observed before M. leidyi invasion were strongly influenced by the excess nutrient addition, an outcome that challenges the relevance of the trophic cascade as described in literature. The concurrent effect of overfishing, climate, and nutrient enrichment likely triggered the outburst of M. leidyi in the late 1980s. Our work shows the potential of loop analysis to grasp the causal relationships between the drivers, the structure of the interactions, and the responses of the variables.
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  • 7
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    Macroalgae may mitigate ocean acidification effects on mussel calcification by increasing pH and its fluctuations (2018)
    ASLO (Association for the Sciences of Limnology and Oceanography) | Wiley
    In:  Limnology and Oceanography, 63 (1). pp. 3-21.
    Details
    Publication Date: 2021-02-08
    Description: Ocean acidification (OA) is generally assumed to negatively impact calcification rates of marine organisms. At a local scale however, biological activity of macrophytes may generate pH fluctuations with rates of change that are orders of magnitude larger than the long-term trend predicted for the open ocean. These fluctuations may in turn impact benthic calcifiers in the vicinity. Combining laboratory, mesocosm and field studies, such interactions between OA, the brown alga Fucus vesiculosus, the sea grass Zostera marina and the blue mussel Mytilus edulis were investigated at spatial scales from decimetres to 100s of meters in the western Baltic. Macrophytes increased the overall mean pH of the habitat by up to 0.3 units relative to macrophyte-free, but otherwise similar, habitats and imposed diurnal pH fluctuations with amplitudes ranging from 0.3 to more than 1 pH unit. These amplitudes and their impact on mussel calcification tended to increase with increasing macrophyte biomass to bulk water ratio. At the laboratory and mesocosm scales, biogenic pH fluctuations allowed mussels to maintain calcification even under acidified conditions by shifting most of their calcification activity into the daytime when biogenic fluctuations caused by macrophyte activity offered temporal refuge from OA stress. In natural habitats with a low biomass to water body ratio, the impact of biogenic pH fluctuations on mean calcification rates of M. edulis was less pronounced. Thus, in dense algae or seagrass habitats, macrophytes may mitigate OA impact on mussel calcification by raising mean pH and providing temporal refuge from acidification stress.
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  • 8
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    Resilience in moving water: Effects of turbulence on the predatory impact of the lobate ctenophore Mnemiopsis leidyi (2018)
    ASLO (Association for the Sciences of Limnology and Oceanography) | Wiley
    In:  Limnology and Oceanography, 63 (1). pp. 445-458.
    Details
    Publication Date: 2021-02-08
    Description: Despite its delicate morphology, the lobate ctenophore Mnemiopsis leidyi thrives in coastal ecosystems as an influential zooplankton predator. Coastal ecosystems are often characterized as energetic systems with high levels of natural turbulence in the water column. To understand how natural wind-driven turbulence affects the feeding ecology of M. leidyi, we used a combination of approaches to quantify how naturally and laboratory generated turbulence affects the behavior, feeding processes and feeding impact of M. leidyi. Experiments using laboratory generated turbulence demonstrated that turbulence can reduce M. leidyi feeding rates on copepods and Artemia nauplii by 〉 50%. However, detailed feeding data from the field, collected during highly variable surface conditions, showed that wind-driven turbulence did not affect the feeding rates or prey selection of M. leidyi. Additional laboratory experiments and field observations suggest that the feeding process of M. leidyi is resilient to wind-driven turbulence because M. leidyi shows a behavioral response to turbulence by moving deeper in the water column. Seeking refuge in deeper waters enables M. leidyi to maintain high feeding rates even under high turbulence conditions generated by wind driven mixing. As a result, M. leidyi exerted a consistently high predatory impact on prey populations during highly variable and often energetic wind-driven mixing conditions. This resilience adds to our understanding of how M. leidyi can thrive in a wide spectrum of environments around the world. The limits to this resilience also set boundaries to its range expansion into novel areas.
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  • 9
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    Thaumarchaeal ecotype distributions across the equatorial Pacific Ocean and their potential roles in nitrification and sinking flux attenuation (2017)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 62 (5). pp. 1984-2003.
    Details
    Publication Date: 2020-02-06
    Description: Thaumarchaea are among the most abundant microbial groups in the ocean, but controls on their abundance and the distribution and metabolic potential of different subpopulations are poorly constrained. Here, two ecotypes of ammonia-oxidizing thaumarchaea were quantified using ammonia monooxygenase (amoA) genes across the equatorial Pacific Ocean. The shallow, or water column “A” (WCA), ecotype was the most abundant ecotype at the depths of maximum nitrification rates, and its abundance correlated with other biogeochemical indicators of remineralization such as NO3 : Si and total Hg. Metagenomes contained thaumarchaeal genes encoding for the catalytic subunit of the urease enzyme (ureC) at all depths, suggesting that members of both WCA and the deep, water column “B” (WCB) ecotypes may contain ureC. Coupled urea hydrolysis-ammonia oxidation rates were similar to ammonia oxidation rates alone, suggesting that urea could be an important source of ammonia for mesopelagic ammonia oxidizers. Potential inducement of metal limitation of both ammonia oxidation and urea hydrolysis was demonstrated via additions of a strong metal chelator. The water column inventory of WCA was correlated with the depth-integrated abundance of WCB, with both likely controlled by the flux of sinking particulate organic matter, providing strong evidence of vertical connectivity between the ecotypes. Further, depth-integrated amoA gene abundance and nitrification rates were correlated with particulate organic nitrogen flux measured by contemporaneously deployed sediment traps. Together, the results refine our understanding of the controls on thaumarchaeal distributions in the ocean, and provide new insights on the relationship between material flux and microbial communities in the mesopelagic.
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  • 10
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    Microbes, macrofauna, and methane: a novel seep community fueled by aerobic methanotrophy (2013)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 58 (5). pp. 1640-1656.
    Details
    Publication Date: 2019-09-23
    Description: During the discovery and description of seven New Zealand methane seep sites, an infaunal assemblage dominated by ampharetid polychaetes was found in association with high seabed methane emission. This ampharetid-bed assemblage had a mean density of 57,000 ± 7800 macrofaunal individuals m−2 and a maximum wet biomass of 274 g m−2, both being among the greatest recorded from deep-sea methane seeps. We investigated these questions: Does the species assemblage present within these ampharetid beds form a distinct seep community on the New Zealand margin? and What type of chemoautotrophic microbes fuel this heterotrophic community? Unlike the other macro-infaunal assemblages, the ampharetid-bed assemblage composition was homogeneous, independent of location. Based on a mixing model of species-specific mass and isotopic composition, combined with published respiration measurements, we estimated that this community consumes 29–90 mmol C m−2 d−1 of methane-fueled biomass; this is 〉 290 times the carbon fixed by anaerobic methane oxidizers in these ampharetid beds. A fatty acid biomarker approach supported the finding that this community, unlike those previously known, consumes primarily aerobic methanotrophic bacteria. Due to the novel microbial fueling and high methane flux rates, New Zealand's ampharetid beds provide a model system to study the influence of metazoan grazing on microbially mediated biogeochemical cycles, including those that involve greenhouse gas emissions
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  • 11
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    The influence of pore-water advection, benthic photosynthesis, and respiration on calcium carbonate dynamics in reef sands (2012)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 57 (3). pp. 809-825.
    Details
    Publication Date: 2016-04-29
    Description: To investigate diel calcium carbonate (CaCO3) dynamics in permeable coral reef sands, we measured pore-water profiles and fluxes of oxygen (O2), nutrients, pH, calcium (Ca2+), and alkalinity (TA) across the sediment-water interface in sands of different permeability at Heron Reef, Australia. Background flushing rates were high, most likely as a result of infaunal burrow irrigation, but flux chamber stirring enhanced pore-water exchange. Light and pore-water advection fueled high rates of benthic primary production and calcification in sunlit surface sediments. In the light, benthic photosynthesis and calcification induced surface minima in Ca2+ and TA and peaks in pH and O2. Oxygen penetration depth in coarse sands decreased from ∼ 1.2 cm during the day to ∼ 0.6 cm at night. Total oxygen uptake (TOU) in dark chambers was three to fourteen times greater than diffusive uptake and showed a direct effect of pore-water advection. Greater sediment oxygen consumption rates were observed in higher permeability sands. In the dark, TA release was not stimulated by increasing TOU because of a damping effect of pore-water advection on metabolic CaCO3 dissolution efficiency. On a daily basis, CaCO3 undergoes net dissolution in Heron Reef sands. However, pore-water advection can reverse the CaCO3 budget and promote CaCO3 preservation under the most energetic conditions.
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  • 12
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    The nutrient stoichiometry of benthic microalgal growth: Redfield proportions are optimal (1999)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 44 . pp. 440-446.
    Details
    Publication Date: 2018-06-25
    Description: Cellular nutrient ratios are often applied as indicators of nutrient limitation in phytoplankton studies, especially the so-called Redfield ratio. For periphyton, similar data are scarce. We investigated the changes in cellular C: N: P stoichiometry of benthic microalgae in response to different levels and types of nutrient limitation and a variety of abiotic conditions in laboratory experiments with natural inocula. C: N ratios increased with decreasing growth rate, irrespective of the limiting nutrient. At the highest growth rates, the C: N ratio ranged uniformly around 7.5. N: P ratios 〈13 indicated N limitation, while N: P ratios 〉22 indicated P limitation. Under P limitation, the C: P ratios increased at low growth rate and varied around 130 at highest growth rates. For a medium with balanced supply of N and P, an optimal stoichiometric ratio of C: N: P = 119 : 17 : 1 could be deduced for benthic microalgae, which is slightly higher than the Redfield ratio (106 : 16 : 1) considered typical for optimally growing phytoplankton. The optimal ratio was stable against changes in abiotic conditions. In conclusion, cellular nutrient ratios are proposed as an indicator for nutrient status in periphyton.
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  • 13
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    Testing the effect of CO2 concentration on dynamics of marine heterotrophic bacterioplankton (2006)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 51 (1). pp. 1-11.
    Details
    Publication Date: 2019-09-23
    Description: To date no study exists that directly addresses changes in dynamics of heterotrophic bacteria in surface waters in relation to partial pressure of CO2 (pCO2). Therefore, we studied the effect of changes in pCO2 on bacterial abundance and activities by using mesocosms with different pCO2 levels (~190, ~370, and ~700 ppmV, representing past, present-day, and future atmospheric pCO2, respectively). Abundance of total bacteria did not differ with increasing pCO2 throughout the whole study period, whereas bacterial protein production (BPP) was highest at highest pCO2. This effect was even more pronounced for cell-specific production rates, especially those of attached bacteria, which were up to 25 times higher than those of free bacteria. During the breakdown of the bloom, however, the abundance of both free and attached bacteria was significantly increased with pCO2. Differences in bacterial growth rate (µ) were smaller than those of BPP, but both µ and BPP of attached bacteria were elevated under high pCO2. Averages of total protease as well as α- and α -glucosidase activities were highest at elevated pCO2 levels, but a statistically significant dependence on pCO2 was only evident for protease activity. There is a measurable but indirect effect of changes in pCO2 on bacterial activities that are mainly linked to phytoplankton and presumably particle dynamics
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  • 14
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    Relative influence of nitrogen and phosphorous availability on phytoplankton physiology and productivity in the Oligotrophic Sub-Tropical North Atlantic Ocean (2008)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 53 (1). pp. 291-305.
    Details
    Publication Date: 2018-08-14
    Description: Nutrient addition bioassay experiments were performed in the low-nutrient, low-chlorophyll oligotrophic subtropical North Atlantic Ocean to investigate the influence of nitrogen (N), phosphorus (P), and/or iron (Fe) on phytoplankton physiology and the limitation of primary productivity or picophytoplankton biomass. Additions of N alone resulted in 1.5-2 fold increases in primary productivity and chlorophyll after 48 h, with larger (~threefold) increases observed for the addition of P in combination with N (NP). Measurements of cellular chlorophyll contents permitted evaluation of the physiological response of the photosynthetic apparatus to N and P additions in three picophytoplankton groups. In both Prochlorococcus and the picoeukaryotes, cellular chlorophyll increased by similar amounts in N and NP treatments relative to all other treatments, suggesting that pigment synthesis was N limited. In contrast, the increase of cellular chlorophyll was greater in NP than in N treatments in Synechococcus, suggestive of NP co-limitation. Relative increases in cellular nucleic acid were also only observed in Synechococcus for NP treatments, indicating co-limitation of net nucleic acid synthesis. A lack of response to relief of nutrient stress for the efficiency of photosystem II photochemistry, Fv :Fm, suggests that the low nutrient supply to this region resulted in a condition of balanced nutrient limited growth, rather than starvation. N thus appears to be the proximal (i.e. direct physiological) limiting nutrient in the oligotrophic sub-tropical North Atlantic. In addition, some major picophytoplankton groups, as well as overall autotrophic community biomass, appears to be co-limited by N and P.
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  • 15
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    Lakes as sentinels of climate change (2009)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 54 (6). pp. 2283-2297.
    Details
    Publication Date: 2017-10-24
    Description: While there is a general sense that lakes can act as sentinels of climate change, their efficacy has not been thoroughly analyzed. We identified the key response variables within a lake that act as indicators of the effects of climate change on both the lake and the catchment. These variables reflect a wide range of physical, chemical, and biological responses to climate. However, the efficacy of the different indicators is affected by regional response to climate change, characteristics of the catchment, and lake mixing regimes. Thus, particular indicators or combinations of indicators are more effective for different lake types and geographic regions. The extraction of climate signals can be further complicated by the influence of other environmental changes, such as eutrophication or acidification, and the equivalent reverse phenomena, in addition to other land-use influences. In many cases, however, confounding factors can be addressed through analytical tools such as detrending or filtering. Lakes are effective sentinels for climate change because they are sensitive to climate, respond rapidly to change, and integrate information about changes in the catchment.
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  • 16
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    Different reactions of Southern Ocean phytoplankton size classes to iron fertilization. (2006)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 51 (3). pp. 1217-1229.
    Details
    Publication Date: 2018-08-14
    Description: During the European Iron Fertilisation Experiment (EIFEX), performed in the Southern Ocean, we investigated the reactions of different phytoplankton size classes to iron fertilization, applying measurements of size fractionated pigments, particulate organic matter, microscopy, and flow cytometry. Chlorophyll a (Chl a) concentrations at 20-m depth increased more than fivefold following fertilization through day 26, while concentrations of particulate organic carbon (POC), nitrogen (PON), and phosphorus (POP) roughly doubled through day 29. Concentrations of Chl a and particulate organic matter decreased toward the end of the experiment, indicating the demise of the iron-induced phytoplankton bloom. Despite a decrease in total diatom biomass at the end of the experiment, biogenic particulate silicate (bPSi) concentrations increased steadily due to a relative increase of heavily silicified diatoms. Although diatoms 〉 20 mu m were the main beneficiaries of iron fertilization, the growth of small diatoms (2-8 mu m) was also enhanced, leading to a shift from a haptophyte- to a diatom-dominated community in this size fraction. The total biomass had lower than Redfield C : N, N : P, and C : P ratios but did not show significant trends after iron fertilization. This concealed various alterations in the elemental composition of the different size fractions. The microplankton 20 mu m) showed decreasing C : N and increasing N : P and C : P ratios, possibly caused by increased N uptake and the consumption of cellular P pools. The nanoplankton (2-20 mu m) showed almost constant C : N and decreasing N : P and C : P ratios. Our results suggest that the latter is caused by a shift in composition of taxonomic groups.
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  • 17
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    Elevating the predatory effect: Sensory‐scanning foraging strategy by the lobate ctenophore Mnemiopsis leidyi (2015)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 60 (1). pp. 100-109.
    Details
    Publication Date: 2018-01-01
    Description: The influential predatory role of the lobate comb jellyfish Mnemiopsis leidyi has largely been attributed to the generation of a hydrodynamically silent feeding current to entrain and initiate high encounter rates with prey. However, for high encounter rates to translate to high ingestion rates, M. leidyi must effectively capture the entrained prey. To investigate the capture mechanisms, we recorded and quantified, using three-dimensional videography, the outcome of encounter events with slow swimming Artemia prey. The auricles, which produce the feeding current of M. leidyi, were the primary encounter structures, first contacting 59% of the prey in the feeding current. Upon detection, the auricles manipulated the Artemia to initiate captures on the tentillae, which are coated with sticky cells (colloblasts). Using this mechanism of sensory-scanning to capture prey entrained in the feeding current, M. leidyi uses a similar foraging strategy to that of feeding-current foraging copepods. As such, M. leidyi has a higher capture efficiency than do medusae, contributing to the greater predatory effect of M. leidyi in both its endemic and invasive ecosystems.
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  • 18
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    Biogeochemical composition of natural sea ice brines from the Weddell Sea during early austral summer (2007)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 52 (5). pp. 1809-1823.
    Details
    Publication Date: 2017-12-31
    Description: Sea ice brines were collected from a single floe composed of different ice types in the western Weddell Sea in December 2004. The chemical composition of the brines (temperature: 23.4°C to 22.1°C; salinity: 40–63) was examined on seven occasions over 25 days with measurements of dissolved oxygen, dissolved inorganic macronutrients (nitrate plus nitrite, ammonium, phosphorus [DIP], and silicic acid), pH, total alkalinity (AT), dissolved organic carbon (DOC) and nitrogen (DON), total dissolved inorganic carbon (CT), and the stable isotopic composition of CT (δ13CT). The in situ pH ranged from 8.41-8.82 on the seawater scale, dissolved oxygen from 212-604 µmol kg−1, nitrate from 0.1-3.1 µmol kg−1, ammonium 0.1-2.4 µmol kg−1, DIP 0.4- 2.0 µmol kg−1, silicic acid 4-80 µmol kg−1, AT 2,690-4,620 µeq kg−1, DOC 115-359 µmol kg−1, DON 8-26 µmol kg−1, CT 2,090-3,550 µmol kg−1, and δ13CT +2.9‰ - +6.4‰. Compared with the chemical composition of surface oceanic water (salinity of 34), the brines had elevated pH, reduced concentrations of dissolved inorganic macronutrients (including carbon), especially dissolved inorganic nitrogen, and were mostly supersaturated with dissolved oxygen with respect to equilibrium with air, whereas the CT was considerably enriched in 13C. The chemical composition of the brines was consistent with internal biological productivity, but there was a lack of a distinctive and uniform relationship among the major dissolved inorganic nutrients typically used for describing biological activity. This was interpreted as the result of varying stoichiometry of biological activity within a very small spatial scale. Modification by abiotic processes was a potential contributing factor, such as degassing acting on the dissolved oxygen concentration. Carbonate mineral formation, acting on AT and CT, was not evident in brines from first-year ice but was apparent in brine from second-year ice.
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  • 19
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    Multiple environmental changes induce interactive effects on bacterial degradation activity in the Arctic Ocean (2015)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 60 (4). pp. 1392-1410.
    Details
    Publication Date: 2019-09-23
    Description: The Arctic Ocean faces multiple environmental changes induced by climate change on both global and regional scale. In addition to global changes in seawater temperature and pH, Arctic waters receive organic matter enrichment due to increasing pelagic primary production, enhanced sea ice melting and increasing terrestrial carbon loads. We experimentally tested individual and combined effects of warming, acidification and organic matter amendment on growth, biomass production and extracellular enzyme activities of bacterioplankton in Fram Strait during early summer. Results reveal pH optima of 6.7–7.6 for extracellular leucine-aminopeptidase and below pH 6.0 for beta-glucosidase in the West Spitsbergen Current. These optima well below the current seawater pH imply increasing hydrolytic activity with ongoing ocean acidification. However, the new synthesis of extracellular enzymes during 4-d incubations obscured the biochemical pH effects. Elevated temperature and carbohydrate supply had strongly interactive effects on bacterial biomass production in both Atlantic Water of the West Spitsbergen Current and Polar Water of the East Greenland Current. Activation energies ranged from 45 kJ mol−1 to 52 kJ mol−1 at in situ substrate concentration, while substantially higher values of 122–174 kJ mol−1 could be estimated from incubations with carbohydrate addition. The net loss of total amino acids in carbohydrate-amended incubations was significantly reduced at elevated temperature in all experiments, suggesting enhanced de novo synthesis. Our findings show that the complexity of combined effects must be considered to better assess the potential of climate change to alter biogenic carbon and energy fluxes in marine systems.
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  • 20
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    Combined effects of CO2 and temperature on carbon uptake and partitioning by the marine diatoms Thalassiosira weissflogii and Dactyliosolen fragilissimus (2015)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 60 (3). pp. 901-919.
    Details
    Publication Date: 2018-06-01
    Description: Carbon uptake and partitioning of two globally abundant diatom species, Thalassiosira weissflogii and Dactyliosolen fragilissimus, was investigated in batch culture experiments under four conditions: ambient (15 degrees C, 400 atm), high CO2 (15 degrees C, 1000 atm), high temperature (20 degrees C, 400 atm), and combined (20 degrees C, 1000 atm). The experiments were run from exponential growth into the stationary phase (six days after nitrogen depletion), allowing us to track biogeochemical dynamics analogous to bloom situations in the ocean. Elevated CO2 had a fertilizing effect and enhanced uptake of dissolved inorganic carbon (DIC) by about 8% for T. weissflogii and by up to 39% for D. fragilissimus. This was also reflected in higher cell numbers, build-up of particulate and dissolved organic matter, and transparent exopolymer particles. The CO2 effects were most prominent in the stationary phase when nitrogen was depleted and CO2(aq) concentrations were low. This indicates that diatoms in the high CO2 treatments could take up more DIC until CO2 concentrations in seawater became so low that carbon limitation occurs. These results suggest that, contrary to common assumptions, diatoms could be highly sensitive to ongoing changes in oceanic carbonate chemistry, particularly under nutrient limitation. Warming from 15 to 20 degrees C had a stimulating effect on one species but acted as a stressor on the other species, highlighting the importance of species-specific physiological optima and temperature ranges in the response to ocean warming. Overall, these sensitivities to CO2 and temperature could have profound impacts on diatoms blooms and the biological pump.
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  • 21
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    The association between biogenic and inorganic minerals and the amino acid composition of settling particles (2010)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 55 (5). pp. 2207-2218.
    Details
    Publication Date: 2017-10-24
    Description: To test the hypothesis that calcium carbonate (rather than opal) carries most organic carbon to the deep sea, total hydrolysable amino acid (THAA) analysis was applied to deep-sea (3000 m) sediment trap material from the northeast Atlantic (PAP Site), a variable but intrinsically carbonate-dominated system. THAAs were analyzed in conjunction with total organic carbon, biogenic silica, calcium carbonate, and inferred lithogenic fluxes. The THAA-based degradation state of organic carbon could not be systematically explained by changes in the flux of different mineral phases, which could account for only 16% of the observed variability. In addition amino acid parameters indicative of source organisms indicate that diatom cell walls are an important residual component of organic carbon reaching the deep ocean, a finding supported by comparison with data from previous studies of diverse oceanic environments. Finally, during 2001, very high organic carbon fluxes were associated with elevated lithogenic fluxes and low organic matter degradation relative to surrounding years. In accordance with other recent experimental and observational studies, the data indicate that under specific export scenarios, lithogenic fluxes can act as highly significant mediators of organic carbon transfer to the deep ocean.
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  • 22
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    Dissolved silicon isotopic compositions in the East China Sea: Water mass mixing vs. biological fractionation (2015)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 60 (5). pp. 1619-1633.
    Details
    Publication Date: 2018-06-20
    Description: We present the first set of dissolved silicon isotope data in seawater (delta Si-30(Si(OH)4)) from the East China Sea, a large and productive marginal sea significantly influenced by the Kuroshio Current and freshwater inputs from the Changjiang (Yangtze River). In summer (August 2009), the lowest surface delta Si-30(Si(OH)4) signatures of +2.1 parts per thousand corresponding to the highest Si(OH)(4) concentrations (similar to 30.0 mu mol L-1) were observed nearshore in Changjiang Diluted Water. During advection on the East China Sea inner shelf, surface delta Si-30(Si(OH)4) increased rapidly to +3.2 parts per thousand while Si(OH)(4) became depleted, indicating increasing biological utilization of the Si(OH)(4) originating from the Changjiang Diluted Water. This is also reflected in the water column profiles characterized by a general decrease of delta Si-30(Si(OH)4) and an increase of Si(OH)(4) with depth on the East China Sea mid-shelf and slope. In winter (December 2009-January 2010), however, the delta Si-30(Si(OH)4) was nearly constant at +1.9 parts per thousand throughout the water column on the East China Sea shelf beyond the nearshore, which was a consequence of enhanced vertical mixing of the Kuroshio subsurface water. Horizontal admixture of Kuroshio surface water, which is highly fractionated in Si isotopes, was observed only beyond the shelf break. Significant seasonal differences in delta Si-30(Si(OH)4) were detected in the surface waters beyond the Changjiang Diluted Water-influenced region on the East China Sea shelf, where the winter values were similar to 1.0 parts per thousand lower than those in summer, despite the same primary Si(OH)(4) supply from the Kuroshio subsurface water during both seasons. This demonstrates significantly higher biological consumption and utilization of Si(OH)(4) in summer than in winter.
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  • 23
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    The modulating effect of light intensity on the response of the coccolithophore Gephyrocapsa oceanicato ocean acidification (2015)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 60 (6). pp. 2145-2157.
    Details
    Publication Date: 2018-10-01
    Description: Global change leads to a multitude of simultaneous modifications in the marine realm among which shoaling of the upper mixed layer, leading to enhanced surface layer light intensities, as well as increased carbon dioxide (CO2) concentration are some of the most critical environmental alterations for phytoplankton. In this study, we investigated the responses of growth, photosynthetic carbon fixation and calcification of the coccolithophore Gephyrocapsa oceanica to elevated inline image (51 Pa, 105 Pa, and 152 Pa) (1 Pa ≈ 10 μatm) at a variety of light intensities (50–800 μmol photons m−2 s−1). By fitting the light response curve, our results showed that rising inline image reduced the maximum rates for growth, photosynthetic carbon fixation and calcification. Increasing light intensity enhanced the sensitivity of these rate responses to inline image, and shifted the inline image optima toward lower levels. Combining the results of this and a previous study (Sett et al. 2014) on the same strain indicates that both limiting low inline image and inhibiting high inline image levels (this study) induce similar responses, reducing growth, carbon fixation and calcification rates of G. oceanica. At limiting low light intensities the inline image optima for maximum growth, carbon fixation and calcification are shifted toward higher levels. Interacting effects of simultaneously occurring environmental changes, such as increasing light intensity and ocean acidification, need to be considered when trying to assess metabolic rates of marine phytoplankton under future ocean scenarios.
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  • 24
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    The effect of submarine CO2vents on seawater: Implications for detection of subsea carbon sequestration leakage (2015)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 60 (2). pp. 402-410.
    Details
    Publication Date: 2015-07-03
    Description: The effect of submarine carbon dioxide (CO2) vents on seawater carbonate chemistry have been determined using hydrographical and marine carbonate data obtained from two submarine hydrothermal vent fields, as well as a reference station, all near the Jan Mayen Island in the Norwegian-Greenland Sea. We have shown that one can successfully determine the excess carbon that enters the seawater from the vents by applying a modified version of a back-calculation technique, which is traditionally used to study the invasion of excess atmospheric CO2 in the surface ocean. As a result of this excess carbon, total dissolved inorganic carbon (CT) in the seawater surrounding the vents was on average 12 μmol kg−1 (1-30 μmol kg−1) higher compared to samples obtained from a reference station outside the venting areas. The observed excess CT was most significant between 100 m and 200 m but was noticeable in all depths with the exception of the upper 10-20 m. The absence of a venting CO2 signal in the surface water and the realism of the results are discussed. We believe the present method is promising for monitoring (detection and quantification) of CO2 leakage into the water column due to its high sensitivity and readiness for automation.
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  • 25
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    A toolbox for secondary quality control on ocean chemistry and hydrographic data (2015)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography: Methods, 13 (11). pp. 601-608.
    Details
    Publication Date: 2021-04-21
    Description: High quality, reference measurements of chemical and physical properties of seawater are of great importance for a wide research community, including the need to validate models and attempts to quantify spatial and temporal variability. Whereas data precision has been improved by technological advances, the data accuracy has improved mainly by the use of certified reference materials (CRMs). However, since CRMs are not available for all variables, and use of CRMs does not guarantee bias-free data, we here present a recently developed Matlab toolbox for performing so-called secondary quality control on oceanographic data by the use of crossover analysis. This method and how it has been implemented in this toolbox is described in detail. This toolbox is developed mainly for use by sea-going scientists as a tool for quickly assessing possible bias in the measurements that can-hopefully-be remedied during the expedition, but also for possible post-cruise adjustment of data to be consistent with previous measurements in the region. The toolbox, and reference data, can be downloaded from the Carbon Dioxide Information Analysis Center (CDIAC): http://cdiac.ornl.gov/ftp/oceans/2nd_QC_Tool_V2/.
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  • 26
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    Effects of increased CO2 concentration on nutrient limited coastal summer plankton depend on temperature (2016)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 61 (3). pp. 853-868.
    Details
    Publication Date: 2019-02-01
    Description: Increasing seawater temperature and CO2 concentrations both are expected to increase coastal phytoplankton biomass and carbon to nutrient ratios in nutrient limited seasonally stratified summer conditions. This is because temperature enhances phytoplankton growth while grazing is suggested to be reduced during such bottom-up controlled situations. In addition, enhanced CO2 concentrations potentially favor phytoplankton species, that otherwise depend on costly carbon concentrating mechanisms (CCM). The trophic consequences for consumers under such conditions, however, remain little understood. We set out to experimentally explore the combined effects of increasing temperature and CO2 concentration for phytoplankton biomass and stoichiometry and the consequences for trophic transfer (here for copepods) on a natural nutrient limited Baltic Sea summer plankton community. The results show, that warming effects were translated to the next trophic level by switching the system from a bottom-up controlled to a mainly top-down controlled one. This was reflected in significantly down-grazed phytoplankton and increased zooplankton abundance in the warm temperature treatment (22.5°C). Additionally, at low temperature (16.5°C) rising CO2 concentrations significantly increased phytoplankton biomass. The latter effect however, was due to direct negative impact of CO2 on copepod nauplii which released phytoplankton from grazing in the cold but not in the warm treatments. Our results suggest that future seawater warming has the potential to switch trophic relations between phytoplankton and their grazers under nutrient limited conditions with the consequence of potentially disguising CO2 effects on coastal phytoplankton biomass.
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    Diatom frustules show increased mechanical strength and altered valve morphology under iron limitation (2011)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 56 (4). pp. 1399-1410.
    Details
    Publication Date: 2017-05-02
    Description: Iron limitation often results in increased cellular silica contents of diatoms, suggesting that diatoms grow thicker and possibly mechanically stronger frustules when limited. We performed stability measurements for six diatom species grown under iron-limitation and iron-sufficient conditions. Frustule strength increased in all species when grown under iron limitation, with this effect being statistically significant for four of them. Valve morphology and silica content of the pennate Fragilariopsis kerguelensis and the centric Coscinodiscus wailesii changed under iron limitation but only valve morphology changes were significant; F. kerguelensis grew thicker costae while C. wailesii had smaller pores, especially in the outer part of the valves. These morphological changes are clearly in agreement with increased mechanical strength. Increased cellular silica concentrations in diatoms grown under iron limitation do result in increased frustule strength, most likely improving their protection against grazers.
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  • 28
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    Changes in primary productivity and chlorophyll a in response to iron fertilization in the Southern Polar Frontal Zone (2002)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 47 . pp. 1324-1335.
    Details
    Publication Date: 2014-01-30
    Description: EisenEx�the second in situ iron enrichment experiment in the Southern Ocean�was performed in the Atlantic sector over 3 weeks in November 2000 with the overarching goal to test the hypothesis that primary productivity in the Southern Ocean is limited by iron availability in the austral spring. Underwater irradiance, chlorophyll a (Chl a), photochemical efficiency, and primary productivity were measured inside and outside of an iron-enriched patch in order to quantify the response of phytoplankton to iron fertilization. Chl a concentration and photosynthetic rate (14C uptake in simulated in situ incubations) were measured in pico-, nano-, and microphytoplankton. Photochemical efficiency was studied with fast repetition rate fluorometry and xenon-pulse amplitude modulated fluorometry. The high-nutrient low-chlorophyll waters outside the Fe-enriched patch were characterized by deep euphotic zones (63-72 m), low Chl a (48-56 mg m-2), low photosynthetic efficiency (Fv/Fm ~ 0.3), and low daily primary productivity (130-220 mg C m-2 d-1). Between 70 and 90% of Chl a was found in pico- and nanophytoplankton. During the induced bloom, Fv/Fm increased up to ;0.55, primary productivity and Chl a reached the maximum values of 790 mg C m-2 d-1 and 231 mg Chl a m-2, respectively. As a consequence, the euphotic depth decreased to ~41 m. Picophytoplankton biomass hardly changed. Nano- and microphytoplankton biomass increased. In the first 2 weeks of the experiment, when the depth of the upper mixed layer was mostly 〈40 m, primary productivity was highly correlated with Chl a. In the third week, productivity was much lower than predicted from Chl a, probably because of a reduction in photosynthetic capacity as a consequence of increased physical variability in the upper water column. These results provide unequivocal evidence that iron supply is the central factor controlling phytoplankton primary productivity in the Southern Ocean, even if the mixing depth is 〉80 m.
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  • 29
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    Disrupted seasonal clockwork in the population dynamics of a freshwater copepod by climate warming (2009)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 54 (6). pp. 2493-2505.
    Details
    Publication Date: 2014-01-30
    Description: Life history responses are expected to accompany climate warming, yet little is known how long-term effects of climate and environmental change affect the seasonal dynamics of planktonic organisms. We used an historical data set from Lake Washington (U.S.A.) to quantify population responses of a calanoid copepod (Leptodiaptomus ashlandi) to long-term changes in temperature and resource availability and explore potential mechanisms for the responses. Increasing water temperatures (annual mean increase of 1.5 degrees C in the upper 10-m water volume) and longer stratification periods (about 4 weeks) were observed between 1962 and 2005, coincident with a pronounced decline in Leptodiaptomus densities. However, production was maintained because of an increase in the production to biomass ratio and a life cycle shift in Leptodiaptomus from an annual to a 6-month cycle. Cross-wavelet analyses demonstrated that the annual thermal forcing of copepod recruitment observed during the first two decades of the study weakened substantially, leading to more stochastic population dynamics during the past two decades. This shift from one to two generations per year was most likely produced by a longer and warmer growing period combined with changing fluctuations in resource (phytoplankton) availability. Climate change can lead to higher-frequency voltinism in ectothermic organisms and to temporal reorganization of their population dynamics.
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    The influence of the Atlantic and regional climate variability on the long-term changes in gelatinous carnivore populations in the northwestern Mediterranean (2008)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 53 (4). pp. 1456-1467.
    Details
    Publication Date: 2018-08-14
    Description: We investigated potential connections between climate and the population size of gelatinous carnivores in the northwestern Mediterranean. The interannual variability of large- and meso-scale climate factors was related to gelatinous carnivore dynamics by means of a multivariate regression model. Gelatinous carnivore population dynamics integrate the climate-related changes in the northwestern Mediterranean with close correlations between climate and the population size of the Calycophoran siphonophores Chelophyes appendiculata and Abylopsis tetragona and the Hydromedusae Rhopalonema velatum and Solmundella bitentaculata. We show that the sensitivity of particular gelatinous carnivore species to environmental forcing relates to their seasonal appearance and peak of abundance. Our results advocate the occurrence of short time windows, during which gelatinous carnivores appear more sensitive to environmental conditions, and favorable conditions may therefore substantially enhance their annual peaks. Furthermore, there were threshold values from which climate effects on gelatinous carnivores become noticeable, indicating that the climate-gelatinous carnivore relationship intensifies according to the strength of climate forcing. The possibility of using the North Atlantic climate variability for assessing and predicting interannual abundance changes of these organisms in the northwestern Mediterranean is considered.
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    Influence of river discharge in the tropical and subtropical North Atlantic Ocean (2009)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 54 (2). pp. 644-648.
    Details
    Publication Date: 2019-09-23
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    Effects of microzooplankton and mixotrophy in an experimental planktonic food web (2004)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 49 . pp. 1435-1445.
    Details
    Publication Date: 2019-09-23
    Description: Microzooplankton have received increased attention as an important trophic link between the microbial loop and calanoid copepods. On the basis of food size spectra overlap in some microzooplankton groups and calanoid copepods, however, such microzooplankton could function as competitors rather than as food for calanoid copepods (intraguild prey). Mixotrophic flagellates presumably represent a link between the microbial loop and the micro and mesozooplankton. We investigated the effects of microzooplankton and mixotrophy by altering the presence of a heterotrophic dinoflagellate and of a mixotrophic nanoflagellate in artificial food webs with calanoid copepods as terminal consumers. Overall system productivity was manipulated by two levels of nutrient enrichment. The heterotrophic dinoflagellate drastically reduced the nanophytoplankton and enhanced the reproduction of the copepods, suggesting that its role as a competitor is negligible compared to its function as a trophic link. In spite of the presence of heterotrophic nanoflagellates, the mixotroph had a strong negative effect on the picophytoplankton and (presumably) on bacterial biomass. At the same time, the mixotroph enhanced the atomic C:N ratio of the seston biomass, indicating a higher efficiency in overall primary production. Copepod reproduction was enhanced in the presence of the mixotrophic nanoflagellate. Results did not support predictions of the intraguild predation theory: The ratios of the intraguild predators and their preys were not affected by overall system productivity
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    A comparative study of responses in planktonic food web structure and function in contrasting European coastal waters exposed to experimental nutrient addition (2006)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 51 . pp. 488-503.
    Details
    Publication Date: 2018-08-14
    Description: We quantify, compare, and generalize responses of experimental nutrient loadings (LN) on planktonic community structure and function in coastal waters. Data were derived from three mesocosm experiments undertaken in Baltic (BAL), Mediterranean (MED), and Norwegian (NOR) coastal waters. A planktonic model with seven functional compartments and 30-32 different carbon flows fit to all three experiments was used as a framework for flow-rate estimation and comparison. Flows were estimated on the basis of time series of measured biomass, some measured flows, and inverse modeling. Biomass and gross uptake rate of carbon of most groups increased linearly with increasing LN in the nutrient input range of 0-1 µmol N L-1 d-1 at all locations. The fate of the gross primary production (GPP) was similar in all systems. Autotrophic biomass varied by two orders of magnitude among locations, with the lowest biomass and response to nutrient addition in MED waters. The variation of GPP among sites was less than one order of magnitude. Mesozooplankton dominated by doliolids (Tunicata), but not those dominated by copepods, presumably exerted efficient control of the autotrophic biomass, thereby buffering responses of autotrophs to high nutrient input. Among the many factors that can modify the responses of autotrophs to nutrients, the time scale over which the enrichment is made and the precise mode of nutrient enrichment are important. We suggest a general concept that may contribute to a scientific basis for understanding and managing coastal eutrophication
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    Diversity in planktonic communities: an experimental test of the intertmediate disturbance hypothesis (1999)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 44 . pp. 1114-1119.
    Details
    Publication Date: 2019-09-23
    Description: According to Connell�s intermediate disturbance hypothesis (IDH), diversity within a community is maximal at intermediate frequencies and intensities of disturbances. In order to test the IDH, disturbances of different frequencies and intensities were imposed on natural plankton communities in controlled field experiments. These disturbances consisted of an artificial deepening of the mixed layer, leading to the dilution of epilimnetic populations and to a higher level of nutrients. Intervals between disturbances ranged from 2 to 12 d. Different intensities of disturbance were caused by differences in the experimental mixing depth (150 and 225% of the original epilimnion depth). Investigation focused on the effect that disturbances had on the diversity of natural phytoplankton communities. Additionally, we were interested in determining the effect of grazing by zooplankton. The results of the field experiments show for the first time the applicability of the IDH to phytoplankton within complete planktonic communities. Diversity showed a clear maximum at the intermediate disturbance interval of 6 d. Similarly, species number peaked at intermediate interval length (6-10 d).
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    The role of the picoeukaryote Aureococcus anophagefferens in cycling of marine high molecular weight dissolved organic nitrogen (2003)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 48 (5). pp. 1825-1830.
    Details
    Publication Date: 2018-06-25
    Description: Environmental evidence suggests that Aureococcus anophagefferens (Pelagophyceae), a eukaryotic picoplankton that blooms in coastal seawaters, can outcompete other organisms because of its ability to use abundant dissolved organic nitrogen (DON). To test this hypothesis, we isolated A. anophagefferens in axenic culture and monitored its growth on high-molecular weight (HMW) DON collected from sediment pore waters, a putative source for DON in bays where blooms occur. HMW DON originating from pore water had a substantially higher protein content than surface seawater DON. We found that A. anophagefferens could deplete 25-36% of the available nitrogen in cultures with HMW DON as the sole source of nitrogen and that this corresponded well with the protein fraction in pore-water HMW DON. High rates of cell surface peptide hydrolysis and no detectable N-acetyl polysaccharide hydrolysis, together with the high percentage of hydrolyzable amino acids compared to hydrolyzable aminosugars present in the HMW DON, pointed to the protein fraction as the more likely source of nitrogen used for growth. Whether or not nitrogen scavenging from protein is a common mechanism in phytoplankton is at present unknown but needs to be investigated
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    Variable sediment oxygen uptake in response to dynamic forcing (2010)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 55 (2). pp. 950-964.
    Details
    Publication Date: 2019-09-23
    Description: Seiche-induced turbulence and the vertical distribution of dissolved oxygen above and within the sediment were analyzed to evaluate the sediment oxygen uptake rate (JO2), diffusive boundary layer thickness (δDBL), and sediment oxic zone depth (zmax) in situ. High temporal-resolution microprofiles across the sediment-water interface and current velocity data within the bottom boundary layer in a medium-sized mesotrophic lake were obtained during a 12-h field study. We resolved the dynamic forcing of a full 8-h seiche cycle and evaluated JO2 from both sides of the sediment-water interface. Turbulence (characterized by the energy dissipation rate, ε), the vertical distribution of dissolved oxygen across the sediment-water interface (characterized by δDBL and zmax), JO2, and the sediment oxygen consumption rate (RO2) are all strongly correlated in our freshwater system. Seiche-induced turbulence shifted from relatively active (ε = 1.2 × 10-8 W kg-1) to inactive (ε = 7.8 × 10-12 W kg-1). In response to this dynamic forcing, δDBL increased from 1.0 mm to the point of becoming undefined, zmax decreased from 2.2 to 0.3 mm as oxygen was depleted from the sediment, and JO2 decreased from 7.0 to 1.1 mmol m-2 d-1 over a time span of hours. JO2 and oxygen consumption were found to be almost equivalent (within ~ 5% and thus close to steady state), with RO2 adjusting rapidly to changes in JO2. Our results reveal the transient nature of sediment oxygen uptake and the importance of accurately characterizing turbulence when estimating JO2.
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    Simultaneous mooring‐based measurements of seawater CO2 and O2 off Cape Hatteras, North Carolina (1997)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 42 (1). pp. 21-28.
    Details
    Publication Date: 2020-03-20
    Description: We deployed CO2 and O2 sensors on the U.S. continental shelf off Cape Hatteras, North Carolina, during late summer 1994. A continuous 32‐d gas record was obtained at 20 m in 25 m of water, below the thermocline for most of the period. Analysis of the correlation between CO2 and O2 indicates that biological and advective processes dominated the gas variability, with small or insignificant fluxes due to air–sea exchange, vertical eddy diffusion, and carbonate dissolution or formation. The observed O2 : CO2 correlation was 1.39, within the range predicted for the photosynthetic quotient. Photosynthesis and respiration appeared to be tightly coupled, resulting in no net community production in these waters during the late summer. It is evident from these results that the combination of mooring‐based CO2 and O2 measurements will be a powerful tool for studying the marine carbon cycle.
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  • 38
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    Benthic iron and phosphorus fluxes across the Peruvian oxigen minimum zone (2012)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 57 (3). pp. 851-867.
    Details
    Publication Date: 2019-09-23
    Description: Benthic fluxes of dissolved ferrous iron (Fe2+) and phosphate (TPO4) were quantified by in situ benthic chamber incubations and pore-water profiles along a depth transect (11°S, 80–1000 m) across the Peruvian oxygen minimum zone (OMZ). Bottom-water O2 levels were 〈 2 µmol L-1 down to 500-m water depth, and increased to ~40 µmol L-1 at 1000 m. Fe2+ fluxes were highest on the shallow shelf (maximum 316 mmol m-2 yr-1), moderate (15.4 mmol m-2 yr-1) between 250 m and 600 m, and negligible at deeper stations. In the persistent OMZ core, continuous reduction of Fe oxyhydroxides results in depletion of sedimentary Fe :Al ratios. TPO4 fluxes were high (maximum 292 mmol m-2 yr-1) throughout the shelf and the OMZ core in association with high organic carbon degradation rates. Ratios between organic carbon degradation and TPO4 flux indicate excess release of P over C when compared to Redfield stoichiometry. Most likely, this is caused by preferential P release from organic matter, dissolution of fish debris, and/or P release from microbial mat communities, while Fe oxyhydroxides can only be inferred as a major P source on the shallow shelf. The benthic fluxes presented here are among the highest reported from similar, oxygen-depleted environments and highlight the importance of sediments underlying anoxic water bodies as nutrient sources to the ocean. The shelf is particularly important as the periodic passage of coastal trapped waves and associated bottom-water oxygenation events can be expected to induce a transient biogeochemical environment with highly variable release of Fe2+ and TPO4.
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  • 39
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    The carbon turnover response to thermal stress of a dominant coralline alga on the fast warming Levant coast (2016)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 61 (3). pp. 1120-1133.
    Details
    Publication Date: 2019-02-01
    Description: Marine organisms in the Mediterranean Sea experience the highest temperatures, salinities and oligotrophic conditions in its easternmost part along the eastern shores of the Levantine basin. Over the past three decades this region has warmed by ca. 1.5–3.0°C with current winter and summer extremums of 17°C and 31°C, respectively. In this study, we tested the response of the native abundant articulated coralline red alga Ellisolandia elongata to this warming. Coralline algae play a key role in coastal ecosystems by structuring marine habitats, providing shelter for a myriad of species, and substantially influencing the coastal carbon budget. Despite being ubiquitous along the Levantine coasts, coralline's ecology, physiology, and biogeochemical role are nearly unknown as well as their performance under different temperatures. Measurements of primary production, respiration and calcification in the temperatures range 15–35°C, which represent past, present and predicted local annual conditions, indicated two physiological tipping points: 1) metabolic breakdown above 31°C; 2) metabolic shift at 23°C, possibly promoting seasonal algal heterotrichy (perennation of the alga without its fronds). Annual production rates were evaluated under the current and predicted temperature regimes indicating a loss of ca. one third of the organic carbon and carbonate production by corallines contributed to the shallow Levantine coast in the upcoming decades. We predict that with continued warming, Eastern Mediterranean corallines will experience a westward range contraction, initiating with phenological shifts, followed by performance declines and population decreases, ending with local extinctions.
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  • 40
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    Silicon and nitrogen cycling in the upwelling area off Peru: A dual isotope approach (2016)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 61 (5). pp. 1661-1676.
    Details
    Publication Date: 2019-09-23
    Description: We present a comparison of the dissolved stable isotope composition of silicate (δ30Si(OH)4) and nitrate (δ15 inline image) to investigate the biogeochemical processes controlling nutrient cycling in the upwelling area off Peru, where one of the globally largest Oxygen Minimum Zones (OMZs) is located. Besides strong upwelling of nutrient rich waters mainly favoring diatom growth, an anticyclonic eddy influenced the study area. We observe a tight coupling between the silicon (Si) and nitrogen (N) cycles in the study area. Waters on the shelf showed high Si(OH)4 concentrations accompanied by diminished inline image concentration as a consequence of intense remineralization, high Si fluxes from the shelf sediments, and N-loss processes such as anammox/denitrification within the OMZ. Correspondingly, the surface waters show low δ30Si(OH)4 values (+2‰) due to low Si utilization but relatively high δ15 inline image (+13‰) values due to upwelling of waters influenced by N-loss processes. In contrast, as a consequence of the deepening of the thermocline in the eddy center, a pronounced Si(OH)4 depletion led to the highest δ30Si(OH)4 values (+3.7‰) accompanied by high δ15 inline image values (+16‰). In the eddy center, high inline image: Si(OH)4 ratios favored the growth of non-siliceous organisms (Synechococcus). Our data show that upwelling processes and the presence of eddies play important roles controlling the nutrient cycles and therefore also exert a major influence on the phytoplankton communities in the Peruvian Upwelling. Our findings also show that the combined approach of δ30Si(OH)4 and δ15 inline image can improve our understanding of paleo records as it can help to disentangle utilization and N-loss processes.
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  • 41
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    Performance of sinking and nonsinking phytoplancton taxa in a gradient of mixing depths (2003)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 48 (5). pp. 1903-1912.
    Details
    Publication Date: 2014-01-30
    Description: According to a recent dynamical model, the depth of a well-mixed water column should have contrasting effects on the abundances of sinking and nonsinking phytoplankton taxa. Because of increasing light limitation, nonsinking taxa should decline monotonically with increasing mixing depth, and because of sinking loss limitation at low mixing depths, sinking taxa should peak at intermediate mixing depths. Along a gradient of mixing depths, the position of this maximum should increase with increasing taxon-specific sinking velocity and decrease with increasing background turbidity. In two field-enclosure experiments, we investigated the effects of mixing depth and background turbidity on a variety of sinking and nonsinking phytoplankton taxa. We exposed the natural, 100-µm screened phytoplankton community of a clear, unproductive, but silica-rich lake to a gradient of mixing depths (1.5- 15 m) during 4-6 weeks. To mimic two different background turbidities, the transparent enclosure walls were surrounded by either white or black foliage. Although diatoms suffered from high sedimentation losses at low mixing depths, they dominated biomass at all mixing depths throughout both experiments. Results were largely in accordance with model predictions. Specific gross growth rates of most common taxa were negatively related to mixing depth. In both experiments, the abundances of most sinking taxa showed a unimodal pattern along the mixing depth gradient, while two of three motile taxa declined monotonically with mixing depth. The depths where these taxa reached their maximal abundances were positively related to taxon-specific sinking velocity and negatively related to background turbidity.
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  • 42
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    Carbon acquisition of marine phytoplankton: Effect of the photoperiodic length (2006)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 51 . pp. 12-20.
    Details
    Publication Date: 2018-08-14
    Description: We investigated the carbon acquisition of three marine microalgae, Skeletonema costatum, Phaeocystis globosa, and Emiliania huxleyi in response to different light regimes. Rates of photosynthetic O2 evolution and CO2 and HCO3- uptake were measured by membrane inlet mass spectrometry in cells acclimated to cycles of 16 : 8 light : dark (LD; h : h) and 12 : 12 LD and were compared with those obtained under continuous light. In addition, cellular leakage was estimated for different photoperiods and ambient CO2 concentrations during growth. Maximum rates of photosynthesis more or less doubled under LD cycles compared with continuous light. In S. costatum and E. huxleyi, a remarkably higher contribution of HCO3- to the overall carbon uptake was observed under LD cycles. In contrast, P. globosa did not change its CO2 :HCO3- uptake ratio in response to daylength. Half saturation concentrations (K1/2) for O2 evolution and inorganic carbon (Ci) uptake were also influenced by the photoperiod. Under LD cycles K1/2 values for photosynthesis in S. costatum and P. globosa were similar or higher compared with continuous light, whereas they were much lower in E. huxleyi. With the exception of CO2 uptake in E. huxleyi and P. globosa, affinities for Ci decreased under the LD cycles. Cellular leakage was highest for E. huxleyi and lowest for S. costatum and generally decreased with increasing CO2 concentration. Although this study confirms species specific differences in the CO2-concentrating mechanisms (CCMs), the effect of daylength on CO2 and HCO3- uptake has hitherto not been described. We put forward the idea that variations in light condition influence the cellular carbon demand, thereby imposing a stronger control on CCM regulation than the naturally occurring changes in CO2 supply.
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  • 43
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    Coccolith strontium to calcium ratios in Emiliania huxleyi: The dependence on seawater strontium and calcium concentrations (2006)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 51 . pp. 310-320.
    Details
    Publication Date: 2019-09-23
    Description: In recent studies the Sr/Ca ratio of coccolithophore calcite was used as a proxy for past coccolithophore growth and calcification rates. Since Sr and Ca concentrations in seawater have not remained constant through time, interpretation of Sr/Ca data from the coccolith-dominated sedimentary record requires knowledge about the incorporation of seawater Sr into coccolith calcite during coccolithogenesis. Here we show that Sr/Ca of Emiliania huxleyi coccoliths is linearly related to seawater Sr/Ca, meaning that the Sr exchange coefficient does not change with changing seawater Sr/Ca. The exchange coefficient for Sr in this study, 0.39, is close to values presented in the literature and is high compared with values obtained by inorganic precipitation experiments. This suggests a strong effect of cell physiology on biogenic calcite precipitation in coccolithophores. We present a conceptual model, based on the transmembrane transport of Sr and Ca, which explains the offset.
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  • 44
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    Uptake and efflux of 64Cu by the marine cyanobacterium Synechococcus (WH7803) (2003)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 48 (1). pp. 179-188.
    Details
    Publication Date: 2014-01-30
    Description: The uptake and efflux of 64Cu was studied in the marine cyanobacterium Synechoccous strain WH7803 (DC2). Uptake followed classical Michaelis-Menten type kinetics in metal-buffered seawater. The maximum uptake rate, Vmax, was 0.236 ± 0.016 × 10-18 mol Cu cell-1 h-1, with the half-saturation constant, KS, of 10-10.81±0.11 mol L-1. An efflux mechanism was also observed in WH7803, whose growth was inhibited by high internal Cu concentrations. Efflux of Cu enabled WH7803 to maintain homeostasis for Cu at typical seawater ambient free copper concentrations ([Cu2+]f). The sensitivity of WH7803 growth to Cu was related to a simple inability to regulate internal Cu concentrations when external concentrations were 〉10-11 mol L-1.
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  • 45
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    CO2 and HCO3- uptake in marine diatoms acclimated to different CO2 concentrations. (2001)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 46 . pp. 1378-1391.
    Details
    Publication Date: 2014-01-30
    Description: Rates of cellular uptake of CO2 and HCO3- during steady-state photosynthesis were measured in the marine diatoms Thalassiosira weissflogii and Phaeodactylum tricornutum, acclimated to CO2 partial pressures of 36, 180, 360, and 1,800 ppmv. In addition, in vivo activity of extracellular (eCA) and intracellular (iCA) carbonic anhydrase was determined in relation to CO2 availability. Both species responded to diminishing CO2 supply with an increase in eCA and iCA activity. In P. tricornutum, eCA activity was close to the detection limit at higher CO2 concentrations. Simultaneous uptake of CO2 and HCO3- was observed in both diatoms. At air-equilibrated CO2 levels (360 ppmv), T. weissflogii took up CO2 and HCO3- at approximately the same rate, whereas CO2 uptake exceeded HCO3- uptake by a factor of two in P. tricornutum. In both diatoms, CO2 :HCO3- uptake ratios progressively decreased with decreasing CO2 concentration, whereas substrate affinities of CO2 and HCO3- uptake increased. Half-saturation concentrations were always 〈=5 mM CO2 for CO2 uptake and 〈700 mM HCO3- for HCO3- uptake. Our results indicate the presence of highly efficient uptake systems for CO2 and HCO3- in both diatoms at concentrations typically encountered in ocean surface waters and the ability to adjust uptake rates to a wide range of inorganic carbon supply.
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  • 46
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    Effect of CO2 concentration on the C:N:P ratio in marine phytoplankton: A species comparison. (1999)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 44 . pp. 683-690.
    Details
    Publication Date: 2014-01-30
    Description: The effect of variable concentrations of dissolved molecular carbon dioxide, [CO2,aq], on C:N:P ratios in marine phytoplankton was studied in batch cultures under high light, nutrient-replete conditions at different irradiance cycles. The elemental composition in six out of seven species tested was affected by variation in [CO2,aq]. Among these species, the magnitude of change in C:N:P was similar over the experimental CO2 range. Differences in both cell size and day length-dependent growth rate had little effect on the critical CO2 concentration below which a further decrease in [CO2,aq] led to large changes in C:N:P ratios. Significant CO2-related changes in elemental ratios were observed at [CO2,aq] 〈 10 mu mol kg-l and correlated with a CO2-dependent decrease in growth rate. At [CO2,aq] typical for ocean surface waters, variation in C:N:P was relatively small under our experimental conditions. No general pattern far CO2-related changes in the elemental composition could be found with regard to the direction of trends. Either an increase or a decrease in C:N and C:P with increasing [CO2,aq] was observed, depending on the species tested. Diurnal variation in C:N and C:P, tested in Skeletonema costatum, was of a similar magnitude as CO2-related variation. In this species, the CO2 effect was superimposed on diurnal variation, indicating that differences in elemental ratios at the end of the photoperiod were not caused by a transient buildup of carbon-rich storage compounds due to a more rapid accumulation of carbohydrates at high CO2 concentrations. If our results obtained under high light, nutrient-replete conditions are representative for natural phytoplankton populations, CO2-related changes in plankton stoichiometry are unlikely to have a significant effect on the oceanic carbon cycle
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  • 47
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    Redfield ratios revisited: Removing the biasing effect of anthropogenic CO2 (2001)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 46 . pp. 964-970.
    Details
    Publication Date: 2014-01-30
    Description: Redfield ratios of remineralization are calculated based on chemical data analysis on isopycnal surfaces. The concentrations of dissolved inorganic carbon used in this study were corrected for the anthropogenic CO2 content as estimated with a back-calculation technique. The corrections increased the apparent carbon remineralization by 25-30%, thus proving important for the reliable estimation of Redfield carbon ratios in the presence of anthropogenic CO2. Best estimates from this study largely confirm the more recently published Redfield ratios of remineralization. The following results were obtained for the latitude range 3-41°N along 20-29°W in the Northeast Atlantic Ocean: Corg: P ratio = 123 ± 10; Corg : N ratio = 7.2 ± 0.8; -O2 :Corg ratio = 1.34 ± 0.06; -O2 : P ratio = 165 ± 15; N: P ratio = 17.5 ± 2.0. These ratios are in close agreement with the average composition of phytoplankton and represent respiration of organic matter consisting on average of 52% protein, 36% polysaccharide, and 12% lipid.
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  • 48
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    The impact of cell shape and chain formation on nutrient acquisition of marine diatoms (1997)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 42 . pp. 1660-1672.
    Details
    Publication Date: 2014-01-30
    Description: Diatoms have evolved a multitude of morphologics, including highly elongated cells and cell chains. Elongation and chain formation have many possible functions, such as grazing protecticn or effects on sinking. Here, a model of diffusive and advective nutrient transport is used to predict impacts of cell shape and chain length on potential nutrient supply and uptake in a turbulent environment. Rigid, contiguous, prolate spheroids thereby represent the shapes of simple chains and solitary cells. At scales larger than a few centimeters, turbulent water motions produce a more or less homogeneous nutrient distribution. At the much smaller stall: of diatom cells, however, turbulence drcates a roughly linear shear and nutrients can locally become strongly dl=pleted because of nutrient uptake by phytoplankton cells. The potential diffusive nutrient supply is greater for elongated than for spherically shaped cells of similar volume but lower for chains than for solitary cells. Although the relative increase in nutrient transport due to turbulence is greater for chains, single cells still enjoy a greater total nutrient supply in turbulent cnvironmerits. Only chains with specialized structures, such as spaces between the cells, can overcome this disadvantage and even obtain a higher nutrient supply than do solitary cells. The mod=1 results are compared to laboratory measurements of nutrient uptake under turbulent conditions and to effects ol’ sinking
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  • 49
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    Temperature and nutrient stoichiometry interactively modulate organic matter cycling in a pelagic algal-bacterial community (2011)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 56 (2). pp. 599-610.
    Details
    Publication Date: 2019-09-23
    Description: A microcosm experiment was conducted to investigate the interactive effects of rising sea-surface temperature and altered nutrient stoichiometry on the biogeochemical cycling of organic matter in a pelagic algal–bacterial assemblage. Natural seawater, containing a mixed bacterial community, was inoculated with an axenic culture of the bloom-forming diatom species Skeletonema costatum. A factorial combination of three temperatures, simulating weak to strong warming as projected for the end of the 21st century, and either nitrogen (N)-replete or -deficient growth conditions were applied. Depending on the type of nutrient limitation, the mixed algal–bacterial communities displayed pronounced differences in the accumulation and microbial utilization of organic matter in response to warming. Under N-deficient conditions, the build-up of organic matter occurred, irrespective of temperature, dominantly in the particulate pool, and only small amounts of dissolved material accumulated. The subsequent bacterial consumption of organic matter was low, as indicated by measurements of bacterial secondary production and extracellular enzyme activities, and remained also largely unaffected by an increase in temperature from 4°C up to 12°C. Contrastingly, warming resulted in a distinct temperature-dependent increase in the accumulation of dissolved organic carbon compounds under N-replete growth conditions. Moreover, rising temperature notably stimulated the bacterial activity, indicating an enhanced flow of organic matter through the microbial loop. These findings suggest that there will be strong shifts in the biogeochemical cycling of organic matter in the upper ocean in response to increased temperature and nutrient loading that will affect pelagic food-web structures and the biological sequestration of organic matter.
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  • 50
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    Strong bottom-up and top-down control of early life stages of macroalgae (2001)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 46 . pp. 749-757.
    Details
    Publication Date: 2018-06-25
    Description: In contrast to most pelagic primary producers, benthic macrophytes pass through morphologically distinct life stages, which can be subject to different ecological controls. Using factorial field experiments, we investigated how grazing pressure (three levels) and nutrient supply (four levels) interact in controlling the passage of marine macroalgae through an apparent recruitment bottleneck at the germling stage. In comparative experiments, we asked whether relative bottom-up and top-down effects on early life stages (〈4 week germlings) vary (1) between the eutrophic Baltic Sea and the oligotrophic NW Atlantic, (2) across seasons in the NW Atlantic, and (3) among annual and perennial macroalgae. In both systems nutrient enrichment favored and grazers suppressed recruitment of green and brown annual algae; however, enrichment effects were much more pronounced in the Baltic, whereas grazer effects dominated in the NW Atlantic. Grazers induced a shift from grazer-susceptible green to more resistant brown algae in the Baltic without reducing total germling density. In the NW Atlantic, grazers strongly reduced overall recruitment rate throughout all seasons. Effects on perennials were similar in both systems with moderate losses to grazing and no effects of nutrient enrichment. Recruit densities and species composition shifted with season in the NW Atlantic. We conclude that the relative effects of grazers and nutrient enrichment depended on the nutrient status of the system, algal life history strategy, and season. Strong bottom-up and top-down controls shape benthic community composition before macroalgae reach visible size
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    Factors controlling the formation of marine snow and its sustained residence in surface waters (1992)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 37 (1). pp. 63-76.
    Details
    Publication Date: 2014-01-30
    Description: The formation of large marine snow macroflocs and stringers was monitored at a fixed station in the southern North Sea. During an 18-month investigation, large marine snow floes occurred in the water column on nine occasions for periods lasting between 5 and 16 d. Their formation coincided with both high phytoplankton biomass and low windspeeds. Retention of marine snow in the surface layer was related to slow sinking, neutral buoyancy, and rising of the large floes. Applying a specially developed instrument showed buoyancy to be caused by gas bubbles incorporated in the floes. Marine snow flotation coincided with periods of oxygen supersaturation in the water column, suggesting that oxygen production could be responsible for gas bubble formation within the floes. Disappearance of the large marine snow floes from the surface layer coincided with drastic increases in windspeed, indicating that enhanced wind-induced turbulent mixing in the surface layer could have resulted in the breakup of these floes. Formation and disappearance of marine snow had a marked influence on the light regime in the water.
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  • 52
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    Solubility of iron in the Southern Ocean (2012)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 57 (3). pp. 684-697.
    Details
    Publication Date: 2019-07-09
    Description: Iron solubility (cFeS) ranged from 0.4 to 1.5 nmol L−1, decreasing from south to north in three different Southern Ocean zones (the Coastal Zone, the Antarctic Zone, and the Polar Frontal Zone plus the Subantarctic Zone). This decrease was at times correlated with an increase in temperature. Organic Fe solubility (cFeS,org), which was obtained by subtracting from total measured Fe solubility the solubility of inorganic species of iron (Fe) at the measurement temperature (20°C), ranged from 0.3 to 1.3 nmol L−1, representing an average of 32 ± 14% of the concentration of ligands in the dissolved size fraction as determined via competitive ligand exchange–absorptive cathodic stripping voltammetry (barring a handful of extremely high values from a transect run to the east of Prydz Bay). Values of cFeS were mainly lower than the predicted value for inorganic Fe solubility at the in situ temperature. Total in situ Fe solubility (cFeS,adj) was therefore estimated by adjusting for inorganic Fe solubility at in situ temperatures (between −2°C and +18°C). Because in situ temperatures in the Antarctic Circumpolar Current were mostly lower than +3°C, such cFeS,adj values, ranging from 0.5 to 1.8 nmol L−1, were roughly twice as large as cFeS,org. The adjustment relies heavily on model calculations of inorganic Fe solubility but, if correct, indicates that the bulk of the solubility of Fe in the cold waters of the Southern Ocean is tied to the solubility of inorganic Fe rather than to Fe ligands in the soluble size fraction.
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    Calcium carbonate precipitation induced by the growth of the marine cyanobacteria Trichodesmium (2010)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 55 (6). pp. 2563-2569.
    Details
    Publication Date: 2014-01-30
    Description: In this laboratory study, we monitored the buildup of biomass and concomitant shift in seawater carbonate chemistry over the course of a Trichodesmium bloom under different phosphorus (P) availability. During exponential growth, dissolved inorganic carbon (DIC) decreased, while pH increased until maximum cell densities were reached. Once P became depleted, DIC decreased even further and total alkalinity (TA) dropped, accompanied by precipitation of aragonite. Under P-replete conditions, DIC increased and TA remained constant in the postbloom phase. A diffusion-reaction model was employed to estimate changes in carbonate chemistry of the diffusive boundary layer. This study demonstrates that Trichodesmium can induce precipitation of aragonite from seawater and further provides possible explanations about underlying mechanisms.
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    Temporal decoupling of carbon and nitrogen dynamics in a mesocosm diatom bloom (2002)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 47 (3). pp. 753-761.
    Details
    Publication Date: 2014-01-30
    Description: Flows of the major biogeochemical elements (C, N, P, Si) and of transparent exopolymer particles (TEP) were traced during a bloom of a natural assemblage of marine diatoms in a mesocosm (l m(3)) to determine whether the exudation and subsequent gelation of carbon-rich phytoplankton exopolymers can account for the formation and potential export of carbon in excess of that predicted by Redfield ratios. Exponential growth of the phytoplankton community in the mesocosm extended for 10 d until nitrate concentration fell below detection and concentrations of dissolved inorganic and particulate organic nitrogen and phosphorus remained stable. Tight covariation of particulate organic elements occurred as long as nutrients were replete. But, after nitrate depletion, decoupling of carbon dynamics from that of nitrogen and phosphorus was observed, with a large flow of carbon into TEP An uptake of 72% more dissolved inorganic carbon (DIC) than inferred from nitrate supply and Redfield stoichiometry (referred to as carbon overconsumption) occurred during the study, largely during the postbloom phase, and was almost entirely traced to the particulate organic matter (POM) pool. Marine snow (aggregates 〉0.5 mm) appeared at the onset of nitrate depletion and coincided with rapid increase in TEP concentrations. Elemental composition of marine snow differed from the Redfield ratio by an enrichment in carbon and a depletion in phosphorus relative to nitrogen. It is suggested that sinking of TEP-rich marine snow could be a possible mechanism for export of carbon above calculations that are based on the Redfield stoichiometry.
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    Nitrate use by plankton in the eastern subtropical North Atlantic, March-April 1989 (1990)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 35 . pp. 1781-1788.
    Details
    Publication Date: 2018-06-25
    Description: Nitrate concentration was measured in seawater samples from the euphotic zone at the beginning and end of 12-h, daytime, in situ incubations. The changes in concentration are considered to be measurements of new production. During periods of 2-3 weeks in March-April 1989, important time scales for NO3- input to the euphotic zone (i.e. residence times) and new production were approximately 26 d at 18-degrees-N, 31-degrees-W and approximately 10 d near 33-degrees-N, 21-degrees-W. The average rate of NO3- use in the two areas was 2.63 and 0.62 mmol N m-2 (12 h)-1, or, in carbon equivalents 209 and 49 mg C m-2 d-1, respectively. These values bracket the large-scale estimate by Jenkins of new production in the nearby beta triangle of 150 mg C m-2 d-1.
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    Selectivity of Idothea chelipes (Crustacea: Isopoda) grazing on benthic microalgae (1997)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 42 (7). pp. 1622-1628.
    Details
    Publication Date: 2018-06-25
    Description: Periphyton grazing by the marine isopod Idothea chelipes was studied by exposing periphyton grown on glass slides to a gradient of grazer densities. An analysis of the algal growth rates and their relationships to grazer density revealed two groups of algae. The unicellular diatoms Licmophora ehrenbergii, Fragilaria tabulata, Navicula spp., Cocconeis costata, and the green alga Ulothrix implexa had high maximal growth rates (0.90–1.47 d−1) and suffered high grazing losses (0.41–0.68 d−1 per grazer ind.). The tube dwelling diatom Amphipleura rutilans and the cyanobacteria Lyngbya confervoides and Spirulina subsalsa had low maximal growth rates (0.38–0.81 d−1) and suffered only moderate grazing losses (0.10–0.27 d−1 per grazer ind.). The species of the first group seemed to be less strongly resource limited than did the species of the second group. Grazing by I. chelipes has the potential to drive succession from the well‐edible to the less edible periphyton species.
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    Distinguishing between the effects of ocean acidification and ocean carbonation in the coccolithophore Emiliania huxleyi (2011)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 56 (6). pp. 2040-2050.
    Details
    Publication Date: 2020-10-16
    Description: The coccolithophore Emiliania huxleyi was cultured under a broad range of carbonate chemistry conditions to distinguish the effects of individual carbonate system parameters on growth, primary production, and calcification. In the first experiment, alkalinity was kept constant and the fugacity of CO2 (fCO2) varied from 2 to 600 Pa (1 Pa ≈ 10 µatm). In the second experiment, pH was kept constant (pHfree = 8) with fCO2 varying from 4 to 370 Pa. Results of the constant-alkalinity approach revealed physiological optima for growth, calcification, and organic carbon production at fCO2 values of ∼ 20 Pa, ∼ 40 Pa, and ∼ 80 Pa, respectively. Comparing this with the constant-pH approach showed that growth and organic carbon production increased similarly from low to intermediate CO2 levels but started to diverge towards higher CO2 levels. In the high CO2 range, growth rates and organic carbon production decreased steadily with declining pH at constant alkalinity while remaining consistently higher at constant pH. This suggests that growth and organic carbon production rates are directly related to CO2 at low (sub-saturating) concentrations, whereas towards higher CO2 levels they are adversely affected by the associated decrease in pH. A pH dependence at high fCO2 is also indicated for calcification rates, while the key carbonate system parameter determining calcification at low fCO2 remains unclear. These results imply that key metabolic processes in coccolithophores have their optima at different carbonate chemistry conditions and are influenced by different parameters of the carbonate system at both sides of the optimum.
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    Implications of increased carbon supply for the global expansion of jellyfish blooms (2011)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography Bulletin, 20 . pp. 38-39.
    Details
    Publication Date: 2018-08-14
    Type: Article , PeerReviewed
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    Transport and consumption of oxygen and methane in different habitats of the Håkon Mosby Mud Volcano (HMMV) (2010)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 55 (6). pp. 2366-2380.
    Details
    Publication Date: 2017-05-02
    Description: The Hakon Mosby Mud Volcano is a highly active methane seep hosting different chemosynthetic communities such as thiotrophic bacterial mats and siboglinid tubeworm assemblages. This study focuses on in situ measurements of methane fluxes to and from these different habitats, in comparison to benthic methane and oxygen consumption rates. By quantifying in situ oxygen, methane, and sulfide fluxes in different habitats, a spatial budget covering areas of 10-1000-m diameter was established. The range of dissolved methane efflux (770-2 mmol m(-2) d(-1)) from the center to the outer rim was associated with a decrease in temperature gradients from 46 degrees C m(-1) to 〈 1 degrees Cm(-1), indicating that spatial variations in fluid flow control the distribution of benthic habitats and activities. Accordingly, total oxygen uptake (TOU) varied between the different habitats by one order of magnitude from 15 mmol m(-2) d(-1) to 161 mmol m(-2) d(-1). High fluid flow rates appeared to suppress benthic activities by limiting the availability of electron acceptors. Accordingly, the highest TOU was associated with the lowest fluid flow and methane efflux. This was most likely due to the aerobic oxidation of methane, which may be more relevant as a sink for methane as previously considered in submarine ecosystems.
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  • 60
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    The paradox of the plankton: Fluctuations of phosphorus availability maintain diversity of phytoplankton in flow-through cultures (1984)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 29 (3). pp. 633-636.
    Details
    Publication Date: 2018-06-06
    Description: Competition experiments with phytoplankton under steady state conditions have largely verified the competitive exclusion principle. Coexistence of species limited by different resources contributes little to the explanation of the natural diversity of phytoplankton. It is shown by multispecies experiments in flow-through cultures that pulsed input of a key nutrient allows the coexistence of species competing for the same resource.
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    Phytoplankton natural community experiments: A reinterpretation (1985)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 30 . pp. 436-440.
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    Publication Date: 2018-05-29
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  • 62
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    Survival probability of larval sprat in response to decadal changes in diel vertical migration behavior and prey abundance in the Baltic Sea (2010)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 55 (4). pp. 1485-1498.
    Details
    Publication Date: 2017-05-02
    Description: We employed a coupled three-dimensional biophysical model to explore long-term inter- and intra-annual variability in the survival of sprat larvae in the Bornholm Basin, a major sprat spawning area in the Baltic Sea. Model scenarios incorporated observed decadal changes in larval diel vertical distribution and climate-driven abiotic and biotic environmental factors including variability in the abundance of different, key prey species (calanoid copepods) as well as seasonal changes, long-term trends, and spatial differences in water temperature. Climate forcing affected Baltic sprat larval survival both directly (via changes in temperature) and indirectly (via changes in prey populations). By incorporating observed changes in larval diel vertical migration, decadal changes in modeled and observed survival of Baltic sprat agreed well. Higher larval survival (spawning stock biomass) was predicted in the 1990s compared to the 1980s. After changing their foraging strategy by shifting from mid-depth, low prey environment to near-surface waters, first-feeding larvae encountered much higher rates of prey encounter and almost optimal feeding conditions and had a much higher growth potential. Consequently, larvae were predicted to experience optimal conditions to ensure higher survival throughout the later larval and early juvenile stages. However, this behavioral shift also increased the susceptibility of larvae to unfavorable winddriven surface currents, contributing to the marked increase in interannual variability in recruitment observed during the past decade.
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    Effects of climate change on methane emissions from seafloor sediments in the Arctic Ocean: A review (2016)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 61 (S1). pp. 301-309.
    Details
    Publication Date: 2019-09-24
    Description: Large quantities of methane are stored in hydrates and permafrost within shallow marine sediments in the Arctic Ocean. These reservoirs are highly sensitive to climate warming, but the fate of methane released from sediments is uncertain. Here, we review the principal physical and biogeochemical processes that regulate methane fluxes across the seabed, the fate of this methane in the water column, and potential for its release to the atmosphere. We find that, at present, fluxes of dissolved methane are significantly moderated by anaerobic and aerobic oxidation of methane. If methane fluxes increase then a greater proportion of methane will be transported by advection or in the gas phase, which reduces the efficiency of the methanotrophic sink. Higher freshwater discharge to Arctic shelf seas may increase stratification and inhibit transfer of methane gas to surface waters, although there is some evidence that increased stratification may lead to warming of sub-pycnocline waters, increasing the potential for hydrate dissociation. Loss of sea-ice is likely to increase wind speeds and seaair exchange of methane will consequently increase. Studies of the distribution and cycling of methane beneath and within sea ice are limited, but it seems likely that the sea-air methane flux is higher during melting in seasonally ice-covered regions. Our review reveals that increased observations around especially the anaerobic and aerobic oxidation of methane, bubble transport, and the effects of ice cover, are required to fully understand the linkages and feedback pathways between climate warming and release of methane from marine sediments.
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    Linked sediment and water-column methanotrophy at a man-made gas blowout in the North Sea: Implications for methane budgeting in seasonally stratified shallow seas (2016)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 61 (S1). S367-S386.
    Details
    Publication Date: 2019-02-01
    Description: Large quantities of the greenhouse gas methane (CH4) are stored in the seafloor. The flux of CH4 from the sediments into the water column and finally to the atmosphere is mitigated by a series of microbial methanotrophic filter systems of unknown efficiency at highly active CH4-release sites in shallow marine settings. Here, we studied CH4-oxidation and the methanotrophic community at a high-CH4-flux site in the northern North Sea (well 22/4b), where CH4 is continuously released since a blowout in 1990. Vigorous bubble emanation from the seafloor and strongly elevated CH4 concentrations in the water column (up to 42 µM) indicated that a substantial fraction of CH4 bypassed the highly active (up to ∼2920 nmol cm−3 d−1) zone of anaerobic CH4-oxidation in sediments. In the water column, we measured rates of aerobic CH4-oxidation (up to 498 nM d−1) that were among the highest ever measured in a marine environment and, under stratified conditions, have the potential to remove a significant part of the uprising CH4 prior to evasion to the atmosphere. An unusual dominance of the water-column methanotrophs by Type II methane-oxidizing bacteria (MOB) is partially supported by recruitment of sedimentary MOB, which are entrained together with sediment particles in the CH4 bubble plume. Our study thus provides evidence that bubble emission can be an important vector for the transport of sediment-borne microbial inocula, aiding in the rapid colonization of the water column by methanotrophic communities and promoting their persistence close to highly active CH4 point sources.
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    Environmental dependence of the correlations between stoichiometric and fatty acid-based indicators of phytoplankton nutritional quality (2017)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 62 (1). pp. 334-347.
    Details
    Publication Date: 2020-02-06
    Description: Marine phytoplankton is simultaneously affected by multiple environmental drivers. To-date integrative assessments of multiple combined effects are rare on the relationship between elemental stoichiometry and biochemicals in marine phytoplankton. We investigated responses of stoichiometric (N:C and P:C ratios) and fatty acid-based (polyunsaturated fatty acid, PUFA) indicators of nutritional quality to three N:P supply ratios (10:1, 24:1, and 63:1 mol mol−1), three temperatures (12, 18, and 24°C) and two pCO2 levels (560 and 2400 μatm) in the marine phytoplankters Rhodomonas sp. and Phaeodactylum tricornutum. Overall, warming and nutrient deficiency showed dramatic effects, but increased pCO2 had modest effects on the two indicators of nutritional quality. Specifically, warming showed strong positive effects on N:C and P:C ratios in Rhodomonas sp. but negative effects on PUFAs in both species. The low N- and low P-media led to low contents of both nutrients but high contents of PUFAs in the biomass of Rhodomonas sp., while the response of P. tricornutum was more complex: N:C ratios were lowest at the intermediate N:P supply but P:C ratios responded negatively to P deficiency and positively to N deficiency. Large variations in the two indicators of nutritional quality can be attributed to species-specific physiological optima and interactions between the three manipulated variables. Our results suggest that stoichiometric and FA-based indicators of nutritional quality may change differentially in response to warming and nutrient deficiency in marine phytoplankton, highlighting the relevance of simultaneous considerations of the two indicators of nutritional quality, when assessing food web dynamics under future ocean scenarios.
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    Even moderate nutrient enrichment negatively adds up to global climate change effects on a habitat-forming seaweed system (2016)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 61 (5). pp. 1891-1899.
    Details
    Publication Date: 2019-02-01
    Description: Coastal marine ecosystems have been under high anthropogenic pressure and it can be assumed that prevalent local perturbation interacts with rising global stressors under proceeding climate change. Understanding their effective pathways and cumulative effects is of high relevance not only with regard to future risk assessment, but also for current ecosystem management. In benthic mesocosms, we factorially tested the effects of one global (combined elevated seawater temperature and CO2 concentration) and one local (nutrient enrichment) stressor on a common coastal Baltic seaweed system (Fucus vesiculosus). Both treatments in combination had additive negative impacts on the seaweed—epiphyte—mesograzer system by altering its regulatory mechanisms. That is, warming decreased the biomass of two mesograzer species (weakened top-down control), whereas moderate nutrient enrichment increased epiphyte biomass (intensified bottom-up control), which ultimately resulted in a significant biomass reduction of the foundation seaweed. Our results suggest that climate change impacts might be underestimated if local pressures are disregarded. Furthermore, they give implication for local ecological management as the mitigation of local perturbation may limit climate change impacts on marine ecosystems.
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    Impact of cell shape and chain formation on nutrient acquisition by marine diatoms (1997)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 42 (8). pp. 1660-1672.
    Details
    Publication Date: 2018-06-25
    Description: Diatoms have evolved a multitude of morphologies, including highly elongated cells and cell chains. Elongation and chain formation have many possible functions, such as grazing protection or effects on sinking. Here, a model of diffusive and advective nutrient transport is used to predict impacts of cell shape and chain length on potential nutrient supply and uptake in a turbulent environment. Rigid, contiguous, prolate spheroids thereby represent the shapes of simple chains and solitary cells. Ar scales larger than a few centimeters, turbulent water motions produce a more or less homogeneous nutrient distribution. At the much smaller scale of diatom cells, however, turbulence creates a roughly linear shear and nutrients can locally become strongly depleted because of nutrient uptake by phytoplankton cells. The potential diffusive nutrient supply is greater for elongated than for spherically shaped cells of similar volume but lower for chains than for solitary cells. Although the relative increase in nutrient transport due to turbulence is greater for chains, single cells still enjoy a greater total nutrient supply in turbulent environments, Only chains with specialized structures, such as spaces between the cells, can overcome this disadvantage and even obtain a higher nutrient supply than do solitary cells. The model results are compared to laboratory measurements of nutrient uptake under turbulent conditions and to effects of sinking.
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    Maximal growth rates of Antarctic phytoplankton: Only weak dependence on cell size (1989)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 34 (6). pp. 1109-1112.
    Details
    Publication Date: 2018-06-25
    Description: Maximal growth rates of 15 Antarctic phytoplankton species at 0 degree C ranged from 0.32 to 0.72/d, showing only a weak dependence on cell size. Comparisons were made with two models for size dependence of temperature-corrected rates of maximal growth. Schlesinger's general phytoplankton model predicts a strong size dependence of growth rates and grossly underestimates the maximal growth rates of the larger species, but gives reasonable estimates for the smallest ones. Banse's marine diatom model assumes a weak size dependence of growth rates and gives generally better predictions.
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    Long range vertical migration of Volvox in tropical Lake Cahora Bassa (Mozambique) (1986)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 31 . pp. 650-653.
    Details
    Publication Date: 2018-06-25
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  • 70
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    Phosphorus-limited Daphnia: Intraspecific facilitation instead of competition (1992)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 37 (5). pp. 966-973.
    Details
    Publication Date: 2018-06-25
    Description: P-limited chemostat cultures of the green alga Scenedesmus acutus were inoculated with the herbivorous zooplankter Daphnia galeata after steady state had been reached. At low dilution rates (up to 0.5 d-l) and consequently low P content of the food algae (P: C 〈 0.0011) Daphnia could not grow. At higher dilution rates (from 0.55 d-l upward) and higher P content ofthe food, Daphnia developed dense populations which were able to reduce algal biomass more than IO-fold. This reduction was accompanied by an increase in the P content of the algae which gave rise to increased birth rates of Daphnia. The birth rates of P-limited Daphnia depended on algal cell quotas in a saturating fashion which was mathematically similar to Droop’s growth equation. During P limitation the birth rates of Daphnia were positively correlated to population density, which suggests intraspecific facilitation instead of competition.
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    Analyses of the in-situ growth rates of Cryptophyceae by use of the mitotic index technique (1985)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 30 . pp. 893-897.
    Details
    Publication Date: 2018-06-06
    Description: Growth rates of Rhodomonas minuta v. nannoplanctica and Cryptomonas ovata were estimated by determining the fraction of cells undergoing cell division during 24 h. Acetocarmine was very useful for identifying stages of division (especially double nuclei). Division stages of R. minuta were found in Lake Constance during the night over a period of 16 or 17 h, of C. ovata over almost the entire 24-h cycle. A comparison of potential growth rates with observed changes in population density indicates that the higher temporal variability of loss rates is more influential for changes in population size
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    Comparison between steady state and non-steady state competition: experiments with natural phytoplankton (1985)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 30 . pp. 335-346.
    Details
    Publication Date: 2018-06-06
    Description: Chemostat competition experiments with natural phytoplankton communities are compared to experiments in which either one (phosphorus) or two (phosphorus and silicon) key nutrients were added discontinuously at l-week intervals. In all types of experiments wide ranges of Si:P ratios were tested. Deviation from steady state was found not only to increase the number of coexisting species, but also to shift the regions of dominance of species and of higher taxa along the gradient of Si:P ratios. Pulsed nutrient addition was mainly to the advantage of green algae and to the disadvantage of diatoms.
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    Nutrient status and nutrient competition of phytoplankton in a shallow, hypertrophic lake (1989)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 34 (7). pp. 1162-1173.
    Details
    Publication Date: 2018-06-25
    Description: The nutritional status of phytoplankton in a shallow, hypertrophic lake was analyzed by stoichiometry of seston and by enrichment bioassays during a 6-month period. Both methods suggested moderate and temporally interrupted nutrient limitation of reproductive rates. Nitrogen was the most frequently limiting nutrient, phosphorus was next, and silicate limitation of three diatom species occurred only once. The nutritional status of the most abundant individual species could be described by the Monod equation. The nutritional status of the entire phytoplankton assemblage could be described by a modified version of the Droop equation. In accordance with competition theory, phytoplankton species were arrayed along resource ratio gradients. These results are consistent with ecophysiological models derived from culture experiments.
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  • 74
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    Nutrient limitation of picophytoplankton photosynthesis and growth in the tropical North Atlantic (2008)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 53 . pp. 1722-1733.
    Details
    Publication Date: 2017-05-02
    Description: Identification of the proximal nutrient limiting primary production is a necessary first step toward evaluating the physiological state of phytoplankton communities and the biogeochemical constraints on the current oceanic carbon cycle. We conducted 48-h nutrient addition bioassay experiments to evaluate nitrogen, phosphorus, and iron limitation of primary productivity, net chlorophyll synthesis, and net increase in cell numbers of the dominant picophytoplankton from the tropical North Atlantic. Our results indicate that N was the proximal limiting factor for primary production during the autumn of 2002, followed by P and then Fe. Net chlorophyll synthesis was significantly stimulated by addition of N alone and further stimulated by addition of P. Analysis of picophytoplankton populations by analytical flow cytometry revealed a more complex response. Cellular red fluorescence, an index of cell chlorophyll content, increased in Prochlorococcus, Synechococcus, and picoeukaryotes in response to addition of NH4NO3 but was not affected by single or combined additions of P and Fe. In contrast, cell abundance in these picophytoplankton populations increased only after combined N and P (63% of comparisons) or N and Fe (21% of comparisons) additions. Thus, our experiments revealed that chlorophyll synthesis and primary production were limited by the availability of nitrogen alone, while net increase in cell abundance was colimited by N and P or N and Fe in the majority of these picophytoplankton populations.
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    Mass deposition event of Pyrosoma atlanticum carcasses off Ivory Coast (West Africa) (2009)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 54 (4). pp. 1197-1209.
    Details
    Publication Date: 2014-01-30
    Description: Thousands of moribund thaliacean carcasses (Pyrosoma atlanticum) were deposited between February and March 2006 at the seafloor in the Ivory Coast area (West Africa). Remotely operated vehicle surveys were conducted in a continuous depth gradient between 20 and 1275 m along an oil pipeline. Video and still photography were used to estimate the carcass distribution, density, and size on the seabed, as well as recording the local megafauna interactions with the gelatinous material. Large patches of dead pyrosomids covered extensive areas on the continental slope, whereas minor aggregations were found on the shelf. The carcasses were in many instances trapped along the pipelines, accumulating extensively in troughs and furrows in the slope, and especially in soft sediment channels. Pyrosoma atlanticum dried samples were used to calculate the carbon content, enabling the extrapolation to the densities and sizes recorded in the video surveys. The average standing stock of organic carbon associated with the carcasses was 〉5 g C m-2 in the whole slope and canyon, with values as high as 22 g C m-2 in certain areas. Eight megafaunal species from three different phyla were observed 63 times directly feeding on the decomposing carcasses. The gelatinous carbon may have contributed substantially to the detrital food web including microbes at the seabed, and certainly to the diet of larger benthic organisms. The organism-level carbon measurements and documented fate of pyrosomid organic carbon is new evidence of the importance of gelatinous material in large-scale processes and elemental cycling.
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    Nutrient limitation of primary producers affects planktivorous fish condition (2007)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 52 . pp. 2062-2071.
    Details
    Publication Date: 2019-09-23
    Description: We investigated whether nutrient limitations of primary producers act upward through food webs only in terms of density effects or if there is a second pathway for nutrient limitation signals channelled upward to higher trophic levels. We used tritrophic food chains to assess the effects of nutrient-limited phytoplankters (the cryptophyte Rhodomonas salina) on herbivorous zooplankters (the calanoid copepod Acartia tonsa) and finally zooplanktivores (larval herring Clupea harengus) living on the herbivores. The primary producers� food quality had a significant effect on fish condition. Our experimental phosphorus-limited food chain resulted in larval fish with a significantly poorer condition than their counterparts reared under nitrogen-limited or nutrient-sufficient conditions. Our results show that mineral nutrient requirements of consumers have to be satisfied first before fatty acids can promote further growth. This challenges the match/mismatch hypothesis, which links larval fish survival probability solely to prey availability, and could imply that reduced nutrient releases into the environment may affect fish stocks even more severely than previously believed.
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  • 77
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    Environmental control on anaerobic oxidation of methane in gassy sediments of Eckernförde Bay (German Baltic) (2005)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 50 (6). pp. 1771-1786.
    Details
    Publication Date: 2014-01-30
    Description: We investigated the effect of seasonal environmental changes on the rate and distribution of anaerobic oxidation of methane (AOM) in Eckernforde Bay sediments (German Baltic Sea) and identified organisms that are likely to be involved in the process. Surface sediments were sampled during September and March. Field rates of AOM and sulfate reduction (SR) were measured with radiotracer methods. Additional parameters were determined that potentially influence AOM, i.e., temperature, salinity, methane, sulfate, and chlorophyll a. Methanogenesis as well as potential rates of AOM and aerobic oxidation of methane were measured in vitro. AOM changed seasonally within the upper 20 cm of the sediment, with rates being between 1 and 14 nmol cm(-3) d(-1). Its distribution is suggested to be controlled by oxygen and sulfate penetration, temperature, as well as methane supply, leading to a shallow AOM zone during the warm productive season and to a slightly deeper AOM zone during the cold winter season. Rising methane bubbles apparently fed AOM above the sulfate-methane transition. Methanosarcinales-related anaerobic methanotrophs (ANME-2), identified with fluorescence in situ hybridization, is suggested to mediate AOM in Eckerntorde Bay. These archaea are known also from other marine methane-rich locations. However, they were not directly associated with sulfate-reducing bacteria. AOM is possibly mediated solely by these archaea that show a mesophilic physiology according to the seasonal temperature changes in Eckernforde Bay.
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  • 78
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    Degree of oligotrophy controls the response of microbial plankton to Saharan dust (2010)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 55 (6). pp. 2339-2352.
    Details
    Publication Date: 2017-05-02
    Description: To determine the effects of Saharan dust on the abundance, biomass, community structure, and metabolic activity of oceanic microbial plankton, we conducted eight bioassay experiments between ca. 30 degrees N and 30 degrees S in the central Atlantic Ocean. We found that, although bulk abundance and biomass tended to remain unchanged, different groups of phytoplankton and bacterioplankton responded differently to Saharan dust addition. The predominant type of metabolic response depended on the ecosystem's degree of oligotrophy and was modulated by competition for nutrients between phytoplankton and heterotrophic bacteria. The relative increase in bacterial production, which was the dominant response to dust addition in ultraoligotrophic environments, became larger with increasing oligotrophy. In contrast, primary production, which was stimulated only in the least oligotrophic waters, became less responsive to dust as the ecosystem's degree of oligotrophy increased. Given the divergent consequences of a predominantly bacterial vs. phytoplanktonic response, dust inputs can, depending on the ecosystem's degree of oligotrophy, stimulate or weaken biological CO2 drawdown. Thus, the biogeochemical implications of changing dust fluxes might not be universal, but variable through both space and time.
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  • 79
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    The invasive ctenophore Mnemiopsis leidyi poses no direct threat to Baltic cod eggs and larva (2011)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 56 (2). pp. 431-439.
    Details
    Publication Date: 2014-05-07
    Description: Since its invasion in to the Baltic Sea in 2006, the ctenophore Mnemiopsis leidyi has been suspected of serious predation on the early life stages of Baltic cod (Gadus morhua callarias L.) due to a temporal and spatial overlap in the most important cod spawning ground, the Bornholm Basin. We conducted laboratory incubation experiments and video observations to quantify feeding rates on Baltic cod eggs and larvae. Ingestion rates increased with cod larvae concentrations up to 8 prey L-1, beyond which ingestion remained constant. Neither Mnemiopsis size nor egg concentration (1-16 prey L-1) affected feeding rates on cod eggs. Observed feeding rates pooled from all experiments conducted at nonsaturating prey concentrations were low, with the highest volume-specific clearance on 〈 4.5-d-old yolk-sac larvae (0.05 ± 0.02 L (mL Mnemiopsis)-1 h-1), and lower rates on 4.5-8-d-old larvae (0.02 ± 0.02 L (mL Mnemiopsis)-1 h-1) and eggs (0.02 ± 0.03 L (mL Mnemiopsis)-1 h-1). When offered Artemia salina and cod eggs simultaneously, Mnemiopsis passively selected against cod eggs. Video recordings showed that eggs did not trigger the capture response that Mnemiopsis shows toward motile prey, and ingested eggs were often ejected (88%, n = 8). Applying our clearance rates to in situ abundances of cod eggs, larvae, and Mnemiopsis for the peak of the spawning season, we demonstrate that the predation pressure of the invasive ctenophore is negligible. We conclude that Mnemiopsis constitutes no direct threat to the Baltic cod population.
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  • 80
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    Seasonal rates of benthic primary production in a Greenland fjord measured by aquatic eddy correlation (2014)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 59 (5). pp. 1555-1569.
    Details
    Publication Date: 2020-11-23
    Description: We present the first year-round estimates of benthic primary production at four contrasting shallow (3–22 m depth) benthic habitats in a southwest Greenland fjord. In situ measurements were performed using the noninvasive aquatic eddy-correlation (EC) oxygen (O2) flux method. A series of high-quality multiple-day EC data sets document the presence of a year-round productive benthic phototrophic community. The shallow-water sites were on average autotrophic during the spring and summer months, up to 43.6 mmol O2 m22 d21, and heterotrophic or close to metabolic balance during the autumn and winter. Substantial benthic gross primary production (GPP) was measured year-round. The highest GPP rates were measured during the spring, up to 5.7 mmol O2 m22 h21 (136.8 mmol O2 m22 d21), and even at low light levels (, 80 mmol quanta m22 s21) during late autumn and winter we measured rates of up to 1.8 mmol O2 m22 h21 (43.2 mmol O2 m22 d21) during peak irradiance. The benthic phototrophic communities responded seasonally to ambient light levels and exhibited year-round high photosynthetic efficiency. In situ downwelling irradiances as low as , 2 mmol quanta m22 s21 induced an autotrophic response and light saturation indices (Ik) were as low as 11 mmol quanta m22 s21 in the winter. On an annual timescale, the average areal rate of benthic GPP was 11.5 mol O2 m22 yr21, which is , 1.4 times higher than the integrated gross pelagic primary production of the , 30–50 m deep photic zone of the fjord. These results document the importance of benthic photosynthesis on an ecosystem level and indicate that the benthic phototrophic compartment should be accounted for when assessing carbon and nutrient budgets as well as responses of coastal Arctic ecosystems to climate change.
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  • 81
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    Taxon- and vector-specific variation in species richness and abundance during the transport stage of biological invasions (2013)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 58 (4). pp. 1361-1372.
    Details
    Publication Date: 2014-08-08
    Description: Recent empirical and statistical evidence suggest that propagule pressure (i.e., number of individuals introduced per event, and the number and frequency of events) and colonization pressure (i.e., number of species released per event, and the number and frequency of events) are of vital importance to invasion success. To explore possible changes in propagule and colonization pressure during the transport stage of the invasion process, we examine abundance and species richness of virus-like particles, bacteria, diatoms, dinoflagellates, and invertebrates transported in commercial ships—a leading vector for global spread of aquatic nonindigenous species. We collected 154 ballast water samples from ships that had performed or were exempt from ballast water exchange (BWE) prior to arrival at Pacific and Atlantic ports in Canada and Laurentian Great Lakes ports. We found that abundance and species richness varied across taxa and regions, with ships arriving to the Atlantic region carrying the highest abundance of taxa. The highest species richness of invertebrates and diatoms was recorded from ships arriving to the Pacific, whereas the richest communities of dinoflagellates occurred in the Atlantic region. We also found that BWE had no effect on abundance or species richness of most taxa (dinoflagellates, diatoms, bacteria, and virus-like particles), whereas the effect on abundance of invertebrates was not clear. Finally, longer voyages resulted in lower abundance of all taxa except dinoflagellates, and lower species richness of diatoms. Paradoxically, the elevated abundance and species richness of dinoflagellates following BWE suggest that this group could have enhanced invasion potential when ships manage ballast water by exchange.
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  • 82
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    Distribution and redox speciation of dissolved iron on the European continental margin (2007)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 52 (6). pp. 2530-2539.
    Details
    Publication Date: 2014-01-30
    Description: To investigate the biogeochemistry of iron in the waters of the European continental margin, we determined the dissolved iron distribution and redox speciation in filtered (〈0.2 μm) open-ocean and shelf waters. Depth profiles were sampled over the shelf slope southeast of the Chapelle Bank area (47.61°N, 4.24°W to 46.00°N, 8.01°W) and a horizontal surface-water transect over the shelf and through the English Channel (la Manche) and the southern North Sea (46°N, 8°W to 52°N, 4°E). An abrupt trace-metal front was found near the shelf slope, indicated by a horizontal gradient of dissolved iron (DFe) and aluminium (DAl), which correlated with changing salinities (r2 = 0.572 and 0.528, respectively, n = 92). Labile Fe(II) concentrations varied from 〈12 pmol L-1 in North Atlantic surface waters to >200 pmol L-1 in the near bottom waters of the shelf break. Labile Fe(II) accounted for ∼5 of the dissolved iron species in surface shelf waters (mean 5.0 ± 2.7), whereas higher Fe(II) fractions (i.e., >8) were observed near the sea bottom on the shelf break and during a midday solar maximum in surface waters in the vicinity of the Scheldt river plume. Benthic processes (resuspension and diagenesis) constituted important sources of Fe(II) and DFe in this region, and photoreduction of Fe(III) species in shelf waters caused enhanced labile Fe(II) concentrations. These processes increased the lability of iron and its potential availability to marine organisms in the shelf ecosystem. © 2007, by the American Society of Limnology and Oceanography, Inc.
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  • 83
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    Seabed foraging by antarctic krill: Implications for stock assessment, bentho-pelagic coupling, and the vertical transfer of iron (2011)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 56 (4). pp. 1411-1428.
    Details
    Publication Date: 2014-08-01
    Description: A compilation of more than 30 studies shows that adult Antarctic krill (Euphausia superba) may frequent benthic habitats year-round, in shelf as well as oceanic waters and throughout their circumpolar range. Net and acoustic data from the Scotia Sea show that in summer 2-20 of the population reside at depths between 200 and 2000 m, and; that large aggregations can form above the seabed. Local differences in the vertical distribution of krill indicate that reduced feeding success in surface waters, either due to predator encounter or food shortage, might initiate such deep migrations and results in benthic feeding. Fatty acid and microscopic analyses of stomach content confirm two different foraging habitats for Antarctic krill: the upper ocean, where fresh phytoplankton is the main food source, and deeper water or the seabed, where detritus and copepods are consumed. Krill caught in upper waters retain signals of benthic feeding, suggesting frequent and dynamic exchange between surface and seabed. Krill contained up to 260 nmol iron per stomach when returning from seabed feeding. About 5 of this iron is labile, i.e., potentially available to phytoplankton. Due to their large biomass, frequent benthic feeding, and acidic digestion of particulate iron, krill might facilitate an input of new iron to Southern Ocean surface waters. Deep migrations and foraging at the seabed are significant parts of krill ecology, and the vertical fluxes involved in this behavior are important for the coupling of benthic and pelagic food webs and their elemental repositories. © 2011, by the American Society of Limnology and Oceanography, Inc.
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  • 84
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    Environmental constraints of the invasive Mnemiopsis leidyi in Scandinavian waters (2013)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 58 (1). pp. 37-48.
    Details
    Publication Date: 2016-01-01
    Description: We studied the seasonal dynamics of the invasive ctenophore Mnemiopsis leidyi over its distribution range from Skagerrak into the Baltic Proper during 1 yr and related this to ambient physical and biological variables. The appearance of M. leidyi was sporadic in the Baltic Proper, with 60-fold lower abundance than in the Skagerrak and Kattegat (mean 0.02 individual m−3 and 1.16 individual m−3, respectively). M. leidyi typically resided above the halocline (10–20 m depth), except in the Baltic Proper where they were found deeper. Smaller size classes were underrepresented in the Baltic Proper, indicating failed reproduction or high mortality of the younger life stages. Ninety percent of the M. leidyi were observed at salinities of 22–29 and 75% of the individuals in water masses warmer than 11°C, i.e., typical late summer to autumn temperatures. Results of a Generalized Additive Model (GAM) indicate that the spatio-temporal distribution of M. leidyi might be explained by advection of M. leidyi from the Skagerrak and Kattegat area to the Baltic Proper. We conclude that the low salinity (〈 9) of the Baltic Proper is likely to restrict successful reproduction and establishment of M. leidyi and that the advection of individuals from higher saline source areas sustained the observed Baltic Proper occurrence.
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  • 85
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    Differential effects of ocean acidification on growth and photosynthesis among phylotypes of Symbiodinium (Dinophyceae) (2011)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 56 (3). pp. 927-938.
    Details
    Publication Date: 2014-06-01
    Description: We investigated the effect of elevated partial pressure of CO2 (pCO2) on the photosynthesis and growth of four phylotypes (ITS2 types A1, A13, A2, and B1) from the genus Symbiodinium, a diverse dinoflagellate group that is important, both free-living and in symbiosis, for the viability of cnidarians and is thus a potentially important model dinoflagellate group. The response of Symbiodinium to an elevated pCO2 was phylotype-specific. Phylotypes A1 and B1 were largely unaffected by a doubling in pCO2; in contrast, the growth rate of A13 and the photosynthetic capacity of A2 both increased by ~ 60. In no case was there an effect of ocean acidification (OA) upon respiration (dark- or light-dependent) for any of the phylotypes examined. Our observations suggest that OA might preferentially select among free-living populations of Symbiodinium, with implications for future symbioses that rely on algal acquisition from the environment (i.e., horizontal transmission). Furthermore, the carbon environment within the host could differentially affect the physiology of different Symbiodinium phylotypes. The range of responses we observed also highlights that the choice of species is an important consideration in OA research and that further investigation across phylogenetic diversity, for both the direction of effect and the underlying mechanism(s) involved, is warranted. © 2011, by the American Society of Limnology and Oceanography, Inc.
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  • 86
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    Spatial and temporal development of phytoplankton iron stress in relation to bloom dynamics in the high-latitude North Atlantic Ocean (2013)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 58 (2). pp. 533-545.
    Details
    Publication Date: 2016-04-01
    Description: The high-latitude North Atlantic (HLNA) is characterized by a marked seasonal phytoplankton bloom, which removes the majority of surface macronutrients. However, incomplete nitrate depletion is frequently observed during summer in the region, potentially reflecting the seasonal development of an iron (Fe) limited phytoplankton community. In order to investigate the seasonal development and spatial extent of iron stress in the HLNA, nutrient addition experiments were performed during the spring (May) and late summer (July and August) of 2010. Grow-out experiments (48–120 h) confirmed the potential for iron limitation in the region. Short-term (24 h) incubations further enabled high spatial coverage and mapping of phytoplankton physiological responses to iron addition. The difference in the apparent maximal photochemical yield of photosystem II (PSII) (Fv : Fm) between nutrient (iron) amended and control treatments (D(Fv : Fm)) was used as a measure of the relative degree of iron stress. The combined observations indicated variability in the seasonal cycle of iron stress between different regions of the Irminger and Iceland Basins of the HLNA, related to the timing of the annual bloom cycle in contrasting biogeochemical provinces. Phytoplankton iron stress developed during the transition from the prebloom to peak bloom conditions in the HLNA and was more severe for larger cells. Subsequently, iron stress was reduced in regions where macronutrients were depleted following the bloom. Iron availability plays a significant role in the biogeochemistry of the HLNA, potentially lowering the efficiency of one of the strongest biological carbon pumps in the ocean.
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  • 87
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    Physical controls of oxygen fluxes at pelagic and benthic oxyclines in a lake (2014)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 59 (5). pp. 1637-1650.
    Details
    Publication Date: 2019-06-27
    Description: We compared oxygen fluxes measured simultaneously at the pelagic and benthic oxycline in a lake and analyze their relation to hydrodynamic forcing conditions. While the mean oxygen fluxes did not differ significantly among both sites, the fluxes were highly variable in time. Short energetic periods contributed disproportionately to the overall oxygen flux above both the benthic and pelagic oxycline. In the pelagic region, mean fluxes across the oxycline were limited by low diffusivities (7 3 1028 m2 s21) and were one to two orders of magnitude smaller than fluxes above the oxycline (0.5 and 32 mmol m22 d21, respectively). A one-dimensional transport model was used to estimate sources and sinks of oxygen potentially causing this imbalance. The model results indicate that 92% of dissolved oxygen transported into the oxycline is used by the respiration of organic material imported into the oxycline from the epilimnion; chemical oxygen consumption associated with the upward flux of reduced substances is negligible. Our findings indicate that under such conditions, dissolved oxygen consumption and therewith mineralization within the oxycline can be comparable with the corresponding rates occurring in the sediments of eutrophic lakes with an oxic hypolimnion.
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  • 88
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    Dissolved cadmium in the Southern Ocean: Distribution, speciation, and relation to phosphate (2014)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 59 (2). pp. 385-399.
    Details
    Publication Date: 2017-05-02
    Description: We report isotope dilution analyses of dissolved cadmium (Cd) and electrochemical Cd speciation measurements in the Atlantic sector of the Southern Ocean. Bioavailable inorganic Cd is 〉 100 times higher in near-surface waters south of the Polar Front compared to the Subantarctic Zone because of upwelling and reduced complexation by organic Cd ligands. To trace local changes in the relation between Cd and P, we examine the deviations from a linear deep-water Cd vs. P relation (Cd*), and find that changes in Cd* coincide with the position of frontal systems and covary with primary productivity and total dissolved Mn and Fe concentrations. These covariations agree with potential local changes in phytoplankton Cd uptake rates, resulting from differences in the availability of Cd, Zn, Mn, and Fe. A band of negative Cd* values is associated with formation of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW). In contrast to SAMW, which may export low Cd : P ratios from the Southern Ocean, the Cd : P ratios in AAIW increase by mixing with underlying Upper Circumpolar Deep Water before being exported from the Southern Ocean. Deep waters show constant Cd : P ratios, and both elements behave conservatively with end-member mixing between deep waters of the Weddell Gyre, the Antarctic Circumpolar Current, and inflowing North Atlantic Deep Water. Overall, our results support the hypothesis that the kink in the global Cd vs. P relation is largely caused by high Cd : P uptake ratios in the trace-nutrient–limited Southern Ocean.
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  • 89
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    Variation in propagule and colonization pressures following rapid human-mediated transport: implications for a universal assemblage-based management model (2014)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 59 (6). pp. 2068-2076.
    Details
    Publication Date: 2017-10-01
    Description: Recent studies have recognized the importance of propagule pressure (number of individuals) and colonization pressure (number of species) for explaining establishment success of nonindigenous species. However, the International Ballast Water Management Convention, when ratified, will require ships to satisfy a numeric discharge standard that focuses only on cumulative propagule pressure of all individuals released. Because of practical constraints, the standard does not differentiate between discharges of single vs. multiple species. The assemblage-based approach, which uses rank-abundance gradients to quantify and manage introduction risk, may compensate for this limitation (e.g., even gradient [relatively consistent propagule pressures among n transported species] or uneven gradient [uneven propagule pressures among n transported species]). Here we explore species abundance distributions of zooplankton during transportation in ballast water to assess variability in the structure of assemblages, with implications for the potential development of an assemblage-based management model. Specifically, we explored species abundance distributions for voyages that lasted 〈 24 h, those from 24 to 48 h, and those from 48 to 72 h (i.e., three time scales). Species abundance distributions within and across transit time scales were highly variable. As transport time increased, we observed a shift from uneven to even rank-abundance gradients. Owing to variation in assemblage structure, the number of organisms necessary to quantify colonization pressure exhibited similarly strong variation within and across time scales. Our study indicates that assemblage-based approaches to estimate introduction risk are warranted, yet the variation inherent in transported assemblages will induce substantial uncertainty within management models.
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  • 90
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    Jelly biomass sinking speed reveals a fast carbon export mechanism (2013)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 58 (3). pp. 1113-1122.
    Details
    Publication Date: 2019-09-23
    Description: Sinking of gelatinous zooplankton biomass is an important component of the biological pump removing carbon from the upper ocean. The export efficiency, e.g., how much biomass reaches the ocean interior sequestering carbon, is poorly known because of the absence of reliable sinking speed data. We measured sinking rates of gelatinous particulate organic matter (jelly-POM) from different species of scyphozoans, ctenophores, thaliaceans, and pteropods, both in the field and in the laboratory in vertical columns filled with seawater using high-quality video. Using these data, we determined taxon-specific jelly-POM export efficiencies using equations that integrate biomass decay rate, seawater temperature, and sinking speed. Two depth scenarios in several environments were considered, with jelly-POM sinking from 200 and 600 m in temperate, tropical, and polar regions. Jelly-POM sank on average between 850 and 1500 m d−1 (salps: 800–1200 m d−1; ctenophores: 1200–1500 m d−1; scyphozoans: 1000–1100 m d−1; pyrosomes: 1300 m d−1). High latitudes represent a fast-sinking and low-remineralization corridor, regardless of species. In tropical and temperate regions, significant decomposition takes place above 1500 m unless jelly-POM sinks below the permanent thermocline. Sinking jelly-POM sequesters carbon to the deep ocean faster than anticipated, and should be incorporated into biogeochemical and modeling studies to provide more realistic quantification of export via the biological carbon pump worldwide.
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  • 91
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    Community succession in hydrothermal vent habitats of the Eastern Lau Spreading Center and Valu Fa Ridge, Tonga (2014)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 59 (5). pp. 1510-1528.
    Details
    Publication Date: 2017-08-04
    Description: Patterns of succession in Lau Basin hydrothermal vent communities determined with high-resolution imagery and in situ physico–chemical data collected over 4 yr and analyzed within a Geographic Information System show that Alviniconcha snails are a pioneering group, the snail Ifremeria nautilei is a mid-successional species, and the heat-intolerant mussel Bathymodiolus brevior dominates when venting declines. The associated fauna also changes as communities progress through the successional stages, and eventually non-vent–endemic deep-sea species appear when venting has mostly subsided. This is a unique example of primary succession in which the primary producers form symbiotic associations with mobile animals, resulting in successional patterns not observed in other systems. I. nautilei dominates newly formed substrates or venting sources where both I. nautilei and Alviniconcha spp. are already established (e.g., by migration), while Alviniconcha spp. seem to be better at colonizing newly active vents (e.g., by settlement) that are remote from colonized vents. Thus, on the scale of a 5–39 m2 diffuse flow area or a single edifice, the mid-successional species dominates new substrates instead of the pioneering group. These communities are remarkably stable over long time periods relative to other hydrothermal vent regions. In addition to the sequential replacements of species as sites age and overall conditions change, Lau vent animals track changes in vent fluids and relocate themselves when local hydrothermal plumbing changes over small spatial scales.
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  • 92
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    Between- and within-population variations in thermal reaction norms of the coccolithophore Emiliania huxleyi (2014)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 59 (5). pp. 1570-1580.
    Details
    Publication Date: 2017-09-02
    Description: Thermal reaction norms for growth rates of six Emiliania huxleyi isolates originating from the central Atlantic (Azores, Portugal) and five isolates from the coastal North Atlantic (Bergen, Norway) were assessed. We used the template mode of variation model to decompose variations in growth rates into modes of biological interest: vertical shift, horizontal shift, and generalist–specialist variation. In line with the actual habitat conditions, isolates from Bergen (Bergen population) grew well at lower temperatures, and isolates from the Azores (Azores population) performed better at higher temperatures. The optimum growth temperature of the Azores population was significantly higher than that of the Bergen population. Neutral genetic differentiation was found between populations by microsatellite analysis. These findings indicate that E. huxleyi populations are adapted to local temperature regimes. Next to between-population variation, we also found variation within populations. Genotype-by-environment interactions resulted in the most pronounced phenotypic differences when isolates were exposed to temperatures outside the range they naturally encounter. Variation in thermal reaction norms between and within populations emphasizes the importance of using more than one isolate when studying the consequences of global change on marine phytoplankton. Phenotypic plasticity and standing genetic variation will be important in determining the potential of natural E. huxleyi populations to cope with global climate change.
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  • 93
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    Invertebrates and their dormant eggs transported in ballast sediments of ships arriving to the Canadian coasts and the Laurentian Great Lakes (2011)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 56 (5). pp. 1929-1939.
    Details
    Publication Date: 2014-08-08
    Description: The most effective strategy for managing nonindigenous species (NIS) is through prevention of their transport via regulation of introduction vectors. We sampled 135 ships arriving to three different regions of Canada to assess abundance and species richness of invertebrates and their dormant eggs transported in ballast sediments. By sampling ships that followed particular pathways, we were able to compare vector strength to different regions, the invasion risk of transoceanic vs. coastal vessels, and the effect of midocean exchange, length of voyage, and amount of sediment on the richness and abundance of species inside ballast tanks. Although standardized ballast management regulations have been implemented across Canada, the resulting invasion risk is not uniform across regions. Ships arriving to the Atlantic region carried a greater sediment load with correspondingly higher abundance and species richness than those arriving to the Pacific and Great Lakes regions. Abundance and species richness of invertebrates and their dormant eggs associated with transoceanic ships did not differ from that of ships operating along coastal areas of North America. Similarly, midocean exchange did not reduce either abundance or species richness of invertebrate dormant eggs in ships. Finally, the length of voyage did not influence taxonomic composition or abundance of invertebrate dormant eggs but was directly related to survival of active macroinvertebrates. Ballast sediments could introduce new NIS to some regions of Canada despite requirements to manage ships' ballast by midocean exchange. Minimizing sediment accumulation may be the only effective management option for this vector.
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  • 94
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    Assessing the dynamics and ecology of marine picophytoplankton: The importance of the eukaryotic component (2004)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 49 (1). pp. 168-179.
    Details
    Publication Date: 2019-03-06
    Description: We assess population dynamics of picophytoplankton groups (≤2 μm diameter; Prochlorococcus, Synechococcus, and picoeukaryote) at a Pacific Ocean coastal site in the Southern California Bight. Weekly sampling (August 2000 to January 2002), dilution experiments, and flow cytometric analysis were combined with an instrument-specific calibration for cell size determination, allowing biovolume and carbon biomass estimation. Synechococcus was almost always numerically dominant, accounting for 60 ± 12 of the total picoplankton cells over time. It had moderately high growth rates (0.52-0.86 d-1) and was subject to low grazing mortality (-0.14 to -0.39 d-1). Prochlorococcus growth and mortality rates were roughly balanced (0.33 ± 0.14 d-1 and -0.36 ± 0.06 d-1, respectively). Picoeukaryotes had the highest growth rates (0.71-1.29 d-1) and were responsible for, on average, 76 of net carbon production (NCP), amounting in up to 32.05 ± 1.31 μg C L-1 d-1 produced and 28.31 ± 2.61 μg C L-1 d-1 consumed. In order to better define the eukaryotic component of these populations, an isolate was characterized via small subunit rRNA gene sequencing, transmission electron microscopy, and growth experiments and was identified as the prasinophyte Ostreococcus, not previously known to the Pacific Ocean. Our results show that although picoeukaryotes do not stand out as particularly important players in this system on the basis of cell abundance, they dominate in terms of picophytoplankton biomass and trophic transfer potential of carbon in this size class.
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  • 95
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    New procedure for determining reactive Fe(III) and Fe(II) minerals in sediments (2003)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 38 (8). pp. 1803-1812.
    Details
    Publication Date: 2019-09-23
    Description: Sediments from the River Elbe estuary and incubated sediments were extracted with 1 N HCI for 24 h at room temperature. The extracted ferric and ferrous iron was determined with DC and AC polarography. Acid-volatile sulfide was determined from H2S trapped in aqueous zinc acetate solution. Analysis of sediment samples and extraction residues with Mossbauer spectroscopy demonstrated that the Fe oxidation state was conserved during extraction and polarographic determination, siderite and vivianite were completely dissolved, and Fe(II) in chlorite was partially extracted with HCl. Incubation experiments showed that extractable Fe was almost completely oxidized to Fe(III) at the oxic sediment surface and reduced to Fe(II) in deeper anoxic layers within a few weeks. Reactive Fe(III), i.e. that fraction of Fe which was reducible on the time scale of the incubation experiment, was completely extracted with HCl.
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  • 96
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    Co-existence of distinct Ostreococcus ecotypes at an oceanic front (2017)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 62 (1). pp. 75-88.
    Details
    Publication Date: 2020-02-06
    Description: Western boundary currents support high primary production and carbon export. Here, we performed a survey of photosynthetic picoeukaryotes in the North Pacific Ocean in four transects crossing the Kuroshio Front. Prasinophyte algae comprised 85% of 18S rRNA gene sequences for photosynthetic taxa in the 〈5 μm size fraction. The picoplanktonic (〈2 μm) genera Micromonas and Ostreococcus comprised 30% and 51% of the total photosynthetic 18S rDNA sequences from five stations. Phylogenetic analysis showed that two Ostreococcus ecotypes, until now rarely found to co‐occur, were both present in the majority of samples. Ostreococcus ecotype OI reached 6,830 ± 343 gene copies mL−1, while Ostreococcus ecotype OII reached 50,190 ± 971 gene copies mL−1 based on qPCR analysis of the 18S rRNA gene. These values are higher than in studies of other oceanographic regions by a factor of 10 for OII. The data suggest that meso‐ and finer‐scale physical dynamics had a significant impact on the populations at the front, either by mingling ecotypes from different source regions at fine scales (∼10s km) or by stimulating their growth through vertical nutrient injections. We investigate this hypothesis with an idealized diffusion‐reaction model, and find that only a combination of mixing and positive net growth can explain the observed distributions and overlap of the two Ostreococcus ecotypes. Our field observations support larger‐scale numerical ocean simulations that predict enhanced biodiversity at western boundary current fronts, and suggest a strategy for systematically testing that hypothesis.
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  • 97
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    Particulate matter flux interception in oceanic mesoscale eddies by the polychaete Poeobius sp. (2018)
    ASLO (Association for the Sciences of Limnology and Oceanography) | Wiley
    In:  Limnology and Oceanography, 63 (5). pp. 2093-2109.
    Details
    Publication Date: 2021-05-19
    Description: Gelatinous zooplankton hold key functions in the ocean and have been shown to significantly influence the transport of organic carbon to the deep sea. We discovered a gelatinous, flux‐feeding polychaete of the genus Poeobius in very high abundances in a mesoscale eddy in the tropical Atlantic Ocean, where it co‐occurred with extremely low particle concentrations. Subsequent analysis of an extensive in situ imaging dataset revealed that Poeobius sp. occurred sporadically between 5°S–20°N and 16°W–46°W in the upper 1000 m. Abundances were significantly elevated and the depth distribution compressed in anticyclonic modewater eddies (ACMEs). In two ACMEs, high Poeobius sp. abundances were associated with strongly reduced particle concentrations and fluxes in the layers directly below the polychaete. We discuss possible reasons for the elevated abundances of Poeobius sp. in ACMEs and provide estimations showing that a single zooplankton species can completely intercept the downward particle flux by feeding with their mucous nets, thereby substantially altering the biogeochemical setting within the eddy.
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  • 98
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    Nitrogen and phosphorus co-limitation of bacterial productivity and growth in the Oligotrophic Sub-Tropical North Atlantic (2008)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 53 . pp. 824-834.
    Details
    Publication Date: 2018-08-14
    Description: Bacterial productivity and biomass are thought to be limited by dissolved organic carbon (DOC) in much of the world’s oceans. However, the mixed layer of oligotrophic oceans is often depleted in dissolved inorganic nitrogen and phosphate, raising the possibility that macronutrients may also limit heterotrophic bacterial growth. We used nutrient bioassay experiments to determine whether inorganic nutrients (N, P, Fe) and/or DOC could limit bacterial productivity and biomass in the central North Atlantic during the spring of 2004 (Mar–Apr). We observed that both heterotrophic bacterial productivity and biomass were co-limited by N and P in the oligotrophic North Atlantic, and additions of labile DOC (glucose) provided no stimulation unless N and P were also added. Flow cytometry results indicated that only a small subset of large cells high in nucleic acid content were responsible for the increased productivity in the combined NP amendments. In contrast, nutrient additions elicited no net change on the dominant component of the bacterial population, composed of small cells with relatively low nucleic acid content. In the combined NP treatments the relative increase in bacterial production was greater than that measured when phytoplankton productivity was relieved of nitrogen limitation. These results suggest that N and P co-limitation in the bacterial community results in increased competition between the heterotrophic and autotrophic components of the surface communities in the Central North Atlantic Ocean, and potentially impacts the cycling of organic matter by the bacterioplankton.
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  • 99
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    Light-dependent carbon isotope fractionation in the coccolithophorid Emiliania huxleyi (2002)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 47 . pp. 120-128.
    Details
    Publication Date: 2014-01-30
    Description: The carbon isotopic composition of marine phytoplankton varies significantly with growth conditions. Aqueous CO2 concentration [CO2] and algal growth rate (µ) have been suggested to be important factors determining isotope fractionation (ep). Here we examine ep of the coccolithophorid Emiliania huxleyi in relation to CO2 concentration and light conditions in dilute batch cultures. Cells were incubated at different irradiance cycles, photon flux densities (PFDs), and [CO2]. Isotope fractionation varied between 6.7 and 12.3‰ under 16 : 8 h light : dark cycle (L :D) and between 14.7 and 17.8‰ at continuous light. ep was largely independent of ambient [CO2], varying generally by less than 2‰ over a range of [CO2] from 5 to 34 mmol L-1. Instantaneous carbon-specific growth rates (µC) and PFDs, ranging from 15 to 150 mmol m-2 s-1, positively correlated with ep. This result is inconsistent with theoretical considerations and experimental results obtained under constant light conditions, suggesting an inverse relationship between ep and µ. In the present study the effect of PFDs on ep was stronger than that of mand thus resulted in a positive relationship between µ and ep. In addition, the L:D cycle of 16 : 8 h resulted in significantly lower ep values compared to continuous light. Since the observed offset of about 8‰ could not be related to daylength dependent changes in µC, this implies a direct influence of the irradiance cycle on ep. These findings are best explained by invoking active carbon uptake in E. huxleyi. If representative for the natural environment, these results complicate the interpretation of carbon isotope data in geochemical and paleoceanographic applications.
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    Effects of eutrophication, grazing, and algal blooms on rocky shores (2006)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 51 . pp. 569-579.
    Details
    Publication Date: 2014-01-30
    Description: Eutrophication can profoundly change rocky shore communities. These changes often cause the replacement of perennial, canopy-forming algae such as Fucus spp. with annual, bloom-forming algae such as Enteromorpha spp. Grazing, however, can counteract eutrophication by eliminating the annual algae�s susceptible recruits. We examine these generalizations across large scales. We use replicated ��bioassay�� experiments to compare the effects of eutrophication and grazing across four paired control versus eutrophied sites in the Northwest Atlantic and four eutrophied sites in the Baltic Sea in spring and summer. At each site, annual algal recruitment and grazing pressure were estimated using tiles seeded with Enteromorpha intestinalis propagules. Tiles were exposed for 3 weeks with grazers excluded or allowed access. Productivity of E. intestinalis recruits was strongly related to eutrophication (10-fold increase) and grazing (80% decrease) and was weakly related to season. While the absolute grazing rate increased in a linear fashion with algal productivity, the relative grazing rate remained surprisingly constant (;80%). Comparative field surveys showed that perennial algae decreased by 30-60%, while annual algae, filter feeders, and grazers increased across a gradient of eutrophication. As eutrophication increased from control to eutrophied to point source sites, rocky shore communities became increasingly dominated by single species of annual algae or filter feeders, and community diversity declined consistently by 24-46%. We conclude that grazers are important controllers of algal blooms but that, ultimately, they cannot override the effects of increasing eutrophication on rocky shore community structure and biodiversity.
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