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  • Other Sources  (53)
  • ASLO (Association for the Sciences of Limnology and Oceanography)
  • Alfred Wegener Institute for Polar and Marine Research & German Society of Polar Research
  • American Association for the Advancement of Science
  • 2015-2019  (28)
  • 2005-2009  (25)
  • 1950-1954
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  • 1
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    Temporal variability of lagoon–sea water exchange and seawater circulation through a Mediterranean barrier beach (2019)
    ASLO (Association for the Sciences of Limnology and Oceanography) | Wiley
    In:  Limnology and Oceanography, 64 (5). pp. 2059-2080.
    Details
    Publication Date: 2022-01-31
    Description: The subterranean flow of water through sand barriers between coastal lagoons and the sea, driven by a positive hydraulic gradient, is a net new pathway for solute transfer to the sea. On the sea side of sand barriers, seawater circulation in the swash-zone generates a flux of recycled and new solutes. The significance and temporal variability of these vectors to the French Mediterranean Sea is unknown, despite lagoons constituting ~ 50% of the coastline. A one-dimensional Ra-224(ex)/Ra-223 reactive-transport model was used to quantify water flow between a coastal lagoon (La Palme) and the sea over a 6-month period. Horizontal flow between the lagoon and sea decreased from ~ 85 cm d(-1) during May 2017 (0.3 m(3) d(-1) m(-1) of shoreline) to ~ 20 cm d(-1) in July and was negligible in the summer months thereafter due to a decreasing hydraulic gradient. Seawater circulation in the swash-zone varied from 10 to 52 cm d(-1) (0.4-2.1 m(3) d(-1) m(-1)), driven by short-term changes in the prevailing wind and wave regimes. Both flow paths supply minor dissolved silica fluxes on the order of ~ 3-10 mmol Si d(-1) m(-1). Lagoon-sea water exchange supplies a net dissolved inorganic carbon (DIC) flux (320-1100 mmol C d(-1) m(-1)) two orders of magnitude greater than seawater circulation and may impact coastal ocean acidification. The subterranean flow of water through sand barriers represents a significant source of new DIC, and potentially other solutes, to the Mediterranean Sea during high lagoon water-level periods and should be considered in seasonal element budgets.
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  • 2
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    Biogeography of zooplankton feeding strategy (2019)
    ASLO (Association for the Sciences of Limnology and Oceanography) | Wiley
    In:  Limnology and Oceanography, 64 (2). pp. 661-678.
    Details
    Publication Date: 2022-01-31
    Description: The trait‐based approach is increasingly used in plankton ecology to understand diversity, community dynamics, and biogeography. While on the global scale phytoplankton traits are fairly well established, zooplankton traits are only beginning to be understood. One taxa‐transcending aspect of zooplankton diversity is the distinction between ambush and active feeding strategies. We present a global‐scale empirical estimate of feeding strategy derived from copepod abundance observations, which for the first time suggests a distinct trait biogeography with ambush feeding as the dominant feeding strategy at higher, but not at lower latitudes. To explain this trait biogeography, we develop a minimalist trade‐off based model of feeding strategies based on encounter rates between zooplankton predators and their phyto‐ and zooplankton prey. Encounter rates are governed by the two traits, size and motility, that trade off against predation risk. Coupled to a three‐dimensional dynamic green ocean model, our idealized encounter model captures the observed feeding strategy biogeography. In the model, this pattern arises from competing dominant food chains within the food web and is shaped by a trophic trait cascade of active vs. passive feeding in adjacent trophic levels. The dominant feeding strategy structures the pathways and efficiency of energy and biomass transfer through the model food web, with consequences for primary production, export and higher trophic levels. Understanding feeding strategies is therefore important for fisheries, biogeochemical cycling, and long‐term predictions of ecosystem dynamics and functioning by global dynamic green ocean models.
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  • 3
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    Nutrient limitation driven dynamics of amino acids and fatty acids in coastal phytoplankton (2019)
    ASLO (Association for the Sciences of Limnology and Oceanography) | Wiley
    In:  Limnology and Oceanography, 64 (1). pp. 302-316.
    Details
    Publication Date: 2022-01-31
    Description: Coastal seas like the North Sea have been subject to major changes in nutrient inputs over the last decades, resulting in shifts of limiting nutrients for phytoplankton communities. Here, we investigated the seasonal and spatial distribution and synthesis patterns of individual amino acids and distinct fatty acid groups and show how these were affected by different nutrient limitations in natural coastal phytoplankton communities. Nitrogen limited communities exhibited substantially slower synthesis of essential amino acids compared to synthesis of nonessential amino acids. In short‐term nutrient addition experiments, this trend was reversed immediately after N addition to levels found under not limiting conditions. On the contrary, phosphorus limited communities showed no such shift in amino acids. Both N and P limitation induced a shift from structural to storage fatty acids with a concurrent decrease in the synthesis of poly‐unsaturated fatty acids. Reversed effects in fatty acid synthesis after N or P addition were only apparent after 72 h, when they could be found in both fatty acid biosynthesis and concentrations. The different strategies of qualitative and quantitative regulation of different biomolecule synthesis under nutrient scarcity may have far‐reaching consequences for the phytoplankton's nutritional value. Higher trophic levels may have to cope with the loss of essential amino acids and poly‐unsaturated fatty acids in nutrient limited phytoplankton, which could induce changes in the structure of food webs.
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  • 4
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    Ecology of a polymetallic nodule occurrence gradient: Implications for deep‐sea mining (2019)
    ASLO (Association for the Sciences of Limnology and Oceanography) | Wiley
    In:  Limnology and Oceanography, 64 (5). pp. 1883-1894.
    Details
    Publication Date: 2022-01-31
    Description: Abyssal polymetallic nodule fields constitute an unusual deep-sea habitat. The mix of soft sediment and the hard substratum provided by nodules increases the complexity of these environments. Hard substrata typically support a very distinct fauna to that of seabed sediments, and its presence can play a major role in the structuring of benthic assemblages. We assessed the influence of seafloor nodule cover on the megabenthos of a marine conservation area (area of particular environmental interest 6) in the Clarion Clipperton Zone (3950–4250 m water depth) using extensive photographic surveys from an autonomous underwater vehicle. Variations in nodule cover (1–20%) appeared to exert statistically significant differences in faunal standing stocks, some biological diversity attributes, faunal composition, functional group composition, and the distribution of individual species. The standing stock of both the metazoan fauna and the giant protists (xenophyophores) doubled with a very modest initial increase in nodule cover (from 1% to 3%). Perhaps contrary to expectation, we detected little if any substantive variation in biological diversity along the nodule cover gradient. Faunal composition varied continuously along the nodule cover gradient. We discuss these results in the context of potential seabed-mining operations and the associated sustainable management and conservation plans. We note in particular that successful conservation actions will likely require the preservation of areas comprising the full range of nodule cover and not just the low cover areas that are least attractive to mining.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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  • 5
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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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  • 6
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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.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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  • 7
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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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  • 8
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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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  • 9
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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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  • 10
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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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  • 11
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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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  • 12
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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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  • 13
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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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  • 14
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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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  • 15
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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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  • 16
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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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  • 17
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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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    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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  • 19
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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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    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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    Carbon sources in the North Sea evaluated by means of radium and stable carbon isotope tracers (2016)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 61 (2). pp. 666-683.
    Details
    Publication Date: 2019-02-01
    Description: A multitracer approach is applied to assess the impact of boundary fluxes (e.g., benthic input from sediments or lateral inputs from the coastline) on the acid-base buffering capacity, and overall biogeochemistry, of the North Sea. Analyses of both basin-wide observations in the North Sea and transects through tidal basins at the North-Frisian coastline, reveal that surface distributions of the δ13C signature of dissolved inorganic carbon (DIC) are predominantly controlled by a balance between biological production and respiration. In particular, variability in metabolic DIC throughout stations in the well-mixed southern North Sea indicates the presence of an external carbon source, which is traced to the European continental coastline using naturally occurring radium isotopes (224Ra and 228Ra). 228Ra is also shown to be a highly effective tracer of North Sea total alkalinity (AT) compared to the more conventional use of salinity. Coastal inputs of metabolic DIC and AT are calculated on a basin-wide scale, and ratios of these inputs suggest denitrification as a primary metabolic pathway for their formation. The AT input paralleling the metabolic DIC release prevents a significant decline in pH as compared to aerobic (i.e., unbuffered) release of metabolic DIC. Finally, long-term pH trends mimic those of riverine nitrate loading, highlighting the importance of coastal AT production via denitrification in regulating pH in the southern North Sea.
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  • 22
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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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    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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    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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    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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    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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    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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    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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    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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    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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    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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    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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    Partitioning Recent Greenland Mass Loss (2009)
    American Association for the Advancement of Science
    In:  Science, 326 (5955). pp. 984-986.
    Details
    Publication Date: 2020-05-28
    Description: Mass budget calculations, validated with satellite gravity observations [from the Gravity Recovery and Climate Experiment (GRACE) satellites], enable us to quantify the individual components of recent Greenland mass loss. The total 2000–2008 mass loss of ~1500 gigatons, equivalent to 0.46 millimeters per year of global sea level rise, is equally split between surface processes (runoff and precipitation) and ice dynamics. Without the moderating effects of increased snowfall and refreezing, post-1996 Greenland ice sheet mass losses would have been 100% higher. Since 2006, high summer melt rates have increased Greenland ice sheet mass loss to 273 gigatons per year (0.75 millimeters per year of equivalent sea level rise). The seasonal cycle in surface mass balance fully accounts for detrended GRACE mass variations, confirming insignificant subannual variation in ice sheet discharge.
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    Effect of zooplankton-mediated trophic cascades on marine microbial food web components (bacteria, nanoflagellates, ciliates) (2009)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 54 (1). pp. 262-275.
    Details
    Publication Date: 2017-05-02
    Description: To examine the grazing effects of copepod-dominated mesozooplankton on heterotrophic microbial communities, four mesocosm experiments using gradients of zooplankton abundance were carried out at a coastal marine site. The responses of different protist groups (nanoflagellates, ciliates) and bacterioplankton in terms of abundance and additionally, for bacteria, diversity, production, and exoenzymatic activity, were monitored during 1 week of incubation. Independent of the initial experimental abiotic conditions and the dominating copepod species, zooplankton caused order-of-magnitude changes in microbial functional groups in a clear community-wide four-link trophic cascade. The strongest predatory effects were observed for protist concentrations, thus generating inverse relationships between mesozooplankton and ciliates and between ciliates and nanoplankton. Copepod grazing effects propagated even further, not only reducing the abundance, production, and hydrolytic activity of bacterioplankton but also increasing bacterial diversity. The overall strength of this trophic cascade was dampened with respect to bacterial numbers, but more pronounced with respect to bacterial diversity and activity. High predation pressure by heterotrophic nanoflagellates, realized at the highest copepod abundance, was probably the underlying mechanism for these structural changes in the bacterial assemblages. Our results thus suggest a mechanism by which changes in higher trophic levels of marine plankton indirectly affect prokaryotic assemblages and microbially mediated ecosystem functions.
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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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  • 36
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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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    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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  • 38
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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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  • 39
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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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  • 40
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    Influence of cell cycle phase on calcification in the coccolithophore Emiliania huxleyi (2008)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 53 (2). pp. 506-512.
    Details
    Publication Date: 2017-05-02
    Description: Calcification of the cosmopolitan coccolithophore species Emiliania huxleyi was investigated in relation to the cell division cycle with the use of batch cultures. With a 12 : 12 h light : dark cycle, the population was synchronised to undergo division as a cohort, simultaneously passing through the G1 (assimilation), S (DNA replication), and G2+M (cell division and mitosis) phases. Cell division was followed with the use of quantitative DNA staining and flow cytometry. Simultaneously, carbon-14 (14C) assimilation in organic and inorganic carbon as well as cell abundance, size, and organic nitrogen content were measured at 2-h intervals. In additional experiments, changes in calcification and cell cycle stages were investigated in nitrogen-, phosphorus-, and light-limited cultures. Calcification occurred only during the G1 cell cycle phase, as seen by the very tight correlation between the percentage of cells in G1 and calcification during the dark period. When growth was limited by nitrogen, cells decreased in size, remained in the G1 phase, and showed a moderate increase in the cell-specific calcite content. Limitation of growth by phosphorus, however, caused a significant increase in cell size and a dramatic increase in cellular calcite. Light limitation, by slowing the growth rate, prolonged the time cells spent in the G1 phase with a corresponding increase in the cellular calcite content. These results help explain the differing responses of coccolithophorid growth to nitrogen, phosphorus, and light limitation.
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  • 41
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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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    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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  • 43
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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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    Structure and Function of the PB1 Domain, a Protein Interaction Module Conserved in Animals, Fungi, Amoebas, and Plants (2007)
    American Association for the Advancement of Science
    In:  Science's STKE, 2007 (401). re6-re6.
    Details
    Publication Date: 2019-07-25
    Description: Proteins containing the PB1 domain, a protein interaction module conserved in animals, fungi, amoebas, and plants, participate in diverse biological processes. The PB1 domains adopt a ubiquitin-like β-grasp fold, containing two α helices and a mixed five-stranded β sheet, and are classified into groups harboring an acidic OPCA motif (type I), the invariant lysine residue on the first β strand (type II), or both (type I/II). The OPCA motif of a type I PB1 domain forms salt bridges with basic residues, especially the conserved lysine, of a type II PB1 domain, thereby mediating a specific PB1-PB1 heterodimerization, whereas additional contacts contribute to high affinity and specificity of the modular interaction. The canonical PB1 dimerization is required for the formation of complexes between p40phox and p67phox (for activation of the NADPH oxidase crucial for mammalian host defense), between the scaffold Bem1 and the guanine nucleotide exchange factor Cdc24 (for polarity establishment in yeasts), and between the polarity protein Par6 and atypical protein kinase C (for cell polarization in animal cells), as well as for the interaction between the mitogen-activated protein kinase kinase kinases MEKK2 or MEKK3 and the downstream target mitogen-activated protein kinase kinase MEK5 (for early cardiovascular development in mammals). PB1 domains can also mediate interactions with other protein domains. For example, an intramolecular interaction between the PB1 and PX domains of p40phox regulates phagosomal targeting of the microbicidal NADPH oxidase; the PB1 domain of MEK5 is likely responsible for binding to the downstream kinase ERK5, which lacks a PB1 domain; and the scaffold protein Nbr1 associates through a PB1-containing region with titin, a sarcomere protein without a PB1 domain. This Review describes various aspects of PB1 domains at the molecular and cellular levels.
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  • 45
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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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  • 46
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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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  • 47
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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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  • 48
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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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    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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  • 50
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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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    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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    Testing the direct effect of CO2 concentration on a bloom of the coccolithophorid Emiliania huxleyi in mesocosm experiments (2005)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 50 . pp. 493-504.
    Details
    Publication Date: 2014-01-30
    Description: We studied the direct effects of CO2 Engel et al. in the seawater was modified by an aeration system. The triplicate mesocosm treatments represented low (190 parts per million by volume (ppmV) CO2 organisms in a mesocosm experiment. In nine outdoor enclosures (11 m3 present (410 ppmV CO2 and high (710 ppmV CO2 pCO2 and related changes in seawater carbonate chemistry on marine planktonic each), the partial pressure of CO2 (pCO2 conditions. After initial fertilization with nitrate and phosphate a bloom dominated by the coccolithophorid Emiliania huxleyi occurred simultaneously in all of the nine mesocosms; it was monitored over a 19-day period. The three CO2 treatments assimilated nitrate and phosphate similarly. The concentration of particulate constituents was highly variable among the replicate mesocosms, disguising direct CO2 within each treatment, however, indicated that the net specific growth rate of E. huxleyi, the rate of calcification per cell, and the elemental stoichiometry of uptake and production processes were sensitive to changes in pCO2 This broad influence of CO2 -related effects. Normalization of production rates . on the E. huxleyi bloom suggests that changes in CO2 physiology with likely effects on the marine biogeochemistry.
    Type: Article , PeerReviewed
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  • 53
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    The physicochemical habitat of Sclerolinum sp., at Hook Ridge hydrothermal vent, Bransfield Strait, Antarctica (2005)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 50 (2). pp. 598-606.
    Details
    Publication Date: 2019-09-23
    Description: At Hook Ridge hydrothermal vent, a new species of Sclerolinum (Monilifera, Siboglinidae) was found at a water depth of 1,045 m. On the basis of investigations of multicores and gravity cores, the species habitat is characterized. Sclerolinum does not occur in sediments that are most strongly influenced by hydrothermal fluids, probably because of high temperature (up to 49°C) and precipitation of siliceous crusts. About 800 individuals m-2 occur in sediments that are only weakly exposed to hydrothermal flow and have the following characteristics: 20°C (15 cm sediment depth) to 21.5°C (bottom water), 18-40 cm yr-1 advection rates, pH 5.5, 〈25 µmol L-1 methane, 〈170 µmol L-1 sulfide, and 〈0.0054 mol m-2 yr-1 sulfide flux. Comparison with geochemical data from other reducing sediments indicates that the two groups of Siboglinidae, Monilifera and Frenulata, occur in sediments with low sulfide concentration and flux. In contrast, sulfurbased chemosynthetic organisms that typically occur at hydrothermal vents and cold seeps (e.g., Vestimentifera, vesicomyid clams, and bacterial mats) occur in sediments with higher sulfide availability; threshold values are around 500 µmol L-1 sulfide and 0.1 mol m-2 yr-1 sulfide fluxes. We did not find typical hydrothermal vent species at Hook Ridge hydrothermal vent, which might be explained by the unfavorable physicochemical habitat: At sites inhabited by Sclerolinum, sulfide availability appears to be too low, whereas at sites with higher sulfide availability, the temperatures might be too high, siliceous crust precipitation could preclude their occurrence, or both.
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