ALBERT

Beschreibung: Cellular responses of Fe-limited Microcystis aeruginosa were investigated under nutrient-depleted and -replete conditions. Cellular growth, Fe quota and Fe uptake kinetics were examined in chemostat systems using nutrient-replete Fraquil * (where all nutrients except for Fe are present at sufficient level to achieve optimal growth) and nutrient-deplete Fraquil * (where some nutrients in addition to Fe are potentially growth-limiting factors). For both nutrient conditions, cellular Fe quota increased with increasing dilution rate in a manner consistent with Droop theory. However, the Fe quota in nutrient-deplete Fraquil * was determined to be lower, indicating lower cellular Fe requirement in the nutrient-depleted condition. Short-term Fe uptake assays indicated that cells acclimated in nutrient-replete conditions adjust to various degrees of Fe stress by solely increasing maximum Fe uptake rate, consistent with expected negative feedback regulation. In contrast, the maximum Fe uptake rate decreased with increasing degree of Fe limitation in the nutrient-depleted chemostat (particularly nitrate and molybdenum in this study). This non-negative feedback regulation is likely associated with lower Fe requirement for specific functions (e.g., intracellular nitrate reduction). Cellular affinity for Fe uptake and cellular size were independent of degree of Fe stress for both nutrient conditions.

Beschreibung: Carbon-to-chlorophyll a ratios (C:Chl a ; weight : weight) were analyzed for 7578 coastal seawater samples collected from Danish waters from 1990 to 2014. The aim was to identify the seasonal and spatial dynamics relative to nutrient richness and to study the effect of reduced nitrogen loadings over time. C:Chl a values were lowest during winter, about 15 across all stations. During the spring, C:Chl a increased to summer values between 20 and 96, depending on the annual mean of total nitrogen concentration. An inter-annual sinusoidal model with monthly time steps described the seasonal C:Chl a pattern well. The amplitudes of the model varied inversely with the annual mean of total nitrogen. Data also showed that a reduction in nitrogen loadings to the area by ∼ 40% over the past 24 yr, resulted in a statistically significant increase in mean annual C:Chl a values of 0.8 ± 0.2 yr −1 . The patterns derived from this large data set can be used to predict C:Chl a values for temperate coastal phytoplankton. Use of the empirical relationships derived from the data set improves predictions of C:Chl a values and thereby, e.g., carbon based food-web calculations and carbon-based ecosystem models, which often are validated using chlorophyll measurements.

Beschreibung: The annual North Atlantic spring bloom influences the ecology and biogeochemistry of the Atlantic Ocean. Diatoms dominate the peak of the bloom and significantly impact productivity and export of organic carbon from the bloom. Despite their key role in a yearly event with global impacts, the genetic diversity and population structure of diatoms that comprise the bloom are unknown. Here, we investigated the population genetics of the diatom Thalassiosira gravida sampled during the 2008 North Atlantic Bloom Experiment using newly developed microsatellite markers. High levels of genetic diversity (clonal diversity: 0.970; expected heterozygosity: 0.884) were observed across all water samples and did not change during the bloom. Four genetically distinct populations were identified ( F ST : 0.036–0.093) but were not associated with different water masses, depths, or time points during the bloom. Instead, all four populations co-existed within individual water samples, spanning different water masses, stages of the bloom, and depths of over 300 m. The co-existence of multiple, genetically distinct populations during the bloom event suggested large-scale admixture, with populations originating via transport from disparate locations combined with potential overwintering capacity in the water column or sediments. The pattern of co-existence suggests that the open ocean may serve as a gene pool that harbors different populations that are then available for selection to act upon, which may contribute to the ecological and biogeochemical success of diatoms and influence their long-term evolutionary survival.

Beschreibung: Gephyrocapsa oceanica is a widespread species of coccolithophore that has a significant impact on the global carbon cycle through photosynthesis and calcium carbonate precipitation. We investigated combined effects of light (50 μ mol m −2 s −1 , 190 μ mol m −2 s −1 , and 400 μ mol m −2 s −1 ) and the nitrogen sources and on its physiological performance under nitrogen-limited conditions. The specific growth rate was highest at the mid-range light level of 190 μ mol m −2 s −1 , where it was further accelerated by relative to . There were no significant growth rate differences between - and -grown cells at the two light levels either above or below this optimum irradiance. Cellular particulate organic carbon (POC) and nitrogen (PON) content were not significantly affected by different light intensities and nitrogen sources. However, both the cellular particulate inorganic carbon (PIC) content and the PIC to POC ratio were greatly decreased by increased light levels, and were further decreased by only at the highest light level. Non-photochemical quenching (NPQ) increased with increasing light intensity, and was higher in rather than in -grown cells at medium and high light intensities. Our results demonstrate that under low, relatively realistic oceanic nitrogen concentrations, increasing light intensity and the replacement of by would have a significant negative effect on the calcification of the coccolithophore G. oceanica . If these findings are also applicable to other coccolithophore species, the future ocean carbon cycle may be greatly affected.

Beschreibung: Our study aims to enhance process understanding of the long-term (decadal and longer) cyclic marsh dynamics by identifying the mechanisms that translate large-scale physical forcing in the system into vegetation change, in particular ( i) the initiation of lateral erosion on an expanding marsh, and ( ii) the control of seedling establishment in front of an eroding marsh-cliff. Short-term sediment dynamics (i.e., seasonal and shorter changes in sediment elevation) at the mudflat causes variation in mudflat elevation over time ( δ z TF ). The resulting difference in elevation between the tidal flat and adjacent marsh (Δ Z ) initiates lateral marsh erosion. Marsh erosion rate was found to depend on sediment type and to increase with increasing Δ Z and hydrodynamic exposure. Laboratory and field experiments revealed that seedling establishment was negatively impacted by an increasing δ z TF . As the amplitude of δ z TF increases towards the channel, expanding marshes become more prone to lateral erosion the further they extend on a tidal flat, and the chance for seedlings to establish increases with the distance that marsh has eroded back towards the land. This process-based understanding, showing the role of sediment dynamics as explanatory factor for marsh cyclicity, is important for protecting and restoring valuable marsh ecosystems. Overall, our experiments emphasize the need for understanding the connections between neighbouring ecosystems such as mudflat and salt marsh.

Beschreibung: Variability of the particulate beam attenuation, scattering, and backscattering coefficients, along with discrete measurements of HPLC derived pigments and particulate organic carbon, is investigated within and beside a finfish aquaculture cage on the west coast of Vancouver Island, Canada. Large bio-optical variability was observed over three seasons: fall, spring, and summer. The use of multiple optical proxies for organic particle concentrations, bulk particle-size distributions, and compositions allowed for the characterization of the particle assemblage over the dynamic conditions. Specifically, in fall, low biological productivity and surface influences from runoff were observed. In the lower-cage, after feeding the fish, optical proxies suggest the dominance of large particles with high indices of refraction, possibly due to the influence of fish fecal particles. Optical variability in spring was driven by diatom bloom conditions ( Chaetoceros and Skeletonema ). Strong bio-optical relationships were observed and the optical proxies suggest a particle-size distribution (PSD) dominated by large particles with low indices of refraction. In turn, summer conditions displayed noticeably high and persistent particulate backscattering in the surface waters, suggesting the presence of an Emiliania huxleyi bloom. Neither spring nor summer showed distinct influences by aquaculture wastes indicating that optical characterization of particulate waste dispersal is likely constrained to low ambient seston conditions. Outside of these conditions bio-optical analysis would be beneficial for environmental monitoring of ambient particles moving through aquaculture systems.

Beschreibung: Microphytobenthos (MPB) are an important nitrogen (N) sink in coastal systems, but little is known about the fate of this N after it has been assimilated. We used an in situ 15 N pulse-chase experiment in subtidal sands to follow the assimilation, trophic transfer, transformation, and flux pathways of MPB-N over 33 d. Throughout the study MPB dominated 15 N uptake, on average representing only 18.1% of the biomass but 63.9% of the 15 N within 0–2 cm sediment. Following assimilation, 15 N was rapidly transferred to deeper sediment, with 32.1% below 2 cm and 16.5% below 5 cm after 60 h. In contrast to MPB, bacteria represented 39.5% of sediment biomass but accounted for only up to 27.3% of assimilated 15 N. Foraminifera accumulated and stored 15 N more than bacteria; their contribution to the 15 N remaining in 0–2 cm sediment at the end of the study was more than double their biomass contribution. Thirty-three days after the 15 N was assimilated by MPB 27% remained in the sediment, 16.5% had been effluxed as , 20.8% had been effluxed as , 20.7% had been effluxed as N 2 and 15.1% was missing. Most (12.6%) of 15 N label that was missing at the end of the study was probably lost as dissolved organic N (DON) fluxes. Of the 15 N remaining in 0–2 cm sediment, 80.4% was in MPB, 2.7% in bacteria, 1% in foraminifera and the remaining 15.9% was uncharacterized. Overall there was little benthic trophic transfer with most of the MPB-assimilated N remineralized over 33 d.

Beschreibung: Rapid changes, including warming and freshening, are occurring in coastal marine ecosystems worldwide. These environmental changes have the potential to alter ecosystem energetics by influencing availability of food sources and organism physiology. We investigated the influence of oceanographic variability on food availability and quality to benthic and pelagic suspension-feeders using detailed observations of phytoplankton, particulate organic matter (POM) detritus, and diverse biomarkers (fatty acids and carbon, nitrogen, and sulfur stable isotopes) along a salinity gradient in the San Juan Archipelago, Washington, U.S.A. We tested the hypothesis that freshwater input from riverine discharge would cause significant changes to oceanographic conditions and reduce food quality (indicated by essential fatty acids; EFA), owing to greater contribution of terrestrial organic matter. Contrary to our expectations, availability of high-quality marine-derived POM increased with freshwater input (reduced salinity). Phytoplankton biomass and biomarker composition responded to oceanographic change similarly across tidal and seasonal scales. Using a meta-analysis spanning a range of spatial and temporal scales, we found that chlorophyll a , temperature, dissolved oxygen (DO) and pH were consistently and significantly higher at reduced salinity. The increase of DO and pH corresponding to higher phytoplankton biomass in low salinity water signifies an important feedback of biological activity on seawater chemistry. This analysis supports the use of salinity as an indicator of processes controlling food availability and oceanographic conditions in this region. Collectively, these results highlight the importance of ecosystem connectivity in coastal environments and produce hypotheses for expected changes related to altered river discharge dynamics.

Beschreibung: The first extensive set of measurements of methane concentrations and fluxes for the Negro River and its major tributaries combined with complementary data for the Solimões and Madeira rivers and several tributaries are presented and their temporal and spatial variations examined. Fluxes were measured using floating chambers, and dissolved CH 4 concentrations were measured by the headspace technique. In the Solimões basin, tributaries had higher fluxes when water levels were low; no statistical difference among periods for lakes and the main stem river was observed. In the Negro basin, rivers had higher fluxes with greater variations among rivers during high water than during low water based on fluxes calculated from the concentration gradient and modelled gas transfer coefficients. We estimate a regional methane emission of 0.31 Tg C yr −1 for large river channels in the lowland Amazon basin.

Beschreibung: A numerical circulation model with a very simple representation of dissolved oxygen dynamics is used to simulate hypoxia in Chesapeake Bay for the 30-yr period 1984–2013. The model assumes that the biological utilization of dissolved oxygen is constant in both time and space in an attempt to isolate the role that physical processes play in modulating oxygen dynamics. Despite the simplicity of the model it demonstrates skill in simulating the observed inter-annual variability of hypoxic volume, capturing 50% of the observed variability in hypoxic volume (〈2 mg L −1 ) for the month of July and 58% of the observed variability for the month of August, over the 30-yr period. Model skill increases throughout the summer suggesting that physical processes play a more important role in modulating hypoxia later in the summer. Model skill is better for hypoxic volumes than for anoxic volumes. In fact, a simple regression based on the integrated January–June Susquehanna River nitrogen load can explain more of the variability in the observed anoxic volumes than the model presented here. Model results suggest that the mean summer (June–August) wind speed is the single-most important physical variable contributing to variations in hypoxic volumes. Previous studies have failed to document the importance of summer wind speed because they have relied on winds measured at Patuxent Naval Air Station, which does not capture the observed inter-annual variations in wind speed that are observed by stations that directly measure wind over the waters of Chesapeake Bay.

Beschreibung: Anthropogenic CO 2 emissions currently decrease open ocean pH, but on multi-millennial time scales intensified continental weathering is expected to contribute to increasing oceanic alkalinity ( A T ) and thus mitigate the acidification signal. The Baltic Sea is an ideal study site for such A T dynamics, due to its direct link to terrestrial processes, short water residence time and long history of A T measurements dating back to the early 20 th century. We compiled an extensive A T data set that revealed the highest data quality and coverage for the past two decades. Within that period, surface water A T levels increased throughout the Baltic Sea. The rates of change were highest in the low-saline, northern areas and decreased gradually toward constant levels in the North Sea. The A T increase observed in the Central Baltic Sea (+3.4 µmol kg −1 yr −1 ) and the Gulf of Bothnia (+7 µmol kg −1 yr −1 ) has compensated CO 2 -induced acidification by almost 50% and 100%, respectively. Further, the A T trends enhanced the CO 2 storage capacity and stabilized the CaCO 3 saturation state of the Baltic Sea over the past two decades. We discuss the attribution of the A T trends to potential changes in precipitation patterns, continental weathering driven by acidic rain and increasing atmospheric CO 2 , agricultural liming and internal A T sources.

Beschreibung: Kelps and fucoids are dominant habitat-forming seaweeds along temperate rocky coastlines. Here, we tested the physiological performance of a dominant kelp ( Ecklonia radiata ) and two fucoids ( Scytothalia dorycarpa and Sargassum fallax ), distributed along the southwest coast of Australia. Photosynthesis and respiration were measured against increments in temperature for seaweeds collected along a latitudinal gradient in ocean temperature from Kalbarri (warm) to Hamelin Bay (cool). We found a similar decrease in photosynthetic activity from cooler to warmer latitudes in all three species. Seaweeds collected from warmer locations had significantly lower chlorophyll a concentration compared to cooler locations which could explain the lower levels of photosynthetic activity at warmer latitudes. The Q 10 values for photosynthesis and respiration tended to decrease from cooler to warmer locations. For all species, the optimum temperature for net photosynthesis remained similar across the locations. However, within locations, the optimum temperature for S . fallax (25.2°C) was significantly higher than E. radiata (24.0°C) and S. dorycarpa (23.6°C). The reduction rates of net photosynthesis observed after optimum temperature showed the greatest variation among the species within and across locations. A thermal performance breadth analysis revealed a broader performance range for S . fallax (21.5–28.4°C) followed by E . radiata (21.2–26.5°C) and S . dorycarpa (21.4–25.8°C). These results highlight the differences in temperature sensitivity among the three species which help explain their current distributional patterns and have potential implications for future responses to future ocean warming.

Beschreibung: We propose a bivariate Bayesian hierarchical model (BBHM), which adds a perspective on a century-long subject of research, nitrogen (N) and phosphorus (P) dynamics in freshwater and coastal marine ecosystems. The BBHM is differentiated from existing approaches by modeling multiple aspects of N-P relationships―N and P concentration variability, ratio, and correlation―simultaneously, allowing these aspects to vary by seasonal and/or spatial components. The BBHM is applied to three aquatic systems, Finnish Lakes, Saginaw Bay, and the Neuse Estuary, which exhibit differing landscapes and complexity of nutrient dynamics. Our model reveals N and P dynamics that are critical to inferring unknown N and P distributions for the overall system as well as for within system variability. For Finnish lakes, strong positive within- and among-lake N and P correlations indicate that the rates of N and P biogeochemical cycles are closely coupled during summer across the different lake categories. In contrast, seasonal decoupling between N and P cycles in Saginaw Bay is evidenced by the large variability in monthly correlations and the seasonal changes in the N distribution. The results underscore the pivotal role that dreissenids have had on the cycling of nutrients and resurgence of eutrophication. The presence of clear seasonality and a spatial gradient in the distributions and N and P in the Neuse Estuary suggest that riverine N input is an important source in the season-space N dynamics, while summer sediment release is a major process regulating seasonal P distribution.

Beschreibung: The dependence of denitrification and dissimilatory nitrate reduction to ammonium (DNRA) on different electron donors was tested in the nitrate-containing layer immediately below the oxic–anoxic interface (OAI) at three stations in the central anoxic basins of the Baltic Sea. Additionally, pathways and rates of fixed nitrogen transformation were investigated with 15 N incubation techniques without addition of donors. Denitrification and anammox were always detected, but denitrification rates were higher than anammox rates. DNRA occurred at two sites and rates were two orders of magnitude lower than denitrification rates. Separate additions of dissolved organic carbon and sulfide stimulated rates without time lag indicating that both organotrophic and lithotrophic bacterial populations were simultaneously active and that they could carry out denitrification or DNRA. Manganese addition stimulated denitrification and DNRA at one station, but it is not clear whether this was due to a direct or indirect effect. Ammonium oxidation to nitrite was detected on one occasion. During denitrification, the production of nitrous oxide (N 2 O) was as important as dinitrogen (N 2 ) production. A high ratio of N 2 O to N 2 production at one site may be due to copper limitation, which inhibits the last denitrification step. These data demonstrate the coexistence of a range of oxidative and reductive nitrogen cycling processes at the Baltic OAI and suggest that the dominant electron donor supporting denitrification and DNRA is organic matter. Organotrophic denitrification is more important for nitrogen budgets than previously thought, but the large temporal variability in rates calls for long-term seasonal studies.

Beschreibung: Freshwater ecosystems can be considerable sources of greenhouse gases (GHG), however, much uncertainty remains in global estimates and understanding of drivers of these emissions. Furthermore, headwater streams have received insufficient attention and may contribute disproportionately to global GHG flux. Our objective was to quantify carbon dioxide (CO 2 ), methane (CH 4 ), and nitrous oxide (N 2 O) flux and assess the impact of changes in dissolved organic carbon (DOC) and NO 3 concentrations on GHG flux in three streams in the Lamprey River watershed in New Hampshire, U.S.A., that contrast in surface water DOC: NO 3 . We measured DOC, NO 3 and dissolved gas partial pressures and estimated gas flux in surface waters monthly from May 2011 to April 2012. We found higher GHG partial pressures and fluxes in the two streams with high DOC concentrations. The stream with high DOC and high NO 3 showed high N 2 O and low CH 4 flux, while the high DOC, low NO 3 stream showed high CH 4 and low N 2 O flux. Our results support a model in which C inputs drive total GHG production, while NO 3 input regulates the relative importance of CH 4 and N 2 O, likely by suppressing methanogenesis and stimulating denitrification. Results suggest streams in this region are small sources of CO 2 , but potentially important sources of CH 4 and N 2 O. Since CH 4 and N 2 O are more powerful than CO 2 at trapping heat in the atmosphere, freshwater emissions of these gases have the potential to partially offset climate benefits of terrestrial carbon sinks, a possibility that has not been sufficiently incorporated into climate models.

Beschreibung: River-floodplain systems are characterized by changing hydrological connectivity and variability of resources delivered to floodplain water bodies. Although the importance of hydrological events has been recognized, the effect of flooding on CH 4 concentrations and emissions from European, human-impacted river-floodplains is largely unknown. This study evaluates aquatic concentrations and emissions of CH 4 from a highly modified, yet partly restored river-floodplain system of the Danube near Vienna (Austria). We covered a broad range of hydrological conditions, including a 1-yr flood event in 2012 and a 100-yr flood in 2013. Our findings demonstrate that river-floodplain waters were supersaturated with CH 4 , hence always serving as a source of CH 4 to the atmosphere. Hydrologically isolated habitats in general have higher concentrations and produce higher fluxes despite lower physically defined velocities. During surface connection, however, CH 4 is exported from the floodplain to the river, suggesting that the main channel serves as an “exhaust pipe” for the floodplain. This mechanism was especially important during the 100-yr flood, when a clear pulse of CH 4 was flushed from the floodplain with surface floodwaters. Our results emphasize the importance of floods differing in magnitude for methane evasion from river-floodplains; 34% more CH 4 was emitted from the entire system during the year with the 100-yr flood compared to a hydrologically “normal” year. Compared to the main river channel, semi-isolated floodplain waters were particularly strong sources of CH 4. Our findings also imply that the predicted increased frequency of extreme flooding events will have significant consequences for methane emission from river-floodplain systems.

Beschreibung: Aquatic plants are important primary producers that affect the nutrient cycling in shallow coastal areas. How benthic communities composed of different plant species affect community metabolism and N fluxes is however, poorly understood, and in situ quantification is scarce. To study primary production and water-sediment N fluxes in a benthic community consisting of various aquatic vascular plant species, diel field experiments were conducted in the Baltic Sea. Nine plant species were incubated in situ with bare sediments as control. The aim was to investigate diel community (including vascular plants, epiphytes, infauna, phytoplankton, and phytobenthos) and biomass-specific metabolism and nitrogen fluxes among different plant species within a benthic community and to compare N fluxes between vegetated and bare sediments. The net production differed between different plant species, while the overall respiration was unaffected by the plant species present. There were no discernible differences in inorganic N fluxes between the different plant species within the benthic community and vegetated and bare sediments, whereas DON fluxes differed significantly. These results demonstrate that aquatic plant species have varying effects on ecosystem processes. Inherent differences between plant species add significant complexity to benthic functional diversity and highlight the important role that species-rich vegetated habitats play for ecosystem functioning in shallow coastal areas.

Beschreibung: The increasing concentrations of methane (CH 4 ) in the atmosphere stress the importance of monitoring and quantifying the fluxes from coastal environments. In nine sampling campaigns between 2013 and 2014, we measured the spatial CH 4 concentrations, identified major sources and calculated the fluxes at the air-water interface in an eutrophic tropical embayment, Guanabara Bay, Rio de Janeiro, Brazil. The bay presented high spatial variability of CH 4 concentrations, without a significant trend with salinity, but observed the influence of the urban areas at its watershed. Although the more polluted sector of the bay accounts for about 10% of the sampled surface area, it contributed to one half of the bay's total CH 4 emissions. In most cases, high CH 4 concentrations seemed be sustained by allochtonous sources such as the sewage network and polluted rivers, especially under high accumulated precipitation conditions. In the most stratified area, at the inner and centre of the Bay, CH 4 concentrations were not significantly higher in bottom hypoxic waters than in surface waters, suggesting that CH 4 diffusion from these sediments was modest, due to the prevalence of sulphate reduction over methanogenesis. Our calculated annual air-sea fluxes (565–980 μmol m −2 d −1 ) are well above those of most estuaries worldwide, showing that urban pollution can be an important source of CH 4 to the coastal waters and even more significant than the presence of organic-rich environments, like salt marshes and mangroves. Comparing the greenhouse gas emissions in terms of CO 2 -equivalent, CH 4 emissions reduced the net CO 2 sink in Guanabara Bay by 16%.

Beschreibung: Freshwater lakes represent a substantial natural source of methane to the atmosphere and thus contribute to global climate change. Microbial methane oxidation is an important control on methane release from these systems, where oxygen appears to be the most essential electron acceptor for this process. However, there is extensive geochemical evidence that methane is also oxidized under anoxic conditions in lakes, though the details about the exact mechanism have still not been resolved. Here, we investigated the fate of methane in the water column of meromictic Lake Zug. We provide evidence for ongoing methane oxidation at the oxic/anoxic boundary and also in the anoxic hypolimnion, both apparently mediated by aerobic methane-oxidizing bacteria. Gammaproteobacterial methanotrophs (gamma-MOB) dominated the indigenous methanotrophic community and were active under all investigated conditions—oxic, sub-oxic and anoxic. Methane oxidation was stimulated by the additions of oxygen or iron and manganese oxides under anoxic conditions. In the latter case, trace amounts of oxygen may have still been required for methane activation, yet these findings indicate that gamma-MOB in Lake Zug might be able to respire electron acceptors other than oxygen. We propose that gamma-MOB are actively removing methane also in anoxic lake waters, thus contributing to methane mitigation from these habitats.

Beschreibung: Physical defenses and grazer avoidance of the bloom-forming microalga Gonyostomum semen may reduce the direct coupling between phytoplankton and higher trophic levels and result in an increased importance of alternative basal food resources such as bacteria and heterotrophic protozoans. To assess the importance of algal and heterotrophic food resources for zooplankton during G. semen blooms and the effects of zooplankton diets on a higher consumer, we analyzed the fatty acid composition of zooplankton and the invertebrate predator Chaoborus flavicans from eight lakes along a gradient in the predominance of G. semen relative to other algae and the duration of G. semen blooms. The proportion of fatty acids of bacterial origin increased significantly along the G. semen gradient in all consumers studied. In addition, the proportion of polyunsaturated fatty acids (PUFA) decreased in cladocerans. These results suggest that heterotrophic pathways can compensate for a reduced trophic coupling between phytoplankton and filter-feeding zooplankton. The lower PUFA content in cladoceran prey from lakes at the higher end of the G. semen gradient did not affect the PUFA content of the predator C. flavicans , suggesting selective assimilation and retention of PUFA and/or feeding on other, more PUFA-rich prey.

Beschreibung: Phytoplankton may serve as a key entry for methylmercury (MeHg) into aquatic food webs however very few studies have quantified the bioconcentration of MeHg in marine phytoplankton from seawater, particularly for non-diatoms. Experiments using 203 Hg to measure MeHg uptake rates and concentration factors in six marine phytoplankton species belonging to different algal classes were conducted and the influence of light, temperature, and nutrient conditions on MeHg bioaccumulation were determined. All algal species greatly concentrated MeHg out of seawater, with volume concentration factors (VCFs) ranging from 0.2 × 10 5 to 6.4 × 10 6 . VCFs were directly related to cellular surface area-to-volume ratios. Most of the cellular MeHg was found in the cytoplasm. Temperature, light, and nutrient additions did not directly affect MeHg uptake in most species, with the exception that the dinoflagellate Prorocentrum minimum displayed significantly greater uptake per cell at 18°C than at 4°C, suggesting an active uptake for this species. Passive transport seemed to be the major pathway for most phytoplankton to acquire MeHg and was related to the surface area-to-volume ratio of algal cells. Environmental conditions that promoted cell growth resulted in more total MeHg associated with cells, but with lower concentrations per unit biomass due to biodilution. The very high bioconcentration of MeHg in marine phytoplankton is by far the largest bioconcentration step in marine food chains and variations in algal uptake may account for differences in the amount of MeHg that ultimately builds up in different marine ecosystems.

Beschreibung: Knowledge on the distribution and habitat use of species is an important precondition for their appropriate management and conservation. This is particularly challenging for highly mobile marine predators such as blue shark that migrate between dynamic and transient oceanic habitats. In addition, blue shark populations have complex spatial structures due to age and sexual segregation, and accurately identifying oceanic nursery habitat is vital to ensure population growth via survival of juveniles. In this study, long-term satellite telemetry data (up to 950 d) was used to estimate the broad-scale habitat utilization of juvenile blue shark and predict their distribution in the North Atlantic. Habitat utilization models were fitted separately for three juvenile life stages that recruit to the pelagic longline fishery (small juvenile males and females, large juvenile and sub-adult females and large juvenile males), and subsequently validated using fisheries data. The models demonstrated that the patterns of spatiotemporal distribution and segregation are shaped to a large extent by differential habitat preferences, notably for distinct ranges of sea surface temperature. For the first time, this modeling approach provides a unifying framework to understand the essential pelagic habitat and dynamic spatial structuring of blue shark at the scale of an entire ocean basin. It represents an important contribution to our understanding of the spatial ecology of pelagic sharks by presenting novel clues to their behavioral strategies in exploiting the most productive oceanic habitats, and offers a promising tool for their management in the face of current intensive shark exploitation.

Beschreibung: Submarine groundwater discharge provides freshwater and nutrients to coastal environments. In some places throughout the world, this direct connection between aquifers and oceans may also allow saltwater intrusion. Saltwater intrusion was studied at a submarine spring within a fringing reef lagoon on the eastern Yucatán Peninsula by observing its intratidal and synoptic-scale variations during wet and dry periods. Saltwater intrusion was linked to wave-driven setup, no rain, high tides, and sea-level rise caused by remote forcing from Yucatán Current variability. Jet discharge velocities were inversely related to tidal oscillations, with maximum velocities at low tides. The wet period produced saltwater intrusion at high tides associated with three different conditions: syzygy tides, wave setup, and Yucatán Current weakening. During the dry period, saltwater intrusion occurred throughout most high tides and was aided by Yucatán Current weakening and wind-driven setup within the lagoon. These results suggested that seasonal precipitation was most important in modulating spring discharge, followed by syzygy tides, Yucatán Current variability, and wave events. The spring discharge was modeled with a modified Bernoulli energy equation that included a friction term. Aquifer elevation and a friction factor were used as free parameters. The dry period produced the best model results because of infrequent rainfall that yielded a relatively steady aquifer level. Precipitation during the wet period most likely led to a more variable aquifer level, reducing the variance explained by the model that assumes a constant aquifer elevation. Nevertheless, the model predicted saltwater intrusion events reasonably well using simplified physics.

Beschreibung: The composition of sinking particles and the mechanisms leading to their transport ultimately control how much carbon is naturally sequestered in the deep ocean by the “biological pump.” While detrital particles often contain much of the sinking carbon, sinking of intact phytoplankton cells can also contribute to carbon export, which represents a direct flux of carbon from the atmosphere to the deep ocean by circumventing the surface ocean food web. Phytoplankton that contributed to carbon flux were identified in sinking material collected by short-term sediment trap deployments conducted along a transect off the eastern shore of South America. Particulate organic carbon flux at 125 m depth did not change significantly along the transect. Instead, changes occurred in the composition and association of phytoplankton with detrital particles. The fluxes of diatoms, coccolithophores, dinoflagellates, and nano-sized cells at 125 m were unrelated to the overlying surface population abundances, indicating that functional-group specific transport mechanisms were variable across locations. The dominant export mechanism of phytoplankton at each station was putatively identified by principal component analysis and fell into one of three categories; (1) transport and sinking of individual, viable diatom cells, (2) transport by aggregates and fecal pellets, or (3) enhanced export of coccolithophores through direct settling and/or aggregation.

Beschreibung: The Changjiang (Yangtze River) and Huanghe (Yellow River) are the two largest rivers in China, and they transport large amounts of terrestrial carbon to the coastal waters of the East China Sea and the Bohai Sea. The sources and cycling of riverine carbon in these two large river estuaries, however, have not been well studied. In this article, we present the results of dual isotope (Δ 14 C and δ 13 C) measurements of dissolved inorganic carbon (DIC) collected in the low reaches of the Changjiang and Huanghe and their estuaries during two cruises in 2014. Our results indicate that both the Changjiang and Huanghe carry very high concentrations of DIC ranging from 1384 μ mol kg −1 to 1732 μ mol kg −1 and 2711 μ mol kg −1 to 4120 μ mol kg −1 , respectively, and DIC levels varied with flow rates during high and low discharge periods. The cycling of DIC exhibited conservative behavior in both the Changjiang and Huanghe estuaries, suggesting DIC levels were controlled mainly by physical mixing processes. Δ 14 C-DIC values indicate that the Changjiang and Huanghe transport aged DIC (1060–1380 yr old). Both Δ 14 C-DIC and δ 13 C-DIC values also showed conservative mixing in the two estuaries. Using a dual carbon isotopic model, we calculated that atmospheric CO 2 consumed mainly by silicate weathering was a major source, contributing 65.2 ± 9.0% and 73.4 ± 3.0% of DIC in the Changjiang and Huanghe, and 96.9–97.7% (by air-sea exchange) of DIC in the coastal waters of the East China Sea (ECS) and Bohai Sea, respectively. Our results indicate that carbonate dissolution was an important (12.3–17.4%) but not major process controlling the high DIC levels in both rivers, as suggested previously. Compared with the large Amazon River, respiration of riverine organic matter (OM) played a less important role, contributing only 15.4–17.2% of DIC in the two Asian rivers. Flux calculations indicate that the Changjiang and Huanghe discharged 1.46 × 10 13 g and 6.28 × 10 11 g DIC into the ECS and Bohai Sea in 2014, which were 9 and 17 times higher than the DOC fluxes in the two rivers. These large fluxes of riverine DIC, especially of aged DIC, could have significant impacts on primary production and carbon cycling in the ECS and Bohai Sea.

Beschreibung: The recent discovery of methane seeps in the Arctic region requires a better understanding of the fate of methane in marine sediments if we are to understand the contributions of methane to Arctic ecosystems and climate change. To this goal, we analyze pore water data from five sites along eastern Vestnesa Ridge, a sediment drift off-north-west Svalbard, to quantify the consumption of dissolved methane across the sulfate-methane-transition-zone which are 3–5 m below seafloor from the investigated sites. We use transport-reaction models to quantify the hydrology as well as the carbon mass balance in the sediments. Pore water profiles and our model results demonstrate that hydrological, microbial, and geochemical processes/reactions efficiently remove methane carbon from fluid over different time scales. We interpret the nonsteady-state behavior of the first 50–70 cm of our pore water profiles from the active sites as an annual scale downward fluid flow due to a seepage-related pressure imbalance. Such downward flow supplies sulfate which enhances methane consumption through anaerobic oxidation of methane (AOM) within this depth range. Our steady-state modeling confirms the efficiency of AOM in consuming dissolved methane in the upper 0.8–1.2 m of sediments. Based on the phosphate profiles, we estimate that AOM at the active pockmarks may have been operating for the last two to four centuries. Precipitation of authigenic carbonate removes more than a quarter of the dissolved inorganic carbon produced by AOM and fixes it as authigenic carbonate in the sediments, a process that sequestrates methane carbon over geological time.

Beschreibung: Recent ocean acidification (OA) studies revealed that seawater [H + ] rather than [CO 2 ] or [ ] regulate short-term responses in carbon fluxes of Emiliania huxleyi . Here, we investigated whether acclimation to altered carbonate chemistry modulates this regulation pattern and how the carbon supply for calcification is affected by carbonate chemistry. We acclimated E. huxleyi to present-day (ambient [CO 2 ], [ ], and pH) and OA conditions (high [CO 2 ], ambient [ ], low pH). To differentiate between the CO 2 and pH/H + effects, we also acclimated cells to carbonation (high [CO 2 ] and [ ], ambient pH) and acidification (ambient [CO 2 ], low [ ], and pH). Under these conditions, growth, production of particulate inorganic and organic carbon, as well as carbon and oxygen fluxes were measured. Under carbonation , photosynthesis and calcification were stimulated due to additional uptake, whereas growth was unaffected. Such stimulatory effects are not apparent after short-term carbonation , indicating that cells adjusted their carbon acquisition during acclimation. Being driven by [ ], these regulations can, however, not explain typical OA effects. Under acidification and OA , photosynthesis stayed constant, whereas calcification and growth decreased. Similar to the short-term responses toward high [H + ], CO 2 uptake significantly increased, but uptake decreased. This antagonistic regulation in CO 2 and uptake can explain why photosynthesis, being able to use CO 2 and , often benefits from OA, whereas calcification, being mostly dependent on , often decreases. We identified H + as prime driver of coccolithophores' acclimation responses toward OA. Acidified conditions seem to put metabolic burdens on the cells that result in decreased growth.

Beschreibung: Dynamic tidal export of dissolved inorganic carbon (DIC) to the coastal ocean from highly productive intertidal marshes and its effects on seawater carbonate chemistry are thoroughly evaluated. The study uses a comprehensive approach by combining tidal water sampling of CO 2 parameters across seasons, continuous in situ measurements of biogeochemically-relevant parameters and water fluxes, with high-resolution modeling in an intertidal salt marsh of the U.S. northeast region. Salt marshes can acidify and alkalize tidal water by injecting CO 2 (DIC) and total alkalinity (TA). DIC and TA generation may also be decoupled due to differential effects of marsh aerobic and anaerobic respiration on DIC and TA. As marsh DIC is added to tidal water, the buffering capacity first decreases to a minimum and then increases quickly. Large additions of marsh DIC can result in higher buffering capacity in ebbing tide than incoming tide. Alkalization of tidal water, which mostly occurs in the summer due to anaerobic respiration, can further modify buffering capacity. Marsh exports of DIC and alkalinity may have complex implications for the future, more acidified ocean. Marsh DIC export exhibits high variability over tidal and seasonal cycles, which is modulated by both marsh DIC generation and by water fluxes. The marsh DIC export of 414 g C m −2 yr −1 , based on high-resolution measurements and modeling, is more than twice the previous estimates. It is a major term in the marsh carbon budget and translates to one of the largest carbon fluxes along the U.S. East Coast.

Beschreibung: Rates of denitrification (isotope pairing) and nitrogen fixation (acetylene reduction) were simultaneously measured in three temperate, intertidal Zostera muelleri meadows and adjacent non-vegetated tidal flats within Western Port, Australia. Net daily nitrogen fluxes ranged from −276 (net denitrification) to 520 μmol N/m 2 d (net nitrogen fixation), and were generally positive, with only two instances of net negative fluxes. The highest fluxes were observed at the sites with the lowest water column nitrate concentrations. No significant differences in net nitrogen fluxes were found between vegetated and non-vegetated sediments ( p  = 0.213). Nitrogen fixation was generally the dominant process occurring, which was stimulated in the presence of vegetation except at the most marine-influenced site, where nitrogen fixation in non-vegetated sediments was higher. Nitrogen fixation rates in non-vegetated sediments were highly correlated to cyanobacterial cell counts (although no mats were present). Rates were ∼65 μmol N/m 2 d at 0 cell counts, suggesting a basal rate driven within the sediment. Additional slurry experiments confirmed significant rates of nitrogen fixation within the sediment, which were stimulated by sucrose and terminated by nitrate ( p  〈 0.05), strongly suggesting sulfate-reducing bacteria contributed to nitrogen fixation. At the bay-wide scale, nitrogen fixation was estimated to contribute ∼430 t N/yr compared to ∼650 t N/yr from catchment and atmospheric inputs and 230 t N/yr lost through denitrification. Sensitivity analysis confirmed that while the loss of seagrass would affect the magnitude of the bay-wide flux of nitrogen on these tidal flats, nitrogen fixation remains the dominant process.

Beschreibung: The method of reconstructing paleoatmospheric CO 2 levels using carbon isotope signatures of organic matter buried in sediments has been questioned due to the dubious foundation that carbon isotope fractionation during phytoplankton photosynthesis ( ε p ) is controlled primarily by aquatic CO 2 concentration ([CO 2 (aq)]). Consequently, what carbon isotope data from bulk sedimentary organic matter reflects is a puzzle. In this study, we determined the carbon isotope compositions of dissolved inorganic carbon and particulate organic carbon in a lake located in a carbonate area. Partial correlation analysis was employed to distinguish between direct and indirect factors in controlling ε p . The results show that ε p is more closely, and more steadily related with pH than with [CO 2 (aq)], which is in accordance with recent advances in our understanding of the physiology of carbon utilization by phytoplankton for CO 2 and . Therefore, we propose that carbon isotope fractionation in phytoplankton is more suitable as a proxy of pH than of [CO 2 (aq)]. One advantage of this amendment is that information on , the main species of carbon uptake by phytoplankton, is likewise included. In the future, culture experiments aiming at revealing the relationship between pH and cellular carbon isotope signatures is necessary to construct a new isotope fractionation formula to couple the different effects of CO 2 and , which is of critical importance to improve the understanding of carbon isotope fractionation, and to more precisely model pH and CO 2 .

Beschreibung: Accumulation of carotenoid pigments in copepods has often been described as a plastic adaptation providing photoprotection against ultraviolet radiation (UVR). However, reports of seasonal carotenoid maxima in winter, when UVR is low, challenge the proposed driving role of UVR. Therefore, we here evaluate the mechanistic connection between UVR and the seasonal pattern of copepod carotenoid pigmentation. We assessed the carotenoids, fatty acid content and reproduction of Leptodiaptomus minutus along with UVR exposure, water temperature, phytoplankton pigments, and fish predation in a boreal lake during 18 months covering two winter seasons. The predominant carotenoid astaxanthin occurred in free form as well as esterified with fatty acids. Mono- and diesters accounted for 62–93% of total astaxanthin and varied seasonally in close correlation with fatty acids. The seasonal variability in total astaxanthin content of the copepods was characterized by net accumulation in late fall of up to 0.034 μ g (mg dry mass) −1 d −1 , which led to the mid-winter maximum of 3.89 ± 0.31 μ g mg −1 . The two periods of net loss (−0.018 μ g mg −1 d −1 and −0.021 μ g mg −1 d −1 ) coincided with peaks of egg production in spring and summer leading to minimum astaxanthin content (0.86 ± 0.03 μ g mg −1 ) in fall. This period was also characterized by the highest predation pressure by young-of-the-year fish. The results suggest that accumulation of astaxanthin in copepods is strongly related to lipid metabolism but not to UVR-photoprotection, and that seasonal changes of fatty acids and carotenoids are related to the reproduction cycle.

Beschreibung: We characterized the intact polar lipid (IPL) composition in the surface waters of 22 lakes from Minnesota and Iowa, ranging in trophic state between eutrophic and oligo-mesotrophic, to investigate the impact of trophic state on IPL composition. A high diversity of IPL classes was detected. Most IPL classes were detected in all lakes, but the eutrophic lakes contained a significantly higher relative abundance of lyso-phosphatidylcholine (PC) than the oligo-mesotrophic lakes, which in turn were characterized by significantly higher relative abundance of hydroxymethyltrimethyl-alanine/trimethyl-homoserine (DGTA/DGTS) betaines, ornithine lipids and the recently discovered trimethyl ornithine (TMO) lipids. The higher relative abundance of ornithines and TMOs may relate to a higher contribution of heterotrophic bacteria relative to phytoplankton while the higher abundance of the DGTA/DGTS betaines may relate to substitution by microorganisms of these non-P lipids for PC under P-stress, as has been observed in other environments. We also detected a variety of heterocyst glycolipids (HGs) derived from N 2 -fixing heterocystous Cyanobacteria in all lakes, suggesting the presence of these Cyanobacteria in the full range of trophic conditions. Correlation of HG abundance with environmental data showed that high productivity lakes have high HG abundances, while other distributional differences in HGs, which did not correlate with environmental parameters, are likely due to differences in species composition. We conclude that the significant differences in IPL composition between the eutrophic and oligo-mesotrophic lakes are either due to adaptation of the membrane composition to nutrient conditions or due to general divergences in microbial composition under the different conditions.

Beschreibung: 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 δ 13 C 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 ( 224 Ra and 228 Ra). 228 Ra 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.

Beschreibung: Through diel vertical migration (DVM), zooplankton add an active transport to the otherwise passive sinking of detrital material that constitutes the biological pump. This active transport has proven difficult to quantify. We present a model that estimates both the temporal and depth characteristic of optimal DVM behavior based on a trade-off between feeding opportunity and predation risk; factors that vary with latitude, time of year, and the size of the migrating animal. This behavioral component, coupled to a nutrient-phytoplankton-zooplankton (NPZ) productivity model provides estimates of the active transport of carbon by different size fractions of the migrating zooplankton population as function of time and space. The approach is motivated by the difficulty in incorporating behavioral aspects of carbon transport into large scale carbon budgets of the world's oceans. The results show that despite their lower abundance, large zooplankton (length circa 1–2 mm) migrate deeper and transport approximately twice as much carbon as do the smaller zooplankton (length circa 0.2–0.3 mm). In mid- latitudes (∼30°N to ∼45°N), where pronounced spring blooms are observed, up to 20% more carbon is transported than at either equatorial or boreal latitudes. We estimate that the amount of carbon transported below the mixed layer by migrating zooplankton in the North Atlantic Ocean constitutes 27% (16–30%) of the total export flux associated with the biological pump in that region.

Beschreibung: Anthropogenic modification watersheds and climate change have altered export from fluvial systems causing changes to the carbonate chemistry of river-influenced near shore environments. To determine the possible effects of riverine discharges on the mussel Perumytilus purpuratus , we performed in situ transplant experiments between river-influenced and open coastal habitats with contrasting seawater carbonate chemistries (i.e., p CO 2 , pH, Ω ar ) across four regions covering a wide latitudinal range (32°55′S–40°10′S). The river-influenced habitats selected for transplant experiments were different than open coastal habitats; with higher p CO 2 (354–1313 μatm), lower pH (7.6–7.9) and Ω ar values (0.4–1.4) than in open coastal area. Growth, calcification, metabolism were measured in a reciprocal transplant experiment to determine physiological responses associated with river-influenced sites and non-influenced control sites. Growth and calcification rates were higher in river-influenced habitats; however the organisms in this area also had lower metabolic rates, possibly due to enhanced food supply from river systems. Further analysis of carbon isotopic composition ( δ 13 C) indicated that the relative contribution of seawater dissolved inorganic carbon (DIC) to the carbonate shells of P. purpuratus was much higher than respiratory carbon. Nevertheless, P. purpuratus incorporated between 7% and 26% of metabolic carbon in the shell depending on season. There was a strong, significant relationship between δ 13 C POC and δ 13 C Tissue , which likely influenced the isotopic composition of the shell carbon.

Beschreibung: The lack of high resolution, geographically diverse proxy records from the marine realm limits our understanding of climate dynamics in the North Atlantic Ocean and Arctic during recent centuries. We investigate the impact of large-scale climate variability on the marine bivalve, Arctica islandica , (Linnaeus 1767) from northern Norway (71°N). We evaluate the use of annual shell growth and geochemical records as proxies for North Atlantic and Arctic climate variability over centennial scales by developing a continuous, 113-yr master shell growth chronology and an oxygen isotope record (δ 18 O) from live caught shell material. A relatively strong inverse relationship is observed between both the shell growth and isotopic proxies and large-scale North Atlantic sea surface temperatures in modern times ( r  = −0.54 to −0.90; p  〈 0.05). This relationship is strengthened when using a combined shell growth/oxygen isotope Multiproxy Index ( r  = −0.72 to −0.90; p 〈0.01). The regional spatial pattern of correlation resembles that of the North Atlantic Current as it bifurcates around 55°N, indicating that large-scale ocean surface current dynamics play an important role in regulating local ecosystem processes and thus shell growth in northern Norway. A combined proxy index created using multiple linear regression exhibits a relatively strong and time-stable relationship with the Atlantic Multidecadal Oscillation (AMO; r  = −0.622; p  〈 0.001) since AD 1900. Variability in the relationship between the shell based records and the North Atlantic Oscillation coincide with variations in the AMO index, suggesting a complex relationship between atmospheric forcing on hydrographic variability and ecosystem dynamics in northern Norway.

Beschreibung: Picophytoplankton, including photosynthetic picoeukaryotes (PPE) and unicellular cyanobacteria, are important contributors to plankton biomass and primary productivity. In this study, phytoplankton composition and rates of carbon fixation were examined across a large trophic gradient in the South East Pacific Ocean (SEP) using a suite of approaches: photosynthetic pigments, rates of 14 C-primary productivity, and phylogenetic analyses of partial 18S rRNA genes PCR amplified and sequenced from flow cytometrically sorted cells. While phytoplankton 〉10 μm (diatoms and dinoflagellates) were prevalent in the upwelling region off the Chilean coast, picophytoplankton consistently accounted for 55–92% of the total chlorophyll a inventories and 〉60% of 14 C-primary productivity throughout the sampling region. Estimates of rates of 14 C-primary productivity derived from flow cytometric sorting of radiolabeled cells revealed that the contributions of PPE and Prochlorococcus to euphotic zone depth-integrated picoplankton productivity were nearly equivalent (ranging 36–57%) along the transect, with PPE comprising a larger share of picoplankton productivity than cyanobacteria in the well-lit (〉15% surface irradiance) region compared with in the lower regions (1–7% surface irradiance) of the euphotic zone. 18S rRNA gene sequence analyses revealed the taxonomic identities of PPE; e.g., Mamiellophyceae ( Ostreococcus ) were the dominant PPE in the upwelling-influenced waters, while members of the Chrysophyceae, Prymnesiophyceae, Pelagophyceae, and Prasinophyceae Clades VII and IX flourished in the oligotrophic South Pacific Subtropical Gyre. Our results suggest that, despite low numerical abundance in comparison to cyanobacteria, diverse members of PPE are significant contributors to carbon cycling across biogeochemically distinct regions of the SEP.

Beschreibung: Despite considerable research on the linkages between dissolved organic matter (DOM) and bacteria, it is not yet clear how the abundance of the main aquatic clades relates to DOM composition in natural aquatic systems. We evaluated this relation using PARAFAC modeling of excitation–emission fluorescence spectroscopy and spectroscopic indexes to characterize DOM composition, and fluorescence in situ hybridization, to quantify the major bacterial groups in a subtropical lagoon. The DOM exhibited marked temporal variations in concentration, molecular weight, aromaticity, color, degree of humification, and freshness, and proportion of the three different fluorescent components identified. All major bacterial clades ( Alphaproteobacteria , Betaproteobacteria , Gammaproteobacteria , and Cytophaga-Flavobacteria ) were significantly linked to DOM concentration and/or composition, being those crucial factors for modeling their abundance in situ. The combination and significance of the factors was specific for each bacterial group, strongly indicating that they behave as coherent and distinctive units. Cytophaga-Flavobacteria and Betaproteobacteria were the groups which correlated with more DOM properties. Alphaproteobacteria and Gammaproteobacteria abundances were significantly explained by low or high dissolved organic carbon concentrations, respectively. The significant relationships between DOM properties and the main bacterial groups delineated a profile of each group regarding DOM preferences/dislikes, in agreement with evidence derived from genome analysis to single-cell substrate uptake. These results highlight the specificities of the main bacterial clades, providing support for a functional classification of the bacterioplankton regarding DOM processing at the level of bacterial classes.

Beschreibung: Three publications recently reported that calanoid copepods, feeding on phytoplankton cells by using a feeding current, perceived such cells by mechanoperception. There was no evidence of remote chemically-mediated perception of those cells. These observations differ from earlier findings that feeding-current producing calanoids are able to detect phytoplankton cells by chemoperception at a distance from their particle-collecting setae of their cephalic appendages. The results on mechanoperception and the earlier published data on chemoperception will be presented and discussed. In addition, the concentration of chemicals within the phycosphere of food cells will be re-examined. We conclude that chemoperception of phytoplankton cells by calanoid copepods in a feeding current is feasible.

Beschreibung: We reply to the comments of Paffenhöfer and Jiang ( ) who argues that remote chemical prey perception is necessary for feeding-current feeding copepods to fulfill their nutritional requirements in a dilute ocean, that remote chemical prey detection may only be observed at very low prey concentrations, and that chemical prey perception is feasible if prey cells release dissolved organic material in short-lasting but intense bursts. We demonstrate that mechanoreception at a very short range is sufficient to sustain a living, even in a dilute ocean. Further, if chemoreception requires that prey cells have short intense leakage burst, only a very small fraction of prey cells would be available to the copepod at any instance in time and, thus would be inefficient at low prey concentration. Finally, we report a few new observations of prey capture in two species of copepods, Temora longicornis and Centropages hamatus , offered a 45- μ m sized dinoflagellate at very low concentration. The observed short prey detection distances, up to a few prey cell radii, are consistent with mechanoreception and we argue briefly that near-field mechanoreception is the most likely and common prey perception mechanism in calanoid copepods.

Beschreibung: Microplankton community structure and particulate matter stoichiometry were investigated in a late summer survey across the Subantarctic and Polar Front in the Indian sector of the Southern Ocean. Microplankton community structure exerted a first order control on PON:POP stoichiometry with diatom-dominated samples exhibiting much lower ratios (4–6) than dinoflagellate and ciliate-dominated samples (10–21). A significant fraction of the total chlorophyll a (30–70%) was located beneath the euphotic zone and mixed layer and sub-surface chlorophyll features were associated to transition layers. Although microplankton community structure and biomass was similar between mixed and transition layers, the latter was characterized by elevated Chl:POC ratios indicating photoacclimation of mixed layer communities. Empty diatom frustules, in particular of Fragilariopsis kerguelensis and Pseudo-nitzschia , were found to accumulate in the Antarctic Zone transition layer and were associated to elevated BSi:POC ratios. Furthermore, high Si(OH) 4 diffusive fluxes (〉1 mmol m 2 d −1 ) into the transition layer appeared likely to sustain silicification. We suggest transition layers as key areas of C and Si decoupling through (1) physiological constraints on carbon and silicon fixation (2) as active foraging sites for grazers that preferentially remineralize carbon. On the Kerguelen Plateau, the dominant contribution of Chaetoceros Hyalochaete resting spores to microplankton biomass resulted in a three-fold enhancement of POC concentration at 250 m, compared to other stations. These findings further highlight the importance of diatom resting spores as a significant vector of carbon export through the intense remineralization horizons characteristing Southern Ocean ecosystems.

Beschreibung: 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 CO 2 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.

Beschreibung: The magnitude and spectral shape of the optical backscattering coefficient of particles, b bp ( λ ), is being increasingly used to infer information about the particles present in seawater. Relationships between b bp and particle properties in the Arctic are poorly documented, and may differ from other oceanic regions which contribute the majority of data used to develop and parameterize optical models. We utilize recent field measurements from the Chukchi and Beaufort Seas to examine relationships between the spectral backscattering coefficient of particles in seawater and the mass concentration, bulk composition, and size distribution of the suspended particle assemblage. The particle backscattering coefficient spanned six orders of magnitude from the relatively clear waters of the Beaufort Sea to extremely turbid waters on the Mackenzie shelf. This coefficient was highly correlated with the mass concentration of particles, and to a lesser extent with other measures of concentration such as particulate organic carbon or chlorophyll a . Increased backscattering and high mass-specific b bp ( λ ) was associated with mineral-rich assemblages that tended to exhibit steeper size distributions, while reduced backscattering was associated with organic-dominated assemblages having a greater contribution of large particles. Our results suggest that algorithms which employ composition-specific relationships can lead to improved estimates of particle mass concentration from backscattering measurements. In contrast to theoretical models, however, we observe no clear relationship between the spectral slope of b bp ( λ ) and the slope of the particle size distribution in this environment.

Beschreibung: Carbon in surface waters is widely recognized as a key element that influences nutrient cycling, metal availability, and water quality. Its degradation in streams occurs primarily by benthic microbial communities that colonize the underlying sediment, which is commonly termed the hyporheic zone (HZ). The biodegradation of a labile dissolved organic carbon (DOC L ), exemplified by sodium benzoate, was studied in a novel laboratory flume system under a combination of different overlying water velocities, losing or gaining fluxes, and biofilm distribution (“surficial” or “homogeneous distribution”). The overall objective of this study was to evaluate the effect of different flow conditions on DOC L biodegradation in the HZ. The results showed that overlying velocity was the dominant factor affecting DOC L biodegradation, regardless of biofilm distribution. Gaining flow conditions also induced a slight increase in the biodegradation rates as compared to losing or neutral flow conditions, due to additional oxygen input from the upwelling water. The aerobic reactive zone under all flow conditions was limited to the upper section of the benthic biofilm (several millimeters), where the surficial biofilm showed the highest activity. Our results demonstrate the processes affecting DOC L biodegradation in the hyporheic zone and will help to implement future modeling of DOC transport in streams.

Beschreibung: The use of resources from multiple habitats has been shown to be important to the production of aquatic consumers. To quantify the support of Great Lakes coastal wetland (WL) and nearshore (NS) habitats to yellow perch, we used otolith microchemistry to trace movements between the habitats. WL and NS water and fish samples were collected from lakes Huron and Michigan for water and otolith trace element analysis. Recently deposited otolith-edge Sr : Ca and Ba : Ca from otoliths were strongly correlated with the chemistry of the water in which fish were caught. In general, Sr : Ca and Ba : Ca in otoliths were significantly greater for individuals collected from WL areas. Because of these observed chemical differences between WL and NS habitats, quadratic discriminant function analysis (QDFA) was used to classify individuals with high accuracy to the habitat from which they were collected. We then combined the predictive abilities of QDFA with the otolith chemistry transect data that represents an individuals' entire life, to classify habitat use through each fish's life. Our results suggest larval use of WL habitats as well as three life histories for adult yellow perch. These strategies include (1) fish utilizing WL once annually (2) WL residents (3) WL residence as juveniles followed by movement to nearshore as adults. This application represents a novel use of transect otolith microchemistry to reconstruct fish movements between freshwater environments across entire life spans at fine scales. These results suggest that regular movements of fish may facilitate the production of coastal fishes in the Great Lakes.

Beschreibung: Surface waters contribute substantially to carbon dioxide (CO 2 ) emissions to the atmosphere. However, global estimates remain uncertain due to methodological difficulties, such as in precisely estimating gas transfer in steep upland streams. Here, we addressed the question of what drives CO 2 evasion from steep mountainous stream network of the European Alps by assessing the spatial and temporal variation of partial pressure of CO 2 ( p CO 2 ) for 148 streams and the gas transfer coefficient for CO 2 ( k CO2 ) for 88 locations within this 254 km 2 watershed. Results show that log k CO2 can be predicted reasonably well ( r 2  = 0.71, p 〈0.001, n  = 88) using a statistical model based on slope, average width, flow velocity and stream discharge. Also, most sites were supersaturated in CO 2 with significant variation in p CO 2 due to season (September vs. December) and time of day (day vs. night), but not stream order. Resulting median CO 2 evasion rates were 145, 119, 46, 43, and 50 mg C m −2 h −1 at 1 st to 5 th order streams, respectively. CO 2 evasion was dependent on season and time of day, with the highest evasion (184.0 kg C h −1 ) during growing season at nighttime, followed by 124.6 kg C h −1 during daytime. Dormant season nighttime evasion was 30.9 kg C h −1 and daytime evasion only 17.1 kg C h −1 . Overall we conclude that CO 2 evasion of steep mountainous streams depends on seasonal and diurnal variation in p CO 2 and reach-specific variability in k CO2 . These controls strongly alter landscape-scale CO 2 evasion estimates, with implications for regional to global carbon budgets.

Beschreibung: Anthropogenic litter (AL; trash) in the environment is increasing and persistent. Rivers are considered a major source of AL to oceans, but AL ecology within rivers is rarely examined. Also, the rapidly developing field of AL research will benefit from fundamental approaches in community and ecosystem ecology. We adapted methods for communities of organisms and movement of organic matter to measure density, mass, assemblages, sources, and flux of AL in riparian and benthic zones at 15 sites in five rivers. We compared riverine AL density, mass, and assemblages to marine habitats worldwide. Benthic zones had greater AL mass and a different assemblage than riparian zones. Reach-scale metrics of human activity (e.g., parking spaces) explained more variation in AL assemblages than total urban land use. AL export was driven by material type and hydrology, and turnover time was ≤ 1 yr. Riparian AL density was similar to beaches, but benthic AL density was higher than marine benthic habitats. Finally, AL assemblages in river benthic and riparian zones were similar to assemblages at beaches rather than marine benthic habitats. AL is abundant and mobile in rivers, which show dynamic periods of AL retention and export. Rivers are likely sites of AL breakdown and burial, with significant biotic interactions which have not yet been studied. Comprehensive assessments of AL across ecosystems require continued adaptation of fundamental ecosystem and community ecology tools. Results will integrate riverine AL dynamics with the growing field of marine AL ecology, and inform management of global AL accumulations.

Beschreibung: As bivalve aquaculture expands worldwide, an understanding of its role in nutrient cycling is necessary to ensure ecological sustainability and determine the potential of using bivalves for nutrient mitigation. Whereas several studies, primarily of epifaunal bivalves, have assessed denitrification, few have considered nutrient regeneration processes such as dissimilatory nitrate reduction to ammonium (DNRA), which competes with denitrification for nitrate and results in nitrogen retention rather than loss. This study compares sediment nitrogen cycling including mineralization, DNRA, and denitrification within U.S. clam aquaculture sediments to nearby uncultivated sediments, seasonally. Clam aquaculture significantly increased sediment ammonium and phosphate effluxes relative to uncultivated sediments. Both DNRA and denitrification were significantly enhanced at clam beds compared to uncultivated sediments in July and November, while in May only DNRA was increased. The ratio of DNRA to denitrification was significantly higher at clam beds compared to uncultivated sediments, demonstrating that DNRA may be favored due to a ready supply of labile organic carbon relative to nitrate and perhaps sulfidic conditions. Functional gene abundances, nrfA (DNRA) and nirS (denitrification) followed similar patterns to nitrate respiration rates with highest nrfA abundances in the clam sediments and similar nirS abundances across seasons and sediment type. Ultimately clam sediments were found to be a significant source of nutrients to the water column whereas uncultivated sediments retained ammonium produced by microbial mineralization. Thus, clam cultivation may promote local eutrophication (i.e., increased primary production) by facilitating nutrient regeneration and retention of ammonium in the sediments.

Beschreibung: There is growing awareness that to improve the understanding of the biological control of silicon (Si) cycling in the oceans, the biogeochemical models need to incorporate Si users other than diatoms. In the last decades, siliceous sponges are coming into sight as important Si users, but the scarce quantitative information on how they use Si is hindering the assessment of their role. We are here investigating Si consumption kinetics in two demosponge species ( Tethya citrina and Hymeniacidon perlevis ) that have contrasting biological features while inhabiting at the same sublittoral habitat. In laboratory experiments, we have determined that both species share some common traits when incorporating Si from seawater: (1) saturable Michaelis-Menten kinetics; (2) maximum velocity of Si consumption occurring at high silicic acid (DSi) concentrations (∼150 μM) that are not available in shallow waters of the modern oceans; (3) the ability to increase consumption rates rapidly and predictably in response to increasing DSi availability; and (4) half-saturation constants that indicate an affinity for DSi lower than those of diatom systems. Across the four sponge species investigated to date, the affinity for DSi varies about 4.5 times. Our results also suggest that at least part of that between-species variability reflects the skeletonization level of the species. Within a given species, there are also between-individual differences in the DSi demand, which appear to reflect the particular physiological condition of each individual (i.e., body size, reproductive vs. non-reproductive stage).

Beschreibung: Bottom-trawl fisheries are wide-spread and have large effects on benthic ecosystems.We investigate the effect of scallop dredging on sand and otter trawling on mud by measuring changes in the infaunal community and the biogeochemical processes which they mediate. We hypothesize that changes in biogeochemistry due to fishing will be larger in mud where macrofauna-mediated processes are expected to play a greater role, than in sand where hydrodynamics mediate the redox system. We sampled benthic infauna, sediment pore-water nutrients, oxygen, chlorophyll a (Chl a ), apparent redox potential discontinuity layer, organic carbon and nitrogen content over a gradient of fishing intensity in sand and mud. The effects of fishing on biogeochemistry were stronger on mud than on sand, where biogeochemistry appeared to be more strongly influenced by tidal currents and waves. On mud, trawling increased sediment-surface Chl a and ammonium concentration beyond 5 cm depth, but decreased ammonium and silicate concentration in the upper sediment layers. The effects of fauna and bioturbation potential on biogeochemistry were very limited in both mud and sand habitats. Our results suggests that otter trawling may be affecting organic-matter remineralization and nutrient cycling through sediment resuspension and burial of organic matter to depth rather than through the loss of bioturbation potential of the benthic community. In conclusion, our hypothesis that the effects of trawling on biogeochemistry are larger in mud is supported, but the hypothesis that these effects are mediated by changes in the infauna is not supported. These results imply that management of trawling on muddy sediments should have higher priority.

Beschreibung: Marine dissolved organic matter (DOM) is a key source of carbon and nutrients to microbial life in the oceans, but rapid biological utilization of labile DOM confounds its compositional characterization. In order to characterize potentially bioavailable DOM produced by phytoplankton, DOM from axenic cultures of Thalassiosira pseudonana cultivated in phosphorus (P) replete and low P conditions was extracted using high-recovery electrodialysis (ED) techniques, which resulted in an average dissolved organic carbon (DOC) recovery of 76% ± 7% from all cultures. Low P concentrations resulted in greater cell-normalized production of DOC relative to P replete culture controls at the same growth phase. Despite the different nutrient conditions, DOC composition and DOM molar ratios of carbon to nitrogen (C : N) were similar in all cultures. In contrast, low P concentrations influenced DOM molar carbon to phosphorus (C : P) ratios and dissolved organic phosphorus (DOP) composition. Under P replete and low P conditions, DOM C : P ratios were 130 (± 22) and 2446 (± 519), respectively. 31 P Nuclear Magnetic Resonance (NMR) spectroscopy identified P esters (〉 90% of DOP) as the dominant P species in DOM produced under P replete conditions, with small or negligible contributions from phosphonates or glycerol P and polyphosphates. However, based on direct fluorometric analysis, DOP from low P cultures was greater than 8 times enriched in dissolved polyphosphate compared to DOP from replete cultures, which is consistent with the growing evidence that polyphosphate is a dynamic component of total P in low P ocean regions.

Beschreibung: The primary drivers of the recent accelerated warming of the Laurentian Great Lakes from 1982 to 2012 are explored through observations, remote sensing, and regional climate model experiments. The study focuses on the abrupt warming from 1997 to 1998 as a proxy for the long-term warming trend. The lake surface warming has been heterogeneous in both space and time, ranging from moderate warming in late spring over the southern lakes and shallow areas of the northern lakes to strong warming in mid-summer over the northern, deep lake areas. The greatest lake warming between 1997 and 1998 occurs over the deepest areas of Lake Superior during mid-summer, primarily arising from enhanced heat accumulation during the mild winter of 1997/1998 and amplified by greater incoming surface solar radiation and air temperature during the spring of 1998, according to model experiments. The mild winter condition, together with the increased solar radiation and air temperature during spring, causes an earlier onset of springtime stratification, resulting in enhanced heat absorption by surface water and thereby contributing to lake surface warming during the subsequent summer in 1998 compared with 1997. In contrast, the modest peak warming over southern lakes and shallow areas of northern lakes from 1997 to 1998 is a rapid response to synchronous increases in solar radiation and air temperature during May between the 2 yr. Changes in antecedent wintertime lake ice cover are found to have played only a minor role in the accelerated warming trend of the Laurentian Great Lakes.

Beschreibung: The distribution of biomass production and its allocation across populations under environmental constraints draw a picture of community dynamics and energy flows in ecosystems. However, microscopic benthic invertebrates (meiofauna) are often overlooked in stream production budgets. We monitored the meiofauna dwelling in the sediment at two headwater stream sites (Ems and Furlbach, NW Germany) during 1 year. The two streams were similar in their granulometries and temperature regimes, but they differed in their flow velocities and nutrient balances, which allowed investigations of the effects of these factors on the density and production of different taxonomic groups of meiofauna. Meiofaunal production in the top 10-cm sediment at Ems and Furlbach was 2.58 and 5.46 gC m −2 yr −1 , respectively; these values are among the highest reported so far for a streambed. Allocation of density and production across taxonomic groups differed between the two streams. Tardigrades, rotifers, oligochaetes, and gastrotrichs thrived in the phosphate-rich, slow-flowing waters of the Ems, whereas nematodes, micro-turbellarians, and harpacticoid copepods were better adapted to the nitrate-rich, fast-flowing waters of the Furlbach. Body-size distribution varied across site and depth and was mostly multi-modal, with important contributions of minute individuals weighing between 0.01 and 0.1 μgC. Our study shows that, despite their small size, meiofauna can produce substantial amounts of biomass and should thus be better considered in budgets and models of stream ecosystems.

Beschreibung: Nitrogen is essential for life but is often a major limiting nutrient for growth in the ocean. Biological dinitrogen fixation is a major source of new nitrogen to surface waters and promotes marine productivity. Yet the fate of diazotroph-derived nitrogen (DDN) in marine ecosystems has been poorly studied, and its transfer to auto- and heterotrophic plankton has not been measured. Here, we use high-resolution nanometer scale secondary ion mass spectrometry (nanoSIMS) coupled with 15 N 2 isotopic labelling and flow cytometry cell sorting to examine the DDN transfer to specific groups of natural phytoplankton and bacteria during three diazotroph blooms dominated by the cyanobacterium Trichodesmium spp. in the South West Pacific. During these experiments, 13% ± 2% to 48% ± 5% of the fixed 15 N 2 was released into the dissolved pool and 6% ± 1% to 8% ± 2% of this DDN was transferred to non-diazotrophic plankton after 48 h. The primary beneficiaries of this DDN were diatoms (45% ± 4% to 61% ± 38%) and bacteria (22% ± 27% to 38% ± 12%), followed by pico-phytoplankton (3% ± 1% to 21% ± 14%). The DDN was quickly converted to non-diazotrophic plankton biomass, in particular that of diatoms, which increased in abundance by a factor of 1.4–15 over the course of the three experiments. The single-cell approach we used enabled quantification of the actual transfer of DDN to specific groups of autotrophic and heterotrophic plankton in the surface ocean, revealing a previously unseen level of complexity in the pathways that occur between N 2 fixation and the eventual export of DDN from the photic zone.

Beschreibung: Eutrophication can initiate sudden ecosystem state change either by slowly pushing lakes toward a catastrophic tipping point beyond which self-reinforcing mechanisms establish an alternate stable state, or through rapid but persistent changes in external forcing mechanisms. In principle, these processes can be distinguished by determining whether historical changes in focal parameters (phytoplankton) exhibit transient (rising then declining) or continuously-elevated variability characteristic of alternate stable states or a “paradox of enrichment,” respectively. We tested this hypothesis in the south basin of Lake Winnipeg, Canada, a site with intense blooms of N 2 -fixing cyanobacteria since 1990, but for which little is known of earlier limnological conditions, causes of eutrophication, or whether modern conditions represent a alternate stable state. Paleolimnological analysis revealed that the basin was naturally mesotrophic (∼15–20 μ g P L −1 ) with diazotrophic cyanobacteria, productive diatoms, and phosphorus-rich sediments. Eutrophication accelerated during ca.1900–ca.1990, when sedimentary nitrogen, phosphorus and carbon contents increased 10–50%, δ 15 N enriched 3–4‰, and concentrations of many fossil pigments increased 300–500%. Nearly 75% of 20 th century variability was explained by concomitant increases in production of livestock and crops, but not by climate. After ca.1990, the basin exhibited a rapid threefold increase in akinetes from Aphanizomenon and Anabaena spp. and 50% declines in pigments from chlorophytes and cyanobacteria because of sudden socio-economic reorganization of agriculture. Phytoplankton variability quantified using Gaussian generalized additive models increased continuously since the onset of agriculture for bloom-forming taxa, did not decline after state change, and suggested that recovery should not be affected by stable-state hysteresis.

Beschreibung: Freshwaters have recently been recognized as important sources of methane emitted to the atmosphere, and microbial methane oxidation at the oxic/anoxic interface is a key process controlling these emissions. We applied proteomics to determine enzyme expression patterns of methanotrophs in response to methane enrichment of lake water. In a small-scale incubation experiment with natural bacterial communities we compared enzymes involved in methane metabolism between control and methane-enriched hypolimnetic water simulating high (∼1 mM) and low (∼0.001 mM) methane concentrations at oxyclines in lakes. Methane was effectively consumed when the supply was high, reducing oxygen levels from 0.40 mM (12.9 mg L −1 ) to 0.09 mM (3.0 mg L −1 ), well below those in the controls. The dominant key enzyme of microbial methane oxidation, particulate methane monooxygenase, was identified in both enriched and control flasks, whereas enzymes potentially involved in methane metabolism via the RuMP pathway and serine cycle were essentially restricted to the enriched flasks. All enzymes had best sequence matches with type I methanotrophs, whereas no indication of type II or type X methanotrophs was found, even though four enzymes of the serine cycle were identified. Overall, our proteomic analysis provides convincing evidence that a suite of genes required for methanotrophy are quickly expressed when the presence of both methane and oxygen creates conditions characteristic of oxyclines in lakes.

Beschreibung: We present results of the thermodynamics and hydrodynamics of an atoll system and their effect on coral cover based on field measurements from 2012 to 2014 on Palmyra Atoll in the central Pacific. We found that spatial variations in coral cover were correlated with temperature variations on time scales of days to weeks. Shallow terrace and backreef sites with high coral cover (〉 50%) had a highly variable temperature distributions, but their average weekly temperature distributions were lower and similar to offshore waters. The mechanism for maintaining this low weekly temperature was mean advection, which varied on a weekly timescale in response to wave forcing. Tides were also important in driving flow on the atoll, but their contribution to the net transport of heat was not significant. Wind and regional forcing were generally not important in driving flow inside the atoll. Buoyancy-driven flows were important within the lagoons, and in driving cross-shore exchange on forereef environments. The physical factors favoring high coral cover percentage varied according to the different prevailing hydrodynamic regimes: low temperatures in backreef habitats, short travel times in lagoon habitats (days since entering the reef system), and lower wave stress on forereef habitats. In light of future warming from climate change, local areas of reefs which maintain lower temperatures through wave-driven mean flows will have the best likelihood of promoting coral survival.

Beschreibung: Seagrasses need dissolved nutrients to maintain their productivity through uptake processes, from substrate pore-water via their roots and/or from the water column via their leaves. Here, we present the first study of exchanges of dissolved nutrients between pore water and the water column in the vicinity of seagrass canopies. We address the following research questions, using a laboratory flume experiment: (1) How does solute exchange between the water column and substrate pore water vary spatially within seagrass patches? (2) How does seagrass leaf length affect this solute exchange? (3) How does the measured rate of solute exchange compare with seagrasses' rates of uptake of dissolved nutrients? Our results indicate that solute intrusion from the water column into the substrate pore water is highest in the area around seagrass patches' leading edges, where flow deceleration is strongest, and decreases approximately linearly with downstream distance into the patch. The decrease in measured flow speed in the canopy fits well the predictions of previously reported models of arrays of rigid obstacles. The length of the region in which the concentration of solute that has infiltrated into the substrate at the upstream end of the seagrass patches is similar to the length scale predicted from model estimates of infiltration rate (based on the substrate permeability) and the length of time over 24-h runs. We conclude that the mechanism we identify only pertains near canopy edges, and therefore that other mechanisms must govern nutrient supply in the interior of seagrass meadows.

Beschreibung: Impacts of invasive species on ecosystems are often context dependent, making empirical assessments difficult when climatic baselines are shifting and extreme events are becoming more common. We documented a mass mortality event of the Asian clam, Corbicula fluminea , an abundant invasive clam, which has replaced native mussels as the dominant filter-feeding bivalve in the southeastern United States. During an extremely hot and dry period in the summer of 2012, over 99% of Corbicula died in our 10-km study reach of the Broad River, Georgia. Because Corbicula were the only filter-feeding organism in the ecosystem with substantial biomass, their death led to the nearly complete cessation of ecosystem services provided by filter-feeding bivalves. We estimate that following the mass mortality event, turnover time within the sampling reach (reach volume/total filtration) rose from approximately 5 h to over 1200 h. In addition to the loss of filtering capacity, concentrations of total dissolved phosphorus (TDP) and soluble reactive phosphorus (SRP) were also higher in areas where die-off was occurring than in an upstream area without mortality. Mass balance calculations and a manipulative mesocosm experiment predicted TDP and SRP concentrations much higher than our observed values, suggesting that rapid biotic or abiotic uptake of phosphorus may have occurred. Our study demonstrates that climate change can increase the temporal variability of populations of aquatic organisms that provide key ecosystem functions, and highlights that even pulsed, short-lived events can markedly affect systems of reduced diversity.

Beschreibung: Unraveling the potentially shifting controls over microbial activity among habitats and across seasonal transitions is critical for understanding how freshwater ecosystems influence broader elemental cycles, and how these systems may respond to global changes. We used nutrient-diffusing substrates to investigate seasonal patterns and constraints on microbial activity of biofilms in streams draining distinct landscape features of the boreal biome (forests, mires, and lakes). Microbial respiration (MR) largely mirrored spatial and temporal variation in water temperature. However, limitation by labile carbon (C) was a constraint to microbial activity during ice-covered periods, when MR of control nutrient-diffusing substrates fell below rates predicted from stream temperature alone. Variation in C limitation among the study streams was reflective of putative organic C availability, with C limitation of biofilms weakest in the dissolved organic C (DOC)-rich, mire-outlet stream and greatest in the relatively DOC-poor, forest stream. Incidences of nutrient limitation were only observed during warmer months. Our study illustrates how variation in processes mediated by heterotrophic biofilms and seasonal shifts in resource limitation can emerge in a stream network draining a heterogeneous landscape. In addition, our results show that, for a large portion of the year, heterotrophic processes in boreal streams can be strongly limited by the availability of labile C, despite high DOC concentrations. Metabolic constraints to dissolved organic matter processing at near-freezing temperatures, coupled with hydrological controls over the delivery of more labile organic resources to streams (e.g., soil freezing and flooding), have potentially strong influences on the productivity of boreal streams.

Beschreibung: Increasing anthropogenic carbon dioxide is altering marine carbonate chemistry through a process called ocean acidification. Many calcium carbonate forming organisms are sensitive to changes in marine carbonate chemistry, especially mollusk bivalve larvae at the initial shell building stage. Rapid calcification, limited energy reserves, and more exposed calcification surfaces, are traits at this stage that increase vulnerability to ocean acidification through our previously argued kinetic-energetic hypothesis. These developmental traits are common to broadcast spawning bivalve species that are the focus of most ocean acidification studies to date. Some oyster species brood their young, which results in slower development of the embryos through the initial shell formation stage. We examined the responses of the brooding Olympia oyster, Ostrea lurida , during their initial shell building stage. We extracted fertilized eggs from, O. lurida , prior to shell development, then exposed developing embryos to a wide range of marine carbonate chemistry conditions. Surprisingly, O. lurida showed no acute negative response to any ocean acidification treatments. Compared to the broadcast spawning Pacific oyster, Crassostrea gigas , calcification rate and standardized endogenous energy lipid consumption rate were nearly 10 and 50 times slower, respectively. Our results suggest that slow shell building may lessen the energetic burden of acidification at this stage and provides additional support for our kinetic-energetic hypothesis. Furthermore, these results may represent an example of exaptation; fitness conveyed by a coopted trait that evolved for another purpose, a concept largely lacking in the current perspective of adaptation and global climate change.

Beschreibung: The occurrence of alkaline phosphatase activity (APA) that hydrolyses organic phosphorus into phosphate (PO 4 ) is commonly related to PO 4 deficiency of oceanic, coastal and fresh waters. APA is almost never investigated in PO 4 -rich estuaries, since very low activities are expected to occur. As a consequence, microbial mineralization of organic phosphorus into PO 4 has often been ignored in estuaries. In this study, we examined the importance of potential APA and the associated microbial dynamics in two estuaries, the Aulne and the Elorn (Northwestern France), presenting two different levels of PO 4 concentrations. Unexpected high potential APA was observed in both estuaries. Values ranged from 50 to 506 nmol L −1 h −1 , which range is usually found in very phosphorus-limited environments. High potential APA values were observed in the oligohaline zone (salinity 5–15) in spring and summer, corresponding to a PO 4 peak and a maximum bacterial production of particle-attached bacteria. In all cases, high potential APA was associated with high suspended particulate matter and total particulate phosphorus. The low contribution of the 0.2–1 μm fraction to total APA, the strong correlation between particulate APA and bacterial biomass, and the close relationship between the production of particle-attached bacteria and APA, suggested that high potential APA is mainly due to particle-attached bacteria. These results suggest that the microbial mineralization of organic phosphorus may contribute to an estuarine PO 4 production in spring and summer besides physicochemical processes.

Beschreibung: We develop an optical classification of marine particle assemblages from an extensive dataset of particle optical properties collected in the Chukchi and Beaufort Seas. Hierarchical cluster analysis of the spectral particulate backscattering-to-absorption ratio partitioned the dataset into seven optically-distinct clusters of particle assemblages, each associated with different characteristics of particle concentration, composition, and phytoplankton taxonomic composition and size. Three phytoplankton-dominated clusters were identified. One was characterized by small-sized phytoplankton that are typically associated with regenerated production, and comprised samples from the subsurface chlorophyll- a maximum in oligotrophic waters of the Beaufort Sea. The other two clusters represented diatom-dominated particle assemblages in turbid shelf waters with differing contributions of photoprotective pigments. Such situations are generally associated with significant new production. Two clusters were dominated by organic nonalgal material, one representing clear waters off the shelf, the other representative of post-diatom bloom conditions in the Chukchi Sea. Another distinct cluster represented mineral-dominated particle assemblages that were observed in the Colville and Mackenzie River plumes and near the seafloor. Finally, samples in a cluster of mixed particle composition were scattered throughout all locations. Optical classification improved performance of predictive bio-optical relationships. These results demonstrate a capability to discriminate distinct assemblages of suspended particles associated with specific ecological conditions from hyperspectral measurements of optical properties, and the potential for identification of ecological provinces at synoptic time and space scales from optical sensors. Analogous analysis of multispectral optical data strongly reduced this capability.

Beschreibung: We present nearly 9 yrs (June 2005–December 2013) of measurements of upper-ocean (0 m to 125 m) dinitrogen (N 2 ) fixation rates, coupled with particulate nitrogen (PN) export at 150 m, from Station ALOHA (22° 45′N, 158°W) in the North Pacific Subtropical Gyre. Between June 2005 and June 2012, N 2 fixation rates were measured based on adding the 15 N 2 tracer as a gas bubble. Beginning in August 2012, 15 N 2 was first dissolved into filtered seawater and the 15 N 2 -enriched water was subsequently added to N 2 fixation incubations. Direct comparisons between methodologies revealed a robust relationship, with the addition of 15 N 2 -enriched seawater resulting in twofold greater depth-integrated rates than those derived from adding a 15 N 2 gas bubble. Based on this relationship, we corrected the initial period of measurements, and the resulting rates of N 2 fixation averaged 230 ± 136 μmol N m −2 d −1 for the full time series ( n  = 71). Analysis of the 15 N isotopic composition of sinking PN, together with an isotope mass balance model, revealed that N 2 fixation supported 26–47% of PN export during calendar years 2006–2013. The N export derived from these fractional contributions and measured N 2 fixation rates ranged between 502 and 919 μmol N m −2  d −1 , which are equivalent to rates of net community production (NCP) of 1.5 to 2.7 mol C m −2  yr −1 , consistent with previous independent estimates of NCP at this site.

Beschreibung: Benthic suspension feeders such as dreissenid mussels ( Dreissena polymorpha and D . rostriformis bugensis ) are often found in remarkably dense aggregations (i.e. 〉 10 5 mussels m − 2 ), which is surprising, given their high clearance rates and limited mixing within the benthic boundary layer. Results from flow visualization in flow chamber experiments indicate that there is indeed limited mixing around mussel aggregations at low flows and that siphonal jets can increase mixing around and above these aggregations. Using particle image velocimetry (PIV) to further investigate the underlying hydrodynamics of these jets, we characterized differences in velocity and vorticity among four siphonal behaviors (e.g., slow flux, streaming, exhalant jets, and inhalant jets), including both continuous and pulsatile jets, the latter of which generate free vortex rings. Incorporating these hydrodynamic characteristics into a computational fluid dynamic (CFD) model revealed that siphonal jets increased mixing, expressed as vertical diffusivity in the benthic boundary layer. These differences were most pronounced at slow vs. fast cross-stream velocities, but those differences diminished several body lengths (i.e. 10 −1 m to 10 −2 m) downstream. The results from PIV measurements and CFD modeling suggest that benthic suspension feeders can influence patterns of local mixing, which would affect mass transport and biogeochemical processes in the near-bed region. This underscores the need for physical-biologically linked models to incorporate the behavior of benthic suspension feeding invertebrates.

Beschreibung: Lakes are a nitrous oxide (N 2 O) source to the atmosphere, but the biogeochemical controls and microbial pathways of N 2 O production are not well understood. To trace microbial N 2 O production (denitrification, nitrifier denitrification, and nitrification) and consumption (denitrification) in two basins of Lake Lugano, we measured the concentrations and N and O isotope compositions of N 2 O, as well as the intramolecular 15 N distribution, i.e., site preference (SP). Our results revealed differential N 2 O dynamics in the two lake basins, with N 2 O concentrations between 12 nmol L −1 and 〉 900 nmol L −1 in the holomictic South Basin, and significantly lower concentrations in the meromictic North Basin (〈13 nmol L −1 ). In the South Basin, the isotope signatures reflected a complex combination of N 2 O production by nitrifying bacteria through hydroxylamine (NH 2 OH) oxidation, N 2 O production through incomplete denitrification, and N 2 O reduction to N 2 , all occurring in close vicinity within the redox transition zone (RTZ). In the North Basin, in contrast, the N 2 O isotopomer signatures suggested that nitrifier denitrification was the main N 2 O source. The pronounced decrease in N 2 O concentrations to undetectable levels within the RTZ, in tandem with an increase in δ 15 N-N 2 O, δ 18 O-N 2 O, and SP indicated quantitative N 2 O consumption by microbial denitrification. In the northern basin this was primarily sulfide-dependent. The apparent N and O isotope enrichment factors associated with net N 2 O consumption were 15 ε ≈ 3.2‰ and 18 ε ≈ 8.6‰, respectively. The according 18 O to 15 N enrichment ratio ( 18 ε : 15 ε ≈ 2.5) is consistent with previous reports for microbial N 2 O reduction, underscoring its robust nature across environments.

Beschreibung: Invertebrate herbivores are often faced with food of different quality, affecting growth, and reproductive rates. Temperature also has a strong influence on these rates, but how temperature and food quality interact is largely unknown. We investigated the interaction between temperature and food quality on the developmental rates of a keystone plankton herbivore, the calanoid copepod Acartia tonsa . We fed the copepod along a gradient of five food qualities from phosphorus-limited to phosphorus-replete algae, exposing them to eight different temperatures from 8°C to 22°C in a full factorial design. We observed that temperature and food quality as well as their interaction significantly affected copepod growth. Food quality effects were strongest at low temperatures and decreased with increasing temperatures. Our results suggest that ectotherms need more carbon relative to phosphorus at higher temperatures to meet their metabolic demands. Thus, the threshold elemental ratio for growth should be higher at higher temperatures and a higher probability of carbon limitation for secondary production when it is warmer.

Beschreibung: We investigated the role of lake sediments as carbon (C) source and sink in the annual C budget of a small (0.07 km 2 ) and shallow (mean depth, 3.4 m), humic lake in boreal Sweden. Organic carbon (OC) burial and mineralization in the sediments were quantified from 210 Pb-dated sediment and laboratory sediment incubation experiments, respectively. Burial and mineralization rates were then upscaled to the entire basin and to one whole year using sediment thickness derived from sub-bottom profiling, basin morphometry, and water column monitoring data of temperature and oxygen concentration. Furthermore, catchment C import, open water metabolism, photochemical mineralization as well as carbon dioxide (CO 2 ) and methane (CH 4 ) emissions to the atmosphere were quantified to relate sediment processes to other lake C fluxes. We found that on a whole-basin and annual scale, sediment OC mineralization was three times larger than OC burial, and contributed about 16% to the annual CO 2 emission. Other contributions to CO 2 emission were water column metabolism (31%), photochemical mineralization (6%), and catchment imports via inlet streams and inflow of shallow groundwater (22%). The remainder (25%) could not be explained by our flux calculations, but was most likely attributed to an underestimation in groundwater inflow. We conclude that on an annual and whole-basin scale (1) sediment OC mineralization dominated over OC burial, (2) water column OC mineralization contributed more to lake CO 2 emission than sediment OC mineralization, and (3) catchment import of C to the lake was greater than lake-internal C cycling.

Beschreibung: Global estimates of methane (CH 4 ) emissions from reservoirs are poorly constrained, partly due to the challenges of accounting for intra-reservoir spatial variability. Reservoir-scale emission rates are often estimated by extrapolating from measurement made at a few locations; however, error and bias associated with this approach can be large and difficult to quantify. Here, we use a generalized random tessellation survey (GRTS) design to generate unbiased estimates of reservoir-CH 4 emissions rates (±95% CI) for areas below tributary inflows, open-waters, and at the whole-reservoir scale. Total CH 4 emission rates (i.e., sum of ebullition and diffusive emissions) were 4.8 (±2.1), 33.0 (±10.7), and 8.3 (±2.2) mg CH 4 m −2 h −1 in open-waters, tributary-associated areas, and the whole-reservoir for the period in August 2014 during which 115 sites were sampled across an 7.98 km 2 reservoir in Southwestern, Ohio, U.S.A. Tributary areas occupy 12% of the reservoir surface, but were the source of 41% of total CH 4 emissions, highlighting the importance of riverine-lacustrine transition zones. Ebullition accounted for 〉 90% of CH 4 emission at all spatial scales. Overall, CH 4 emission rates were high for a temperate zone reservoir, possibly because earlier studies underestimated ebullition or did not include emission hot spots. Confidence interval estimates that incorporated spatial pattern in CH 4 emissions were up to 29% narrower than when spatial independence is assumed among sites. The use of GRTS, or other probabilistic survey designs, can improve the accuracy and precision of reservoir emission rate estimates, which is needed to better constrain uncertainty in global scale emission estimates.

Beschreibung: American eel (Anguilla rostrata) complete their life cycle by migrating from the east coast of North America to Sargasso Sea, where they spawn planktonic eggs and dye. Larvae that develop from eggs need to return to North American coastal waters within the first year of life and are influenced by the oceanic currents during this journey. A coupled physical–biological model is used to investigate the extent to which inter-annual changes in the ocean circulation affect the success rates of larvae in reaching coastal nursery habitats. Our results suggest that natural oceanic variability can lead to changes in larval success rates by a factor of 2. Interannual variation in success rates are strongly affected by the Gulf Stream inertial overshoot events, with the largest success in years with an inertial overshoot and the smallest in years with a straighter and more southern configuration of the Gulf Stream downstream of Cape Hatteras. The mean Gulf Stream length and latitude between 75W and 70W longitude can be used as proxies for characterizing the overshoot events and can be converted into success rates using linear regression.

Beschreibung: Methane (CH 4 ) emissions from aquatic systems should be coupled to CH 4 production, and thus a temperature-dependent process, yet recent evidence suggests that modeling CH 4 emissions may be more complex due to the biotic and abiotic processes influencing emissions. We studied the magnitude and regulation of two CH 4 pathways—ebullition and diffusion—from 10 shallow ponds and 3 lakes in Québec. Ebullitive fluxes in ponds averaged 4.6 ± 4.1 mmol CH 4 m −2 d −1 , contributing ∼56% to total (diffusive + ebullitive) CH 4 emissions. In lakes, ebullition only occurred in waters 〈 3 m deep, averaging 1.1 ± 1.5 mmol CH 4 m −2 d −1 , and when integrated over the whole lake, contributed only 18% to 22% to total CH 4 emissions. While pond CH 4 fluxes were related to sediment temperature, with ebullition having a stronger dependence than diffusion (Q 10 , 13 vs. 10; activation energies, 168 kJ mol −1 vs. 151 kJ mol −1 ), the temperature dependency of CH 4 fluxes from lakes was absent. Combining data from ponds and lakes shows that the temperature dependency of CH 4 diffusion and ebullition is strongly modulated by system trophic status (as total phosphorus), suggesting that organic substrate limitation dampens the influence of temperature on CH 4 fluxes from oligotrophic systems. Furthermore, a strong phosphorus-temperature interaction determines the dominant emission pathway, with ebullition disproportionately enhanced. Our results suggest that aquatic CH 4 ebullition is regulated by the interaction between ecosystem productivity and climate, and will constitute an increasingly important component of carbon emissions from northern aquatic systems under climate and environmental change.

Beschreibung: We characterized the annual luminescent and skeletal density banding patterns in 51 massive Porites corals from 15 reefs from six locations around the Thai-Malay Peninsula in Southeast Asia, and explored the seasonal environmental cues/drivers of band formation. Location-specific recurrent annual luminescent banding patterns were found at all study locations with a brighter band occurring toward the end of the year (∼October/November/December) (at five locations) and in ∼June (one location). Annual density banding patterns could only be discerned at four locations, and were categorized into those that formed a dense band commencing ∼November/December, and those starting ∼May/June. Overall, compared to luminescence, variations in skeletal density provided a less clear signal for demarcation of annual growth increments. Seasonal variations in luminescence showed clearest relationships with salinity, as a proxy for freshwater/river runoff. No convincing relationship between intra-annual luminescence intensity and density variations was found, which supports the notion that luminescent banding is due to inclusions of fluorophores into the coral skeleton rather than variations in skeletal architecture. The relationships between seasonal density variations and significant wave height and rainfall suggest density banding in this region is likely related to wave energy, or some other correlated environmental parameter/s. The large variability in skeletal banding patterns not only highlights the current relatively poor understanding of their nature and causes, but also the need for replication in their interpretation, especially in settings with complex seasonal hydrodynamic/hydrological patterns such as those found around the Thai-Malay Peninsula.

Beschreibung: The deep ocean benthic environment plays a role in long-term carbon sequestration. Understanding carbon cycling in the deep ocean floor is critical to evaluate the impact of changing climate on the oceanic systems. Linear inverse modeling was used to quantify carbon transfer between compartments in the benthic food web at a long time-series study site in the abyssal northeastern Pacific (Station M). Linear inverse food web models were constructed for three separate years in the time-series when particulate organic carbon (POC) flux was relatively high (1990: 0.63 mean mmol C m −2 d −1 ), intermediate (1995: 0.24) and low (1996: 0.12). Carbon cycling in all years was dominated by the flows involved in the microbial loop; dissolved organic carbon uptake by microbes (0.80–0.95 mean mmol C m −2 d −1 ), microbial respiration (0.52–0.61), microbial biomass dissolution (0.09–0.18) and the dissolution of refractory detritus (0.46–0.65). Moreover, the magnitude of carbon flows involved in the microbial loop changed in relation to POC input, with a decline in contribution during the high POC influxes, such as those recently experienced at Station M. Results indicate that during high POC episodic pulses the role of faunal mediated carbon cycling would increase. Semi-labile detritus dominates benthic faunal diets and the role of labile detritus declined with increased total POC input. Linear inverse modeling represents an effective framework to analyze high-resolution time-series data and demonstrate the impact of climate change on the deep ocean carbon cycle in a coastal upwelling system.

Beschreibung: To better predict ecological consequences of changing Arctic sea ice environments, we aimed to quantify the contribution of ice algae-produced carbon ( α Ice ) to pelagic food webs in the central Arctic Ocean. Eight abundant under-ice fauna species were submitted to fatty acid (FA) analysis, bulk stable isotope analysis (BSIA) of nitrogen ( δ 15 N) and carbon ( δ 13 C) isotopic ratios, and compound-specific stable isotope analysis (CSIA) of δ 13 C in trophic marker FAs. A high mean contribution α Ice was found in Apherusa glacialis and other sympagic (ice-associated) amphipods (BSIA: 87% to 91%, CSIA: 58% to 92%). The pelagic copepods Calanus glacialis and C. hyperboreus , and the pelagic amphipod Themisto libellula showed substantial, but varying α Ice values (BSIA: 39% to 55%, CSIA: 23% to 48%). Lowest α Ice mean values were found in the pteropod Clione limacina (BSIA: 30%, CSIA: 14% to 18%). Intra-specific differences in FA compositions related to two different environmental regimes were more pronounced in pelagic than in sympagic species. A comparison of mixing models using different isotopic approaches indicated that a model using δ 13 C signatures from both diatom-specific and dinoflagellate-specific marker FAs provided the most conservative estimate of α Ice . Our results imply that ecological key species of the central Arctic Ocean thrive significantly on carbon synthesized by ice algae. Due to the close connectivity between sea ice and the pelagic food web, changes in sea ice coverage and ice algal production will likely have important consequences for food web functioning and carbon dynamics of the pelagic system.

Beschreibung: Reactive oxygen species (ROS) are key players in the health and biogeochemistry of the ocean and its inhabitants. The vital contribution of microorganisms to marine ROS levels, particularly superoxide, has only recently come to light, and thus the specific biological sources and pathways involved in ROS production are largely unknown. To better understand the biogenic controls on ROS levels in tropical oligotrophic systems, we determined rates of superoxide production under various conditions by natural populations of the nitrogen-fixing diazotroph Trichodesmium obtained from various surface waters in the Sargasso Sea. Trichodesmium colonies collected from eight different stations all produced extracellular superoxide at high rates in both the dark and light. Colony density and light had a variable impact on extracellular superoxide production depending on the morphology of the Trichodesmium colonies. Raft morphotypes showed a rapid increase in superoxide production in response to even low levels of light, which was not observed for puff colonies. In contrast, superoxide production rates per colony decreased with increasing colony density for puff morphotypes but not for rafts. These findings point to Trichodesmium as a likely key source of ROS to the surface oligotrophic ocean. The physiological and/or ecological factors underpinning morphology-dependent controls on superoxide production need to be unveiled to better understand and predict superoxide production by Trichodesmium and ROS dynamics within marine systems.

Beschreibung: A feedback between seagrass presence, suspended sediment and benthic light can induce bistability between two ecosystem states: one where the presence of seagrass reduces suspended sediment concentrations to increase benthic light availability thereby favoring growth, and another where seagrass absence increases turbidity thereby reducing growth. This literature review identifies (1) how the environmental and seagrass meadow characteristics influence the strength and direction (stabilizing or destabilizing) of the seagrass-sediment-light feedback, and (2) how this feedback has been incorporated in ecosystem models proposed to support environmental decision making. Large, dense seagrass meadows in shallow subtidal, non-eutrophic systems, growing in sediments of mixed grain size and subject to higher velocity flows, have the greatest potential to generate bistability via the seagrass-sediment-light feedback. Conversely, seagrass meadows of low density, area and height can enhance turbulent flows that interact with the seabed, causing water clarity to decline. Using a published field experiment as a case study, we show that the seagrass-sediment-light feedback can induce bistability only if the suspended sediment has sufficient light attenuation properties. The seagrass-sediment-light feedback has been considered in very few ecosystem models. These models have the potential to identify areas where bistability occurs, which is information that can assist in spatial prioritization of conservation and restoration efforts. In areas where seagrass is present and bistability is predicted, recovery may be difficult once this seagrass is lost. Conversely, bare areas where seagrass presence is predicted (without bistability) may be better targets for seagrass restoration than bare areas where bistability is predicted.

Beschreibung: Lake Fryxell, McMurdo Dry Valleys, Antarctica contains a constantly cold water column and perennial ice-cover. Although carbon and sulfur cycling in this amictic lake have been studied previously, a paired investigation of 16S rRNA gene based microbial diversity and geochemistry of Lake Fryxell is lacking. Here, we used a combination of radiotracer-based rate measurements, geochemical measurements, and molecular microbial community analysis to investigate the anaerobic oxidation of methane (AOM) and associated processes in Lake Fryxell. The results show that while AOM and sulfate reduction appear coupled in the upper regions of the anoxic water column, in deep anoxic waters, where AOM rates are highest, sulfate is unlikely to be the electron acceptor for AOM. Despite significant rates of AOM in these waters, no putative AOM-associated Archaea or Bacteria were observed. Due to a lack of documented AOM electron acceptors and putative ANMEs, we suggest novel modes of AOM dominate in this extreme environment. First, the notable abundance of the bacterial genus Dehalococcoides suggests that reductive dehalogenation could fuel AOM. Further, taxa of the candidate phylum OP9, the Atribacteria, and the Bathyarchaeota (formerly known as the Miscellaneous Crenarchaeotal Group) both commonly observed at cold methane-seeps globally, may mediate AOM, possibly using humic acids as electron shuttles, in Lake Fryxell.

Beschreibung: We present a comparison of the dissolved stable isotope composition of silicate (δ 30 Si(OH) 4 ) and nitrate (δ 15 ) 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 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 δ 30 Si(OH) 4 values (+2‰) due to low Si utilization but relatively high δ 15 (+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 δ 30 Si(OH) 4 values (+3.7‰) accompanied by high δ 15 values (+16‰). In the eddy center, high : 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 δ 30 Si(OH) 4 and δ 15 can improve our understanding of paleo records as it can help to disentangle utilization and N-loss processes.

Beschreibung: Sediment porewater was analyzed at several sampling dates in two adjacent basins of an oligotrophic boreal lake, one basin perennially oxygenated (Basin A) and the other occasionally anoxic (Basin B). Depth concentration profiles of methane (CH 4 ), dissolved inorganic carbon (DIC), and electron acceptors were modeled with a one-dimensional transport-reaction equation to constrain the depth intervals (zones) where solutes are produced/consumed in the top 10 cm of the sediment column, and to obtain the net reaction rates in each zone. This multicomponent geochemical modeling reveals that CH 4 was produced below 4–7 cm depth at lower rates in Basin A (250–800 fmol cm −2 s −1 ) than in Basin B (1900–6500 fmol cm −2 s −1 ) and that methanogenesis accounted for 30–64% and 84–100% of the sediment organic matter (OM) mineralization in Basins A and B, respectively. We show that methanogenesis did not always yield equimolar amount of CH 4 and DIC, as would be expected from the fermentation of the model molecule CH 2 O. While ∼50% of the CH 4 produced in Basin A is oxidized in the sediment column, this proportion decreases to ∼20% in Basin B. Dioxygen is by far the main electron acceptor for CH 4 and OM oxidations in both basins. Methanotrophy in the sediment, however, is not limited to the ∼4-mm thick surface layer in which O 2 diffuses from bottom water but occurs down to 4–7 cm depth where O 2 is transported through bioirrigation. Thermodynamic calculations suggest that, in addition to O 2 , Fe oxyhydroxides, and sulfate may serve as oxidants for methanotrophy in that zone. We predict that Basin B sediments release more CH 4 than DIC whereas Basin A sediments mainly export DIC. This study highlights that small changes in hypolimnetic O 2 levels may significantly alter the magnitude of OM mineralization pathways and the fate of CH 4 in boreal lake sediments.

Beschreibung: Seagrass meadows are important marine carbon sinks, yet they are threatened and declining worldwide. Seagrass management and conservation requires adequate understanding of the physical and biological factors determining carbon content in seagrass sediments. Here, we identified key factors that influence carbon content in seagrass meadows across several environmental gradients in Moreton Bay, SE Queensland. Sampling was conducted in two regions: (1) Canopy Complexity , 98 sites on the Eastern Banks, where seagrass canopy structure and species composition varied while turbidity was consistently low; and (2) Turbidity Gradient , 11 locations across the entire bay, where turbidity varied among sampling locations. Sediment organic carbon content and seagrass structural complexity (shoot density, leaf area, and species specific characteristics) were measured from shallow sediment and seagrass biomass cores at each location, respectively. Environmental data were obtained from empirical measurements (water quality) and models (wave height). The key factors influencing carbon content in seagrass sediments were seagrass structural complexity, turbidity, water depth, and wave height. In the Canopy Complexity region, carbon content was higher for shallower sites and those with higher seagrass structural complexity. When turbidity varied along the Turbidity Gradient , carbon content was higher at sites with high turbidity. In both regions carbon content was consistently higher in sheltered areas with lower wave height. Seagrass canopy structure, water depth, turbidity, and hydrodynamic setting of seagrass meadows should therefore be considered in conservation and management strategies that aim to maximize sediment carbon content.

Beschreibung: Altitudinal and latitudinal gradients are excellent venues for investigating the direct and indirect effects of air temperature, solar irradiance, and insularity on spatial patterns of aquatic biodiversity. The findings can be used to predict how lake communities will respond to increasingly extreme climate events. We explored hypotheses of energy/climate, geography, and glacial history explaining patterns in species richness in a historical dataset of crustacean zooplankton communities from 436 lakes in the Canadian Rocky Mountains. GIS-based estimates of solar and thermal energy inputs combined with habitat area and insularity provided the best prediction of local species richness. Energetic and geographic factors explained a moderate proportion of the total variation in species richness (Generalized R 2  = 0.50), and were sufficient to account for both altitudinal and latitudinal gradients in zooplankton diversity. History of deglaciation was not supported as a predictor of patterns in species richness. A post hoc analysis with a smaller dataset also found strong support for lake pH, and some support for fish presence as predictors of species richness, but these only increased the proportion of the total variation explained very slightly relative to the model including only energetic and geographic factors (Generalized R 2  = 0.55 vs. 0.53). Our findings highlight the multiplicity of local and regional factors of zooplankton species richness in mountain lakes, forecasting that it will increase under a scenario of warmer and drier (i.e., less cloud cover) conditions, especially in high connectivity lakes that cease to be fed by rapidly disappearing glaciers.

Beschreibung: Ongoing changes in sea ice distribution will have major implications for the ecology of the Arctic Ocean. First year ice (FYI) supports abundant ice-algae communities that produce dissolved and particulate carbohydrates, including extracellular polymeric substances (EPS), which are significant carbon sources, influence ice formation and microbial survival within sea ice, and water column carbon cycling following ice melt. Key drivers of the distribution and composition of these carbohydrates are poorly characterised. Carbohydrates and chlorophyll a concentrations were linearly related in springtime bottom FYI at 36 sites in the Canadian Archipelago region, with high levels of spatial heterogeneity. Nanoeukaryote cell density and phosphate concentration were strong drivers of total and dissolved carbohydrate and uronic acid concentrations. Particulate carbohydrates were strongly related to total bacterial abundance. Dissolved carbohydrates contributed 77% of total carbohydrate: the most abundant (51%) size fraction being dissolved carbohydrates 〈 8 kDa in size, with dissolved EPS contributing 7% to total carbohydrate. Carbohydrate fractions differed in monosaccharide profiles; dissolved components being glucose rich; particulate EPS containing more mannose, xylose, fucose and arabinose. These profiles corresponded to those of cultured sea-ice diatoms. Microbial abundance, silicate, nitrate and phosphate concentration and ice thickness were important environmental drivers, with thicker ice containing relatively more particulate EPS, with thinner ice containing high amounts of glucose-rich smaller-sized carbohydrate moieties. Changes in ice characteristics will alter the relative balance of labile and refractory carbohydrates generated within bottom ice layers, with implications for food webs and carbon turnover in the warming Arctic Ocean.

Beschreibung: Largely due to size differences, mesozooplankton are important exporters of carbon and prey for larger organisms, while microzooplankton are important recyclers of nutrients, dominant grazers of phytoplankton, and a key link in the microbial loop. We investigated the relative importance of meso- and microzooplankton grazing in the western tropical North Atlantic Ocean (WTNA) and Amazon River plume. Sampling as part of the ANACONDAS project occurred in spring (May–June) 2010 during the peak outflow of the Amazon River and in fall (September–October) 2011 during the plume seasonal retroflection. Mesozooplankton grazing rates decreased with increasing salinity in both seasons, but during the fall both day and nighttime grazing rates were significantly negatively correlated with salinity. Mesozooplankton grazing was highest in plume-influenced surface waters (0–25 m), and usually dominated by smaller size classes (0.2–0.5 mm and 0.5–1.0 mm). Microzooplankton grazing accounted for approximately 68% of bulk phytoplankton growth across all stations. Comparison of meso- and microzooplankton grazing suggests a transition in food web dynamics from a mesozooplankton dominated “export” structure in the plume transitioning to a microzooplankton dominated “retention” structure at mesohaline and oceanic stations above sea surface salinity of 33. Comparison between the seasons suggests a seasonal planktonic succession of low mesozooplankton grazing during the spring peak discharge followed by higher grazing rates and impact by mesozooplankton during the fall retroflection. These results provide important baseline information required for examining effects of climate change on the planktonic food web of the WTNA and for use in biogeochemical models of the region.

Beschreibung: Increasing seawater temperature and CO 2 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 CO 2 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 CO 2 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 CO 2 concentrations significantly increased phytoplankton biomass. The latter effect however, was due to direct negative impact of CO 2 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 CO 2 effects on coastal phytoplankton biomass.

Beschreibung: We investigated microbial pathways of nitrogen transformation in highly permeable sediments from the German Bight (South-East North Sea) by incubating sediment cores percolated with 15 N-labeled substrates under near in situ conditions. In incubations with added , production of occurred while the sediment was oxic, indicating ammonia oxidation. Similarly, production during incubations indicated nitrite oxidation. Taken together these findings provide direct evidence of high nitrification rates within German Bight sands. The production of 15 N-N 2 on addition of revealed high denitrification rates within the sediment under oxic and anoxic conditions. Denitrification rates were strongly and positively correlated with oxygen consumption rates, suggesting that denitrification is controlled by organic matter availability. Nitrification and denitrification rates were of the same magnitude and the rapid production of 15 N-N 2 in incubations with added confirmed close coupling of the two processes. Areal rates of N-transformation were estimated taking advective transport of substrates into account and integrating volumetric rates over modeled oxygen and nitrate penetration depths, these ranged between 22 μmol N m −2 h −1 and 94 μmol N m −2 h −1 . Furthermore, results from the 15 N-labeling experiments show that these subtidal permeable sediments are, in sharp contrast to common belief, a substantial source of N 2 O. Our combined results show that nitrification fuels denitrification by providing an additional source of nitrate, and as such masks true N-losses from these highly eutrophic sediments. Given the widespread occurrence of anthropogenically influenced permeable sediments, coupled benthic nitrification–denitrification might have an important but so far neglected role in N-loss from shelf sediments.

Beschreibung: The Chesapeake Bay is a eutrophic estuary that undergoes seasonal bottom water hypoxia. Methane (CH 4 ) gas bubbles are found within the upper sediment layers, but whether this greenhouse gas escapes to the water column or the atmosphere is not well known. Here, we hypothesize that when bottom waters become anoxic, CH 4 is released from the sediments, builds up under the pycnocline, and escapes to the atmosphere at the end of hypoxia. Osmotically powered pumps (OsmoSamplers) were used for the first time to test this hypothesis. They were deployed from April to October 2013 and continuously collected bottom water into small diameter, copper tubing. Upon recovery, the tubing was cut into 4 d increments, and the enclosed water extracted and measured for CH 4 concentrations. Results showed that in April, CH 4 concentrations were low (∼1 μ M) when bottom waters were fully oxygenated and increased as anoxic conditions set in. By mid-July, CH 4 concentrations peaked and reached as high as 40 μ M, most likely coming from the sediments. By early August, concentrations decreased until they returned to background levels when normal oxygen conditions returned in late September. While most of the built-up CH 4 was gone by the end of anoxia, CH 4 concentrations measured in discrete surface water samples in June and September suggest that there was still a significant flux of CH 4 to the atmosphere. Taken together, our time-series data shows that the CH 4 flux from the Chesapeake Bay is temporally variable, potentially significant, and dependent on physical processes (e.g., storms) occurring in the Bay.

Beschreibung: Salinity is a key variable used to explain the distribution of species within estuaries and the functioning of estuarine ecosystems. Despite the relevance of average conditions and extreme events, the existing schemes of zonation are only based on average values; the extreme values obtained through high-resolution long-term data have not been still integrated in an appropriate methodology capable of recognizing salinity types. Therefore, the background variability of salinity has been frequently underestimated in ecological studies. The primary goal of this research is the identification of ecologically significant salinity zones capable of encompassing the entire estuarine regime. A two-step methodological approach was developed: (1) the reconstruction of long-term salinity series using the Deflt3D hydrodynamic model, the analog method and a subset of short-term representative states of river flow and tidal level and (2) the identification of different zones using eight descriptors of the salinity regime (i.e., the median, the range and the intensity, duration and frequency of extreme events) and the application of a combination of two clustering techniques of unsupervised learning (Self-Organizing Maps (SOM) and K-Means). Thus, five ecologically significant salinity types that are representative of estuarine variability were identified based on a salinity series reconstructed using a validated method. Differences in the mean values of salinity among typologies allow explaining patterns in the general descriptors of benthic macroinvertebrates assemblages (i.e., richness and diversity). If extreme salinity conditions are also considered, typologies increase their ecological significance and they are able to recognize differences in species composition.

Beschreibung: Surface water samples of size-selected seston (0.7–20 μ m) were collected from April 2013 to September 2013 at three similar coarse-sand benthic habitats. Additionally, seston sampling was performed at a fixed location throughout a complete tidal cycle (2014). A combination of fatty acid (FA), isotope, and flow cytometry analyses were used to determine the quality and quantity of nano- and pico-sized particulate organic matter (POM). High variability was found between fatty acid replicate samples. Similar temporal patterns were observed at two sheltered sites, while the exposed site displayed less pronounced seasonal changes. Lower concentrations of 16C and 18C polyunsaturated fatty acids were found during low tide sampling. Globally, POM was dominated by picoeukaryotes, with concentrations exceeding 50,000 cells mL −1 , and (16:4 ω 3 + 18:3 ω 3)/Σ ω 3 is proposed as novel biomarker of picoeukaryotes in this region.

Beschreibung: Microbial endoliths, which inhabit interior pores of rocks, skeletons and coral, are ubiquitous in terrestrial and marine environments. In this study, various colored layers stratified the endolithic environment within the skeleton of Isopora palifera ; however, there was a distinct green-pigmented layer in the skeleton (beneath the living coral tissue). To characterize diversity of endolithic microorganisms, 16S ribosomal RNA gene amplicon pyrosequencing was used to investigate bacterial communities in the green layer of eight I. palifera coral colonies retrieved from two locations on Green Island, Taiwan. The dominant bacterial group in the green layer belonged to the bacterial phylum Chlorobi, green sulfur bacteria capable of anoxygenic photosynthesis and nitrogen fixation. Specifically, bacteria of the genus Prosthecochloris were dominant in this green layer. To our knowledge, this is the first study to provide a detailed profile of endolithic bacteria in coral and to determine the prevalence of Prosthecochloris in the green layer. Based on our findings, we infer that these bacteria may have an important functional role in the coral holobiont in the nutrient-limited coral reef ecosystem.

Beschreibung: Nitrogen (N) loss from different benthic habitats via net denitrification and burial was quantified, and first-order N budgets were constructed, for three geomorphically distinct shallow warm temperate South-East Australian barrier estuaries. Seagrass communities were the most important benthic habitats for N loss via net denitrification due to a combination of their area and high denitrification rates. Similarly, the largest N loss via burial occurred in the seagrass communities in the Hastings River Estuary and Wallis Lake, but in contrast, the largest annual loss of N via burial in the Camden Haven occurred in the subtidal muds due to their large area. N inputs to the river-dominated Hasting River Estuary were dominated by diffuse sources from the catchment. Budget deficits in Camden Haven and Wallis Lake suggest that the largest input of N may have been from the ocean, although missing N-fixation and/or groundwater cannot be excluded. Export to the ocean was the largest loss of N in the Hasting River Estuary followed by net denitrification and then burial. Net denitrification was the largest loss of N in the Camden Haven and Wallis Lake followed by burial. As the systems mature (evolve) the burial of N per m 2 , the loss of N via denitrification per m 2 and the % of the total N load that is removed as fish per m 2 , all decrease. Overall N loss via denitrification for a given residence time may be higher in shallow and oligotrophic coastal systems with extensive seagrass habitats than deeper temperate systems.

Beschreibung: Despite a growing awareness of the importance of inland waters in regional and global carbon (C) cycles, particularly as sources of the greenhouse gases carbon dioxide (CO 2 ) and methane (CH 4 ), very little is known about C sources and fluxes in the Laurentian Great Lakes, Earth's largest surface freshwater system. Here, we present a study of CH 4 dynamics in Lake Erie, which has large spring algae blooms linked to fertilizer runoff and followed by hypoxia, as well as an extensive network of natural gas wells and pipelines in Canadian waters. Lake Erie is a positive source of CH 4 to the atmosphere in late summer, even in shallow regions without water column hypoxia. Stable isotopic measurements indicate that both biogenic and thermogenic CH 4 contribute to emissions from Lake Erie. We estimate that Lake Erie emits 1.3 ± 0.6 × 10 5 kg CH 4 -C d −1 in late summer, with approximately 30% of CH 4 derived from natural gas infrastructure. Additional work is needed to determine the spatial and temporal dynamics of CH 4 emissions from Lake Erie and to confirm estimates of source contribution. Studies of the C cycle in large lakes are not as straightforward as those in smaller lakes, as, in addition to O 2 availability, subsurface currents and high winds may exert significant control over dissolved CH 4 patterns. If climate warming and increasing precipitation intensity lead to increased algal biomass and/or greater extent and duration of hypoxia, this may increase emissions of CH 4 from Lake Erie in a positive feedback to climate change.

Beschreibung: Microbial nitrite reduction plays an important role in the nitrogen cycle, producing the first gaseous product in the denitrification pathway. The role of nitrite reduction in the environment can be assessed using stable isotope measurements of nitrite. Here, we present estimates for nitrogen (N) and oxygen (O) isotope fractionation during nitrite reduction catalyzed by copper-containing nitrite reductase (Cu-NIR) and cytochrome cd1 -containing nitrite reductase (Fe-NIR). A Rayleigh fractionation model was used to calculate the N and O isotope effects, 15 ε and 18 ε respectively, from time-course measurements of nitrite concentration and isotopic composition in batch culture experiments. For three strains of denitrifier carrying the Cu-NIR, 15 ε  = 22 ± 2‰ and 18 ε  = 2 ± 2‰ (95% confidence interval). For three strains of denitrifier carrying the Fe-NIR, 15 ε  = 8 ± 2 and 18 ε  = 6 ± 2‰ (95% confidence interval). These isotope effects for nitrite reduction are significantly different from each other. Furthermore, 15 ε and 18 ε do not show a 1 : 1 relationship, as has been assumed. The difference between the isotope effects for these two families of enzymes is likely due to a mechanical difference in how the enzymes bind nitrite. The Cu-NIR binds to both O atoms and the Fe-NIR only binds to the N, allowing either N O bond to be cleaved and imparting a larger isotope effect for O than for the Cu-NIR. Utilizing these new N isotope effects for nitrite reduction in oxygen minimum zone N cycle models results in higher rates of nitrite oxidation than previously modeled.

Beschreibung: Eastern boundary current systems (EBCSs) experience dynamic fluctuations in seawater pH due to coastal upwelling and primary production. The lack of high-resolution pH observations in EBCSs limits the ability to relate field pH exposures to performance of coastal marine species under future ocean change (acidification, warming). This 3-yr study describes spatio-temporal pH variability across the northern Channel Islands, along a persistent temperature gradient (1–4 ° C) within the eastern boundary California Current System. pH and Conductivity, Temperature, Depth, and Oxygen sensors were deployed on island piers in eelgrass and kelp habitat and on a subtidal mooring. Due to event-scale primary production, the temperature gradient across the islands did not manifest in a pH gradient. We resolved spatial pH variability on diel (ΔpH T 0.05–0.2: photosynthesis), event-scale (ΔpH T  〈0.1–0.2: upwelling, phytoplankton blooms, wind relaxation), and seasonal (ΔpH T 0.06: warming) time frames. In the kelp forest, summer mean pH T (8.01–8.02) and magnitude of diel pH T cycles (ΔpH T 0.12–0.10) were comparable year-to-year, despite 2.1 ° C warming from 2012 to 2014. Compared to nearby mainland sites, the northern Channel Islands experienced few low pH events. The majority of pH T observations were 〉7.9. The lowest pH observations (〉1 SD below mean pH T ) occurred under either warm (respiration during warm nights) or cold (advection of upwelled water) temperatures. We emphasize the importance of incorporating site-specific environmental variability in studies of ocean change biology, particularly in the design of multistressor experiments.

Beschreibung: Phosphorus (P) concentrations and primary production in the Great Lakes have declined since the 1980s, but changes in biogeochemical cycling pathways of different P species remain poorly understood. Water samples were collected for the measurements of different P species to examine the dynamics of P in the water column across the river-lake interface from the Milwaukee River to open Lake Michigan. Dissolved inorganic P (DIP) concentrations were as high as 3049 nM in river waters, but decreased dramatically to as low as 11 ± 7 nM in open lake waters. Total dissolved P was mostly measured in the form of DIP in river waters (73% ± 18%), whereas dissolved organic P (DOP) became the predominant species (85% ± 18%) in lake waters. Colloidal P (〉1 kDa) comprised 58% ± 16% of the bulk DOP in river waters, but decreased to 23% ± 5% in ologotrophic lake waters, showing again contrasting biogeochemical regimes between river and lake waters. Biological processes and coagulation/sedimentation were largely responsible for the removal of river-derived P species and active transformation between inorganic and organic P occurred in both dissolved and particulate phases across the river-lake interface. Increased water column P inventory over the winter of 2013–2014 likely resulted from an accumulative effect of both atmospheric and riverine inputs in the upper water column, and benthic nepheloid layer processes coupled with quagga mussels excretion for the lower water column, respectively. High partition coefficient values (log K d of 4.33–7.01) and a “particle concentration effect” on the partitioning of P between dissolved and particulate phases attested the particle-reactive nature of P in lake waters.

Beschreibung: We quantified benthic community structure on shallow (10 m isobath) reefs separated by 3–130 km on four islands in the south Pacific, and evaluated the roles of disturbances vs. coral recruitment as causes of spatial heterogeneity. Reefs were surveyed in 2013 on Moorea, Tahiti, Tetiaroa, and Maiao, with community structure sampled at two sites on each island using photoquadrats. The effects of coral recruitment on population structure were evaluated through genetic analyses of Pocillopora on three of the islands. Benthic community structure with functional group resolution differed among islands and generally among sites, but coral community structure (generic resolution) differed among islands, but generally not among sites. Genetic analyses of Pocillopora using the open reading frame of host mtDNA revealed varying relative abundances of Pocillopora meandrina/Pocillopora eydouxi, Pocillopora verrucosa, Pocillopora effuses , and two unnamed haplotypes on each island. These results suggest that corals on each island represent unique samplings of genetically discrete larval assemblages rather than random samplings of a single larval assemblage. Together, our findings emphasize the extent to which coral community structure varies over a scale of 〈200 km, and suggests that recruitment from spatially discrete pools of coral larvae plays an important role in creating spatial variation in community structure, even where reefs are connected by prevailing currents.

Beschreibung: Abstract In costal ecosystems, copepods coexist with toxin‐producing phytoplankton. The presence of copepods can amplify the phytoplankton toxin production and thereby increase the overall toxicity of a bloom. Copepods are not always affected by the toxins and can vector the toxins to higher trophic levels. To investigate the interactions between toxin producers and their grazers, we determined the kinetics of grazer‐induced increases in toxin production and the subsequent toxin reduction in a domoic acid (DA)‐producing diatom, Pseudo‐nitzschia seriata. The cellular DA level of the diatom was within the range of in situ measurements. Ten days after removal of the copepods, 28% ± 8% of the cellular DA still remained in the cells. Simultaneously, we monitored the toxicokinetics of DA in two grazers; Calanus finmarchicus and Calanus glacialis. After 144 h of grazing on the toxic diet, the copepods accumulated and retained high concentrations of DA. Nine hours after exposure to the toxic diet was terminated, the copepods had depurated 70% ± 10% of the DA. The depuration lasted 4 ± 2 d and was independent of Calanus species and treatment. We explored the possible physiological responses in copepods after feeding on a purely toxic diet from gene expression profiles of C. finmarchicus. Expression of genes regulating several major metabolic and cellular processes was reduced in copepods feeding on DA‐containing diatoms, and we hypothesize that this is because of exposure to DA.

Beschreibung: Abstract Recent research has highlighted how Lake Baikal, Siberia, has responded to the direct and indirect effects of climate change (e.g., ice‐cover duration), nutrient loading, and pollution, manifesting as changes in phytoplankton/zooplankton populations, community structure, and seasonal succession. Here, we combine and compare analyses of chlorophyll a (an estimate of total algal biomass), carotenoid pigments (biomarkers of algal groups), and lake water silicon isotope geochemistry (δ30SiDSi) to differentiate spatial patterns in dissolved silicon (DSi) uptake at Lake Baikal. A total of 15 sites across the three basins (south, central, and north) of Lake Baikal were sampled in August 2013 along a depth gradient of 0–180 m. Strong, significant correlations were found between vertical profiles of photic zone DSi concentrations and δ30SiDSi compositions (r = −0.81, p 〈 0.001), although these are strongest in the central basin aphotic zone (r = −0.98, p 〈 0.001). Data refute the hypothesis of DSi uptake by picocyanobacteria. Algal biomass profiles and high surface δ30SiDSi compositions suggest greater productivity in the south basin and more oligotrophic conditions in the north basin. δ30SiDSi signatures are highest at depth (20 m) in central basin sites, indicating greater (10–40%) DSi utilization at deep chlorophyll maxima. DSi limitation occurs in the pelagic central basin, probably reflecting a high diatom biomass bloom event (Aulacoseira baicalensis). Meanwhile in the more hydrologically restricted, shallow Maloe More region (central basin), both high δ30SiDSi compositions and picocyanobacteria (zeaxanthin) concentrations, respectively point to the legacy of an “Aulacoseira bloom year” and continuous nutrient supply in summer months (e.g., localized eutrophication).

Beschreibung: Abstract Interactions between biological and physical processes, so‐called bio‐physical feedbacks, are important for landscape evolution. While these feedbacks have been quantified for isolated patches of vegetation in aquatic ecosystems, we still lack knowledge of how the location of one patch affects the occurrence of others. To test for patterns in the spatial distribution of vegetation patches in streams, we first measured the distance between Callitriche platycarpa patches using aerial images. Then, we measured the effects of varying patch separation distance on flow velocity, turbulence, and drag on plants in a field manipulation experiment. Lastly, we investigated whether these patterns of patch alignment developed over time following locations of reduced hydrodynamic forces, using 2‐yr field observations of the temporal patch dynamics of Ranunculus penicillatus in a lowland chalk stream. Our results suggest that vegetation patches in streams organize themselves in V‐like shapes to reduce drag forces, creating an optimal configuration that decreases hydrodynamic forces and may therefore encourage patch growth. Downstream patches are more frequently found at the rear and slightly overlapping the upstream patch, in locations that are partially sheltered by the established upstream vegetation while ensuring exposure to incoming flow (important for nutrient availability). Observations of macrophyte patch dynamics over time indicated that neighboring patches tend to grow in a slightly angled line, producing a spatial pattern resembling the V‐formation in migratory birds. These findings point to the general role of bio‐physical interactions in shaping how organisms align themselves spatially to aerodynamic and hydrodynamic flows at a range of scales.

Beschreibung: Abstract In this study, we successfully implemented a total alkalinity (TA) analyzer in a flow‐through setup, in combination with a FerryBox. The high‐frequency (10 min) measurements along our ship's route revealed that in coastal systems, where carbon fluxes are dynamic, TA can differ significantly (by up to 100 μmol kg−1) between the nearshore and adjacent coastal regions. Even though this study could not account for the net yearly TA production in the coastal region, it demonstrated that there was a seasonal increase in TA of 100–150 μmol kg−1 in coastal waters of the North Sea, equivalent to TA production of 11.7–26.8 mmol m−2 d−1 during the spring and summer months. This seasonal change could not be accounted for by riverine contributions, but instead was probably related to seasonal organic matter production and processing in coastal and nearshore regions. Bottom sediments and the tidally coupled biogeochemical reactor between coastal (North Sea) and nearshore (Wadden Sea) regions are mediating this TA change, and the ~ 4 months lag between the seasonal increase in alkalinity and the peak organic matter production could be explained by the supply of (labile) organic matter and its temperature‐dependent remineralization via both aerobic and anaerobic pathways.