ALBERT

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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%.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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 .

Description: 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.

Description: 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.

Description: A multitracer approach is applied to assess the impact of boundary fluxes (e.g., benthic input from sediments or lateral inputs from the coastline) on the acid-base buffering capacity, and overall biogeochemistry, of the North Sea. Analyses of both basin-wide observations in the North Sea and transects through tidal basins at the North-Frisian coastline, reveal that surface distributions of the 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: Abstract Small ice‐covered lakes are stratified by temperature and solutes. Using time series measurements and profiles of temperature, specific conductance (SC), and dissolved oxygen obtained during spring 2014 and 2015, we identified the physical processes occurring under the ice and at ice‐off in two ~ 2 ha, 10‐m‐deep arctic lakes. The lakes are distinguished from other freshwater, ice‐covered lakes by solutes initially stabilizing the density stratification when temperature decreased in the lower water column and, with one exception, stabilizing it during warming. With an ice cover 1 m thick, wind‐forced internal waves occurred, with 2nd vertical mode waves prevalent where stratification was weak. Snowmelt induced near‐surface chemical stratification such that diurnal thermoclines formed with stable temperature stratification in a ~ 4‐m‐thick layer. Horizontal exchange was mediated by internal waves and gravity currents induced by greater heating near shore and as incoming snowmelt displaced water in shallow regions. Toward ice‐off, the gravity currents reduced temperature stratification between the snowmelt‐induced near‐surface pycnocline and the bottom pycnocline but slight increases in SC precluded radiatively driven convection. Snowmelt retention was greater with rapid spring heating. The lakes did not mix by ice‐off. With moderate winds, Wedderburn numbers decreased below 3 at ice‐off, and the near‐surface pycnocline upwelled and then deepened due to internal wave‐induced mixing. The concomitant downward mixing of heat caused a rapid onset of thermal stratification, and that, combined with incomplete mixing under the ice, led to persistence of near‐bottom depletion in oxygen and increased density and dissolved solutes.

Description: Abstract The globally distributed heterotrophic dinoflagellate Noctiluca scintillans (Macartney) Kofoid & Swezy is well known for its dense blooms and prominent displays of bioluminescence. Intriguingly, along the west coast of the U.S.A., its blooms are not bioluminescent. We investigated the basis for this regional loss of bioluminescence using molecular, cellular, and biochemical analyses of isolates from different geographic regions. Several prominent differences were identified in the nonbioluminescent strains: (1) the fused luciferase and luciferin binding protein gene (lcf/lbp) was present but its transcripts were undetectable; (2) lcf/lbp contained multiple potentially deleterious mutations; (3) the substrate luciferin was absent, based on the lack of luciferin blue autofluorescence and the absence of luciferin‐derived metabolites; (4) although the cells possessed scintillons, the organelles that contain the luminescent chemistry, electron microscopy revealed additional scintillon‐like organelles with an atypical internal structure; and (5) cells isolated from the California coast were 43% smaller than bioluminescent cells from the Gulf of Mexico. Phylogenetic analyses based on large subunit rDNA did not show divergence of the nonbioluminescent population in relation to bioluminescent N. scintillans from the Pacific Ocean and Arabian Sea. This study demonstrates that gene silencing and the lack of the luciferin substrate have resulted in the loss of an important dinoflagellate functional trait over large spatial scales in the ocean. As the bioluminescence system of dinoflagellates is well characterized, nonbioluminescent N. scintillans provide an ideal model to explore the evolutionary and ecological mechanisms that lead to intraspecific functional divergence in natural dinoflagellate populations.

Description: Abstract Global oceanic pH is lowering, which is causing great concern for the natural functioning of marine ecosystems. Current pH predictions are based on open ocean models; however, coastal zones are dynamic systems with seawater pH fluctuating temporally and spatially. To understand how coastal ecosystems will respond in the future, we first need to quantify the extent that local processes influence the pH of coastal zones. With this study, we show that over a single diurnal cycle, the total pH can fluctuate up to 0.2 units in a shallow seagrass‐dominated bay, driven by the photosynthesis and respiration of the vegetation. However, these biologically controlled pH fluctuations vary significantly over small distances. Monitoring conducted at neighboring sites with contrasting hydrodynamic regimes highlights how water motion controls the extent that the local pH is altered by the metabolism of vegetation. The interactive effects of hydrodynamics and vegetation were further investigated with an in situ experiment, where the hydrodynamics were constrained and thus the local water residence time was increased, displaying the counteractive effect of hydrodynamics on the pH change caused by vegetation. With this research, we provide detailed in situ evidence of the spatial variation of pH within marine ecosystems, highlighting the need to include hydrodynamic conditions when assessing the pH‐effects of vegetation, and identifying potential high‐pH refuges in a future low pH ocean.

Description: Abstract In cold and temperate climates, ice typically covers the surface of waterbodies during winter. Many of these systems are also weakly saline where, unlike seawater, the temperature of maximum density, , is higher than its freezing temperature, . This feature of the equation of state results in a stable temperature stratification when surface waters cool below . Conversely, salts excluded from the growing ice can destabilize the underlying water. Previous laboratory and field experiments demonstrated that excluded salts generate localized overturning and downward transport of salt, despite the persistence of a stable temperature gradient. Those experiments were not able to determine the processes responsible for this transport. Here, we use direct numerical simulations to visualize and characterize the plumes generated when ice excludes salt into a stable temperature gradient. We restrict our analysis to times much earlier than the diffusion timescale of temperature over the domain. We define a mass flux parameter ℜ that considers the strength of the reverse‐temperature stratification relative to the rate of salt exclusion. We identify two types of plumes whose characteristics depend mainly on ℜ: double‐diffusive salt‐fingering plumes and convective plumes. The former encourages transport of salt to the bottom without significantly mixing the temperature stratification, while the latter tends to mix the water column. We apply a scaled mass flux parameter to published laboratory and field observations in low‐salinity systems. These limited observations compare favorably with our numerical analysis.

Description: Abstract Increased stratification and mixed layer shoaling of the surface ocean resulting from warming can lead to exposure of marine dinitrogen (N2)‐fixing cyanobacteria to higher levels of inhibitory ultraviolet (UV) radiation. These same processes also reduce vertically advected supplies of the potentially limiting nutrient phosphorus (P) to N2 fixers. It is currently unknown how UV inhibition and P limitation interact to affect the biogeochemical cycles of nitrogen and carbon in these biogeochemically critical microbes. We investigated the responses of the important and widespread marine N2‐fixing cyanobacteria Crocosphaera (strain WH0005) and Trichodesmium (strains IMS 101 and GBR) to UV‐A and UV‐B under P‐replete and P‐limited conditions. Growth, N2 fixation, and carbon dioxide (CO2) fixation rates of Trichodesmium IMS 101 and Crocosphaera were negatively affected by UV exposure. This inhibition was greater for Trichodesmium IMS 101 than for Crocosphaera, which fixes N2 only during the night and so avoids direct UV damage. Negative effects of UV on both IMS 101 and Crocosphaera were less in P‐limited cultures than in P‐replete cultures. In contrast, no UV inhibition was observed in GBR, regardless of P availability. UV inhibition was related to different amounts of UV‐absorbing compounds produced by these isolates. Responses to UV radiation and P availability interactions were taxon‐specific, but our results indicated that in general, UV radiation effects on Trichodesmium and Crocosphaera range from negative to neutral. UV inhibition and its interactions with P limitation may thus have a substantial influence on the present day and future nitrogen and carbon cycles of the ocean.

Description: Abstract The carbonate chemistry of freshwater systems can range from inorganic carbon‐limited to supersaturated with respect to the atmosphere, and the pH of these systems can vary temporally and spatially from alkaline to acidic. Determining how these heterogeneous systems respond to increases in atmospheric CO2 is critical to understanding global impacts of these changes. Here, we synthesize 22 studies from a variety of systems to explore the effects of elevated CO2 on freshwater chemistry and microalgae, which form the base of autotrophic food webs. Across the variability in freshwater systems, elevated CO2 significantly affected water chemistry by decreasing pH and increasing dissolved inorganic carbon. Microalgae were also affected by elevated CO2 with measured increases in (1) nutrient acquisition through microalgal carbon‐to‐nutrient ratios, (2) photosynthetic activity, and (3) growth. While these effects were measured from controlled experiments, the results indicate a wide range of potential freshwater ecosystem effects from elevated atmospheric CO2. Our synthesis also identified several knowledge gaps. Generally, larger sample sizes and studies of longer duration are needed for more robust analyses and conclusions. Additionally, more field experiments across a range of freshwater ecosystem types and studies involving benthic species and multiple trophic levels are needed to strengthen global predictions across the broad variability found within and among freshwater systems.

Description: Abstract The effects of climate change, including ocean acidification and ocean heatwaves, on biological communities in estuaries are often uncertain. Part of the uncertainty is due to the complex suite of environmental factors in addition to acidification and warming that influence the growth of shells and skeletons of many estuarine organisms. The goal of this study was to document spatial and temporal variation in water column properties and to measure the in situ effects on larval and recently settled stages of ecologically important Olympia oysters (Ostrea lurida) and commercially important Pacific oysters (Crassostrea gigas) in a low‐inflow estuary with a Mediterranean climate in Northern California. Our results reveal that seasonal inputs of upwelled or riverine water create important and predictable gradients of carbonate system parameters, temperature, salinity, dissolved oxygen (DO), and other variables that influence oyster performance, and that the influence of these gradients is contingent upon the location in the estuary as well as seasonal timing. During upwelling events (dry season), temperature, carbonate chemistry, and DO had the greatest impact on oyster performance. During runoff events (wet season), gradients in salinity, nutrient concentrations, and total alkalinity driven by river discharge were comparatively more important. These results suggest that the spatial importance of carbonate chemistry and temperature are seasonally variable and are two of several other factors that determine oyster performance. We use these results to discuss future impacts on oysters given projected regional changes in the frequency and magnitude of upwelling and precipitation‐driven runoff events.

Description: Abstract Marine sinking particles transport carbon from the surface and bury it in deep‐sea sediments, where it can be sequestered on geologic time scales. The combination of the surface ocean food web that produces these particles and the particle‐associated microbial community that degrades them creates a complex set of variables that control organic matter cycling. We use targeted metabolomics to characterize a suite of small biomolecules, or metabolites, in sinking particles and compare their metabolite composition to that of the suspended particles in the euphotic zone from which they are likely derived. These samples were collected in the South Atlantic subtropical gyre, as well as in the equatorial Atlantic region and the Amazon River plume. The composition of targeted metabolites in the sinking particles was relatively similar throughout the transect, despite the distinct oceanic regions in which they were generated. Metabolites possibly derived from the degradation of nucleic acids and lipids, such as xanthine and glycine betaine, were an increased mole fraction of the targeted metabolites in the sinking particles relative to surface suspended particles, while algal‐derived metabolites like the osmolyte dimethylsulfoniopropionate were a smaller fraction of the observed metabolites on the sinking particles. These compositional changes are shaped both by the removal of metabolites associated with detritus delivered from the surface ocean and by production of metabolites by the sinking particle‐associated microbial communities. Furthermore, they provide a basis for examining the types and quantities of metabolites that may be delivered to the deep sea by sinking particles.

Description: Abstract To better understand lateral dispersion of buoyant and nonbuoyant pollutants within the surface waters of large lakes, two lateral dispersion experiments were carried out in Lake Michigan during the stratified period: (1) a dye tracking experiment lasting 1 d; and (2) a drifter tracking experiment lasting 24 d. Both the dye patch and drifters were surface‐released at the center of Lake Michigan's southern basin. Near‐surface shear induced by near‐inertial Poincaré waves partially explains elevated dye dispersion rates (1.5–4.2 m2 s−1). During the largely windless first 5 d of the drifter release, the drifters exhibited nearly scale‐independent dispersion ( K ∼ L0.2), with an average dispersion coefficient of 0.14 m2 s−1. Scale‐dependent drifter dispersion ensued after 5 d, with K ∼ L1.09 and corresponding dispersion coefficients of 0.3–2.0 m2 s−1 for length scales L = 1500–8000 m. The largest drifter dispersion rates were found to be associated with lateral shear‐induced spreading along a thermal front. Comparisons with other systems show a wide range of spreading rates for large lakes, and larger rates in both the ocean and the Gulf of Mexico, which may be caused by the relative absence of submesoscale processes in offshore Lake Michigan.

Description: Abstract Chemical pollution mixtures enter aquatic environments and interact with microorganisms in eclectic ways with disparate consequences for microbial ecosystem services. Can using a thermodynamic framework help to determine the net influence of a chemical mixture on the functional capacity of benthic microbial communities? We examined this question by comparing benthic stream microbial communities exposed to a gradient of neutral‐alkaline coalmine effluent. Using a combination of approaches (metagenomics, quantitative polymerase chain reaction [qPCR], and functional assays), we show that functional genes and pathways of microbial communities growing in mine effluent differed in composition, but not diversity. The majority of functional genes and pathways that changed decreased at sites exposed to mine effluent, resulting in lower abundances of nitrogenase and methanogen genes and fermentation pathways. However, selenate reductase gene abundance increased with water and sediment concentration of an ecologically important contaminant at mined sites: selenium. Denitrification genes nosZ and nirK differed between sites: metagenome‐based nosZ increased with dissolved nitrate concentration and qPCR‐based nirK had a hump‐shaped pattern across the mining gradient. Osmoprotectant gene abundance did not change. Extracellular enzyme assays and alkaline phosphatase gene relative abundance suggested that mined stream microbial communities may be constrained by phosphorus bioavailability. Subsidies and stressors related to changes in a set of functional genes and pathways, but differences were not consistently predictable using thermodynamic expectations. This suggests that pairing hypotheses for expected subsidies and stressors with post hoc explorations can yield valuable directions for future study of how microbial functional capacity responds to pollutant mixtures.

Description: Abstract Gas ebullition from aquatic systems to the atmosphere represents a potentially important fraction of primary production that goes unquantified by measurements of dissolved gas concentrations. Although gas ebullition from photosynthetic surfaces has often been observed, it is rarely quantified. The resulting underestimation of photosynthetic activity may significantly bias the determination of ecosystem trophic status and estimated rates of biogeochemical cycling from in situ measures of dissolved oxygen. Here, we quantified gas ebullition rates in Zostera marina meadows in Virginia, U.S.A. using simple funnel traps and analyzed the oxygen concentration and isotopic composition of the captured gas. Maximum hourly rates of oxygen ebullition (3.0 mmol oxygen m−2 h−1) were observed during the coincidence of high irradiance and low tides, particularly in the afternoon when oxygen and temperature maxima occurred. The daily ebullition fluxes (up to 11 mmol oxygen m−2 d−1) were roughly equivalent to net primary production rates determined from dissolved oxygen measurements indicating that bubble ebullition can represent a major component of primary production that is not commonly included in ecosystem‐scale estimates. Oxygen content comprised 20–40% of the captured bubble gas volume and correlated negatively with its δ18O values, consistent with a predominance of mixing between the higher δ18O of atmospheric oxygen in equilibrium with seawater and the lower δ18O of oxygen derived from photosynthesis. Thus, future studies interested in the metabolism of highly productive, shallow water ecosystems, and particularly those measuring in situ oxygen flux, should not ignore the bubble formation and ebullition processes described here.

Description: Abstract Evaluating marine species' population connectivity through larval transport can provide insight into the reliance of geographically separated areas on each other's recruitment and metapopulation resiliency. Using larval transport modeling, we assessed the significance of different regions in supporting the Narragansett Bay Northern quahog (Mercenaria mercenaria) population. We aimed to identify how areas with varying adult quahog biomass and implemented management strategies (based on water quality and commercial harvest) contribute to the overall stock's larval supply. Larval trajectories were modeled by integrating the currents from a realistic physical circulation model with quahog larval behavior applied to particles during spawning periods of 2006, 2007, and 2014. Modeled larval transport suggested that settlement occurs throughout Narragansett Bay, with 35% of spawned larvae swept out of the Bay to the coastal ocean and leaving the stock bounds. Quahogs in areas where shellfishing is prohibited due to water quality concerns produce a significant portion of the Bay's spawned larvae, theoretically serving as de facto spawning sanctuaries. The Providence River, located at the head of the Bay with high mature quahog biomass and currently closed to fishing due to water quality, is a significant source of quahog larvae for the stock. Simulated larval quahog settlement locations corresponded predominantly to sandy bottoms, with less spatial correspondence to commercial fisheries landings. Our work provides insight into the population connectivity of quahogs in Narragansett Bay and highlights the importance of considering oceanography and species' life history characteristics when constructing effective fisheries management plans.

Description: Abstract Ratios of nitrogen (N), phosphorus (P), and dissolved silica (DSi) influence how algal communities respond to nutrient loading, and DSi limitation can facilitate cyanobacterial dominance. The indicator of coastal eutrophication potential (ICEP), described previously by other researchers, predicts production by diatoms vs. nonsiliceous taxa based on deviation of nutrient loads from the Redfield ratio of 106C:16 N:20Si (N‐ICEP) or 106C:1P:20Si (P‐ICEP). The ICEP was calculated for the Mississippi‐Atchafalaya River basin, and four subbasins: the Ohio‐Tennessee, Missouri, Upper Mississippi, and Arkansas‐Red basins from 1979 to 2015. The P‐ICEP indicated a stoichiometric imbalance that favored cyanobacteria for all but the Arkansas‐Red subbasin. The N‐ICEP indicated conditions favorable for cyanobacteria in the Upper Mississippi, Ohio‐Tennessee, and the northern Gulf of Mexico. Agriculture is the predominant land use in the Upper Mississippi and Ohio‐Tennessee subbasins and these subbasins controlled the stoichiometry of the nutrients delivered to the northern Gulf of Mexico. The imbalance in N, P, and DSi inputs to the Gulf was greatest during spring and early summer, and in most years transitioned to favoring diatoms by August or September. Comparing the 1980–1994 and 2001–2015 periods, there was a significant increase in the P‐ICEP for the Upper Mississippi, Ohio‐Tennessee, and Missouri subbasins that appeared to arise mainly from increased P loading to surface waters in the those basins. The ICEP revealed patterns in stoichiometry of N, P, and DSi loads among the major tributaries to the Mississippi River, and an increasing risk of cyanobacterial blooms for inland waters in much of the Mississippi‐Atchafalaya River basin.

Description: Abstract Measurements of the particulate volume scattering function, βp(ψ), at light wavelength of 532 nm, particle size distribution, PSD, and several metrics of particulate concentration and composition were made on eight contrasting seawater samples from nearshore and coastal oceanic environments including river estuary and offshore locations. Both βp(ψ) and PSDs were measured on original (unfiltered) samples and particle size‐fractionated samples obtained through filtration using mesh filters with pore sizes of 5 and 20 μm. We present results based on direct size‐fractionated measurements and data adjusted for imperfect fractionation, which provide insights into the roles played by particle size and composition in angle‐resolved light scattering produced by highly variable natural assemblages of aquatic particles. Despite intricate interplay between the effects of particle size and composition, small particles (〈 5 μm in size) consistently produced a major or dominant contribution (~ 50–80%) to the particulate backscattering coefficient, bbp, in organic, either phytoplankton or nonalgal, dominated samples regardless of significant variations in PSD between these samples. The notable exception was a sample dominated by large‐celled diatoms from microphytoplankton size range, which exemplifies a scenario when large particles (〉 20 μm) can produce a considerable contribution (~ 40%) to bbp. We also observed a trend for inorganic‐dominated samples exhibiting consistently lower contributions (~ 30–40%) of small particles to bbp. The particle size‐based budget for the particulate scattering coefficient, bp, indicates a significant decrease in the role of small particles accompanied by an increase in the role of larger particles compared to the bbp budget.

Description: Abstract There is a growing concern about the implications of accelerated thawing of permafrost for regional biogeochemical cycling of carbon and other bioreactive elements. One such element of concern is nitrogen, and in this study, we investigated the diversity and biogeography of potential diazotrophs within a series of thaw ponds representing different ontogenetic stages in pond development. Using metagenomic sequence data from subarctic thaw ponds, we identified an array of nitrogenase genes across the ponds. The iron‐only nitrogenase gene (anfH) was positively correlated with sulfate, while there was no correlation with methane despite previous findings that organisms carrying anfH can simultaneously participate in nitrogen fixation and methanogenesis. Sulfate is known to inhibit microbial uptake of molybdate, an element essential for the activity of the nifH (molybdenum‐iron) nitrogenase and this may explain the high potential for nitrogen fixation utilizing anfH in sulfate‐rich ponds. NifH was particularly abundant in the hypolimnion of the deeper and older ponds, with Deltaproteobacteria and Chlorobi as the putative dominant diazotrophs. In the epilimnetic waters, nifH composition was more variable, with various Gammaproteobacteria as abundant representatives, while cyanobacterial diazotrophs were scarce. Interestingly, nifH gene abundance was significantly positively correlated with in situ methane concentration. Based on genome‐resolved metagenomics, we hypothesize that diazotrophs and methanogens engage in syntrophic interactions in anoxic waters, possibly via propionate oxidation or (in Geobacter) by interspecies electron transfer. Our results also suggest that nitrogen fixers may supply bioreactive nitrogen compounds to the thaw pond communities, thereby enhancing growth and activity of methanogens.

Description: Abstract Deposition of aerosolized desert dust can affect marine microbial community structure and function through pulsed addition of limiting micro‐ and macronutrients. However, few studies have captured responses to dust deposition in situ following trans‐oceanic transport. We conducted a 26‐d time series evaluating biogeochemical and microbial community response to Saharan dust deposition in surface waters in the subtropical western Atlantic (Florida Keys National Marine Sanctuary, U.S.A.). Following periods of elevated atmospheric dust concentrations, particulate and dissolved iron concentrations increased in surface waters. Autotrophic picoeukaryote abundance increased rapidly, followed by increases in the abundance of heterotrophic bacteria and Synechococcus. Concomitant to cell count changes, we observed successional shifts in bacterial community composition. The relative abundances of Prochlorococcus and Pelagibacter declined with dust arrival, while relative abundance of heterotrophic bacteria increased, beginning with Vibrionales and followed sequentially by Chrysophyceae, Rhodobacteriaceae, and Flavobacteriaceae. Finally, a peak in Synechococcus cyanobacteria was observed. These results provide new insight into microbial community succession in response to Saharan dust deposition, their association with temporal dynamics in surface water dissolved and particulate iron concentrations, and a potential role for bioprocessing of dust particles in shaping marine microbial responses to deposition events.

Description: Abstract The on‐going discussion concerning how environmental factors determine phytoplankton size structure has centered around two hypotheses: (H1) The resource‐size relationship predicts that normalized biovolume size spectrum (NBSS) slopes for phytoplankton are progressively shallower with increasing resource availability and (H2) The temperature‐size relationship predicts that phytoplankton NBSS slopes steepen with increasing water temperature. To test these hypotheses, we examined 72 phytoplankton assemblage collections in the Kuroshio east of Taiwan. Total phytoplankton biomass was used as a proxy for resource availability instead of nutrients because nutrients are depleted and do not represent resource availability for oligotrophic seas. We found no significant relationship between NBSS slopes with temperature, providing little support for the temperature‐size rule. In contrast, a positive relationship between NBSS slopes and total biomass for most of the year lends general support to the resource‐size relationship, except during the winter and early spring. To explain this exception, we hypothesize that resource pulses occurring during the cold seasons are used more efficiently by small cells and promote faster growth of small relative to large phytoplankton because these pulses take place after a long period of resource depletion in oligotrophic seas; thus, the NBSS slopes become much steeper than would be expected from a positive resource‐size relationship. This deviation can be considered as nonsteady state in terms of phytoplankton size structure relative to resources. Nevertheless, we cannot rule out the possibility that grazing effects also play an important role in controlling phytoplankton size structure.

Description: Abstract Suspended marine particles constitute most of the particulate organic matter pool in the oceans, thereby providing substantial substrates for heterotrophs, especially in the mesopelagic. Conversely, sinking particles are major contributors to carbon fluxes defining the strength of the biological carbon pump (BCP). This study is the first to investigate the differential influence of eukaryotic communities to suspended and sinking particles, using 18S rRNA gene sequencing on particles collected with a marine snow catcher in the mixed layer and upper mesopelagic of the Scotia Sea, Southern Ocean. In the upper mesopelagic, most eukaryotic phytoplankton sequences belonged to chain‐forming diatoms in sinking particles and to prymnesiophytes in suspended particles. This suggests that diatom‐enriched particles are more efficient in carbon transfer to the upper mesopelagic than those enriched in prymnesiophytes in the Scotia Sea, the latter more easily disintegrating into suspended particles. In the upper mesopelagic, copepods appeared most influential on sinking particles whereas soft‐tissue metazoan sequences contributed more to suspended particles. Heterotrophic protists and fungi communities were distinct between mixed layer and upper mesopelagic, implying that few protists ride along sinking particles. Furthermore, differences between predatory flagellates and radiolarians between suspended and sinking particles implied different ecological conditions between the two particles pools, and roles in the BCP. Molecular analyses of sinking and suspended particles constitute powerful diagnostic tools to study the eukaryotic influence on the BCP in a more holistic manner compared to classic carbon export studies focusing on sinking particles.

Description: Limnology and Oceanography, Volume 64, Issue 5, Page i-iii, September 2019.

Description: Abstract Biogeochemical silicon (Si) cycling in coastal systems is highly influenced by anthropogenic perturbations in recent decades. Here, we present a systematic study on the distribution of stable Si isotopes of dissolved silicate (δ30SiDSi) in a highly eutrophic coastal system, the Baltic Sea. Besides the well‐known processes, diatom production and dissolution regulating δ30SiDSi values in the water column, we combined field data with a box model to examine the role of human disturbances on Si cycling in the Baltic Sea. Results reveal that (1) damming led to increased δ30SiDSi values in water but had little impacts on their vertical distribution; (2) decrease in saltwater inflow due to enhanced thermal stratification had negligible impacts on the δ30SiDSi distribution. An atypical vertical distribution of δ30SiDSi with higher values in deep water (1.57–1.95‰) relative to those in surface water (1.24–1.68‰) was observed in the central basin. Model results suggest the role of enhanced biogenic silica (BSi) deposition and subsequently regenerated dissolved silicate (DSi) flux from sediments. Specifically, eutrophication enhances diatom production, resulting in elevated exports of highly fractionated BSi to deep water and sediments. In situ sedimentary geochemical processes, such as authigenic clay formation, further fractionate Si isotopes and increase pore‐water δ30SiDSi values, which then leads to pore‐water DSi flux carrying higher δ30SiDSi compositions into deep water. Our findings provide new quantitative information on how the isotope‐based Si cycle responds to human perturbations in coastal seas and shed lights on shifts of Si export to open ocean.

Description: Abstract Dissolved organic matter (DOM) plays a key role in the carbon cycle of freshwater ecosystems. We explored DOM composition from six lakes encompassing a typical latitudinal gradient of lakes in northeast of China (41.9°N–48.6°N). Generally, the relative contents of protein‐like components decreased while humic‐like components increased with increasing latitude. Additionally, the results from modified two‐dimensional correlation spectroscopy indicated that humic‐like components with relatively simple structure were formed earlier than those with relatively complex structure. Furthermore, structural equation models suggested that at higher latitude, lake DOM tended to stabilize as protein‐like component was transformed into humic‐like component. We also found that nutrients (e.g., nitrogen and phosphorus) were key factors affecting DOM composition in low‐latitudinal lakes, whereas temperature was the key factor affecting DOM composition in high‐latitudinal lakes. In conclusion, this study identified the factors which explained the observed latitudinal pattern of DOM in northeast lakes of China. This study provided a theoretical basis for DOM management of lakes at different latitudes.

Description: Abstract The dynamics of freeze and thaw events in Antarctic sea ice impart chemical changes in the underlying sea water. Trace metals in sea ice and accumulated through deposition of dust are released into sea water as sea ice breaks up in spring. Clams such as Laternula elliptica incorporate a record of these and associated chemical dynamics in their carbonate shells. In 2012, we collected samples of L. elliptica from three sites along a sea ice persistence gradient in McMurdo Sound, Ross Sea Antarctica. Concentrations of trace metals in the chondrophore of each shell were measured by laser ablation inductively coupled plasma mass spectrometry. Ablations transected annual growth increments creating time series ranging in length from 13 to 25 yr. An 8‐yr time period of persistent sea ice, associated with presence of the B‐15 and C‐19 icebergs at the entrance of McMurdo Sound, was clearly resolved in the trace element time series. Conservative trace metals (Sr, Ba) were found at higher concentrations, and highly scavenged elements (Pb, Cu) were found at lower concentrations at sites with more persistent sea ice and during the 8‐yr period of iceberg‐influenced sea ice persistence. Bioactive trace metals (Fe, Ni) were found in higher concentrations during ice free conditions, associated with a period of high pelagic productivity. Our results provide an important case study for understanding the chemical signature of changing sea ice dynamics as reflected in bivalve shell material under a changing Antarctic climate.

Description: Abstract Rhizosphere microbiota has received much attention due to their associations with plant growth and their fundamental importance in terrestrial ecosystems. However, relatively few studies have focused on rhizosphere microbial communities associated with aquatic macrophytes in freshwater lakes. We hypothesized that the rhizosphere microbiome would reflect the presence of macrophyte roots and the concomitant microhabitat conditions the roots create. Here, high‐throughput sequencing and network analysis were employed to compare the composition and structure of bacterial communities in the rhizosphere of two common emergent macrophytes, Zizania latifolia and Phragmites australis, with the surrounding sediments in Lake Taihu (China). Results indicated that bacterial diversity, community composition, and co‐occurrence networks differed between the communities of bulk sediments and the communities of rhizosphere and surface sediments. Richness and phylogenetic diversity were higher and more taxa were enriched in the rhizosphere and surface sediment communities compared with bulk sediment communities. No differences were detected between bacterial communities in rhizosphere and surface sediments, nor between rhizospheres sediment communities of the two macrophyte species. Anaerobic taxa were more abundant in bulk sediment communities. Among the co‐occurrence networks, more nodes (operational taxonomic units) and edges (connections) with higher average degree as well as more topologically important nodes were found in rhizosphere and surface sediment communities relative to bulk communities. These findings suggest that rhizosphere microbiome communities are influenced by the presence of macrophyte roots, with oxygenated rhizosphere and surface sediment communities being more diverse, and organized into more interconnected co‐occurrence networks.

Description: Limnology and Oceanography, Volume 64, Issue 5, Page iii-iii, September 2019.

Description: Limnology and Oceanography, Volume 64, Issue 5, Page i-ii, September 2019.

Description: Abstract Field data, a three‐dimensional hydrodynamic numerical model, and modal decomposition were used to examine the baroclinic response to wind in two interconnected multiarm lakes, Knewstubb and Natalkuz Lakes (British Columbia, Canada). Similar to lakes of simpler geometry, the first‐vertical first‐horizontal mode (V1H1) of Knewstubb and Natalkuz Lakes was found to be the most energetic mode. This V1H1 dominance is attributed to similarity between the relatively uniform distributions of along‐thalweg wind and surface‐layer flow for the V1H1 mode. Two bathymetric features, the geometric constriction between Knewstubb and Natalkuz Lakes, and changes in thalweg orientation relative to the wind field act to create subbasins in which wind forcing causes local metalimnetic tilts. These subbasin scale tilts deepen the metalimnion at the downwind ends of the subbasins, raise the metalimnion at the upwind ends, and are superimposed on the lake‐wide V1H1 tilt. The metalimnetic tilts in the subbasins are attributed to higher horizontal modes that are equivalent to V1H1 modes of the subbasins. Because the thickness of the metalimnion is significant, the subbasin metalimnetic tilts occur with variations in the metalimnetic thickness along the subbasins; the metalimnion is compressed at the downwind end and expanded at the other end of each subbasin. This variability in thickness leads to metalimnetic intrusions following the relaxation of wind forcing. The findings of this study contribute to characterizing the baroclinic response to wind in lakes of complex bathymetry. In particular, this study helps in understanding the effect of bathymetric features on modal composition of the baroclinic response.

Description: Abstract Very few studies have focused on the biogeography and genetic diversity of aquatic invertebrate zooplankton from the Qinghai‐Tibetan Plateau, an important biogeographic hotspot. The geological uplift of this region created special environments requiring adaptive genetic diversification of the resident lineages. Here, we developed and used the first set of high‐resolution microsatellite markers for the Daphnia similis species complex. We screened this species in 23 (out of 303) lakes across China, covering a large geographical and ecological range of habitats. High population clonal diversity of the D. similis species complex was found, regardless of their geographical origin. However, four Tibetan Daphnia sinensis populations showed substantially lower clonal diversity. Interestingly, D. sinensis populations from two regions, Eastern China and the Qinghai‐Tibetan Plateau of Western China, were found to be clearly differentiated from each other. Lakes inhabited by Western D. sinensis were of a lower trophic level, higher altitude, and more likely to freeze in the winter. This study contributes to an understanding of adaptation to their special environments by the freshwater invertebrates in the Qinghai‐Tibetan Plateau.

Description: Abstract Single‐lake studies offer an opportunity for understanding, predicting, and mitigating local or regional threats to lake ecosystems. Our goal was to understand how concurrent environmental stressors such as climate change, eutrophication, and salinization affect long‐term lake water quality. We report epilimnetic changes in 18 water‐quality parameters collected at seven sites from 1980 to 2016 in Lake George, a large oligotrophic lake in the Adirondack Park, New York, USA. Improvements and deteriorations in water quality occurred over 37 years. We observed a 32% increase in chlorophyll a associated with an increase in orthophosphate, but not total phosphorus or a warming epilimnion (0.05°C/year). Salinization from road deicing salts contributed to the largest deterioration in water quality. However, chloride concentrations and the current rate of increase are low enough that few ecological impacts are likely to occur over the next few decades. Increasing calcium concentrations were not high enough to facilitate the persistence of invasive species in the lake such as zebra mussels (Dreissena polymorpha) but are sufficient for Asian clams (Corbicula fluminea) and the spiny water flea (Bythotrephes longimanus). Similar to other lakes, environmental legislation has supported recovery from acidification, indicated by reduced sulfate and nitrate, and increased alkalinity and pH. Declines in water quality were minor relative to other lakes, suggesting that decades of tourism and development occurred without major deterioration in water quality, but management efforts are needed to curb salinization in the Lake George watershed, particularly as it relates to sodium concentrations to prevent a loss of drinking water quality.

Description: Abstract Colonies of the N2‐fixing cyanobacterium Trichodesmium can harbor distinct chemical microenvironments that may assist the colonies in acquiring mineral iron from dust. Here, we characterized O2 and pH gradients in and around Trichodesmium colonies by microsensor measurements on 〉 170 colonies collected in the Gulf of Eilat over ∼ 2 months. O2 concentrations and pH values in the center of single colonies decreased in the dark due to respiration, reaching minimum values of 70 μmol L−1 and 7.7, whereas in the light, O2 and pH increased due to photosynthesis, reaching maximum values of 410 μmol L−1 and 8.6. Addition of dust and bacteria and increasing colony size influenced O2 and pH levels in the colonies, yet values remained within the range observed in single natural colonies. However, lower values down to 60 μmol L−1 O2 and pH 7.5 were recorded in the dark in dense surface accumulations of Trichodesmium. Using radiolabelled ferrihydrite, we examined the effect of these conditions on mineral iron dissolution and availability to Trichodesmium. Dark‐incubated colonies did not acquire iron from ferrihydrite faster than light‐incubated colonies, indicating that the dark‐induced decrease in pH and O2 within single colonies is too small to significantly increase mineral iron bioavailability. Yet, ligand‐promoted dissolution of ferrihydrite, a mechanism likely applied by Trichodesmum for acquiring mineral iron, did increase at the lower pH levels observed in surface accumulations. Thus, Trichodesmium surface blooms in their final stage may harbor chemical conditions that enhance the dissolution and bioavailability of mineral iron to the associated microbial community.

Description: Abstract In warm monomictic lakes, the hypolimnion is important for accumulating and decomposing organic matter derived from surface production, and the regenerated nutrients will be supplied to the epilimnion through winter vertical mixing. So far, we know little about microbial community composition and function in the hypolimnion when the significant thermal stratification disappears. In this study, we investigated microbial community compositions and functional gene contents by means of metagenomics along a depth profile in the warm monomictic alpine Lake Fuxian during holomictic period. Overall, bacteria were the dominant microbial group at different water depths, while phages had their high relative abundance in the epilimnion. We observed slight thermal but strong chemical stratification even during this typical winter overturn. The anaerobic respiration with nitrate and sulfate as the terminal electron acceptors was accumulated at bottom of hypolimnionin as indicated through metabolic pathway reconstruction. We were able to get 440 metagenome‐assembled genomes (MAGs) and unraveled a high genomic diversity of freshwater pelagic microbiomes along this depth profile. We furthermore defined a new class of “Plancto_FXH1” of Planctomycetes from these MAGs, of which a distinct nitrate reduction operon was identified. Representatives of this phylum mainly thrive in the hypolimnion as previously suspected, but few lineages were detected in the epilimnion. In summary, metagenomics enabled us to find a new group of Planctomycetes, probably involved in denitrification in the hypolimnion in Lake Fuxian, which expand our knowledge on denitrifying bacterial diversity and their denitrification potential in deep freshwater lakes.

Description: Coastal marine ecosystems have been under high anthropogenic pressure and it can be assumed that prevalent local perturbation interacts with rising global stressors under proceeding climate change. Understanding their effective pathways and cumulative effects is of high relevance not only with regard to future risk assessment, but also for current ecosystem management. In benthic mesocosms, we factorially tested the effects of one global (combined elevated seawater temperature and 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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).

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.