Showing papers in "Limnology and Oceanography in 2016"

Optical properties of dissolved organic matter (DOM): Effects of biological and photolytic degradation

Abstract: Advances in spectroscopic techniques have led to an increase in the use of optical properties (absorbance and fluorescence) to assess dissolved organic matter (DOM) composition and infer sources and processing. However, little information is available to assess the impact of biological and photolytic processing on the optical properties of original DOM source materials. Over a 3.5 month laboratory study, we measured changes in commonly used optical properties and indices in DOM leached from peat soil, plants, and algae following biological and photochemical degradation to determine whether they provide unique signatures that can be linked to original DOM source. Changes in individual optical parameters varied by source material and process, with biodegradation and photodegradation often causing values to shift in opposite directions. Although values for different source materials frequently overlapped, multivariate statistical analyses showed that unique optical signatures could be linked to original DOM source material, with 17 optical properties determined by discriminant analysis to be significant (p < 0.05) in distinguishing between DOM source and environmental processing. These results demonstrate that inferring source material from optical properties is possible when parameters are evaluated in combination even after extensive biological and photochemical alteration.

Pluses and minuses of ammonium and nitrate uptake and assimilation by phytoplankton and implications for productivity and community composition, with emphasis on nitrogen-enriched conditions

Abstract: Anthropogenic activities are altering total nutrient loads to many estuaries and freshwaters, resulting in high loads not only of total nitrogen (N), but in some cases, of chemically reduced forms, notably NH4+. Long thought to be the preferred form of N for phytoplankton uptake, NH4+ may actually suppress overall growth when concentrations are sufficiently high. NH4+ has been well known to be inhibitory or repressive for NO3‐ uptake and assimilation, but the concentrations of NH4+ that promote vs. repress NO3‐ uptake, assimilation, and growth in different phytoplankton groups and under different growth conditions are not well understood. Here, we review N metabolism first in a “generic” eukaryotic cell, and the contrasting metabolic pathways and regulation of NH4+ and NO3− when these substrates are provided individually under equivalent growth conditions. Then the metabolic interactions of these substrates are described when both are provided together, emphasizing the cellular challenge of balancing nutrient acquisition with photosynthetic energy balance in dynamic environments. Conditions under which dissipatory pathways such as dissimilatory NO3−/ NO2− reduction to NH4+ and photorespiration that may lead to growth suppression are highlighted. While more is known about diatoms, taxon-specific differences in NH4+ and NO3− metabolism that may contribute to changes in phytoplankton community composition when the composition of the N pool changes are presented. These relationships have important implications for harmful algal blooms, development of nutrient criteria for management, and modeling of nutrient uptake by phytoplankton, particularly in conditions where eutrophication is increasing and the redox state of N loads is changing.

Methane ebullition and diffusion from northern ponds and lakes regulated by the interaction between temperature and system productivity

Abstract: Methane (CH4) emissions from aquatic systems should be coupled to CH4 production, and thus a temperature-dependent process, yet recent evidence suggests that modeling CH4 emissions may be more complex due to the biotic and abiotic processes influencing emissions. We studied the magnitude and regulation of two CH4 pathways—ebullition and diffusion—from 10 shallow ponds and 3 lakes in Quebec. Ebullitive fluxes in ponds averaged 4.6 ± 4.1 mmol CH4 m−2 d−1, contributing ∼56% to total (diffusive + ebullitive) CH4 emissions. In lakes, ebullition only occurred in waters < 3 m deep, averaging 1.1 ± 1.5 mmol CH4 m−2 d−1, and when integrated over the whole lake, contributed only 18% to 22% to total CH4 emissions. While pond CH4 fluxes were related to sediment temperature, with ebullition having a stronger dependence than diffusion (Q10, 13 vs. 10; activation energies, 168 kJ mol−1 vs. 151 kJ mol−1), the temperature dependency of CH4 fluxes from lakes was absent. Combining data from ponds and lakes shows that the temperature dependency of CH4 diffusion and ebullition is strongly modulated by system trophic status (as total phosphorus), suggesting that organic substrate limitation dampens the influence of temperature on CH4 fluxes from oligotrophic systems. Furthermore, a strong phosphorus-temperature interaction determines the dominant emission pathway, with ebullition disproportionately enhanced. Our results suggest that aquatic CH4 ebullition is regulated by the interaction between ecosystem productivity and climate, and will constitute an increasingly important component of carbon emissions from northern aquatic systems under climate and environmental change.

Phytoplankton growth and the interaction of light and temperature: A synthesis at the species and community level

Abstract: Temperature strongly affects phytoplankton growth rates, but its effect on communities and ecosystem processes is debated. Because phytoplankton are often limited by light, temperature should change community structure if it affects the traits that determine competition for light. Furthermore, the aggregate response of phytoplankton communities to temperature will depend on how changes in community structure scale up to bulk rates. Here, we synthesize experiments on 57 phytoplankton species to analyze how the growth-irradiance relationship changes with temperature. We find that light-limited growth, light-saturated growth, and the optimal irradiance for growth are all highly sensitive to temperature. Within a species, these traits are co-adapted to similar temperature optima, but light-limitation reduces a species' temperature optimum by ∼5°C, which may be an adaptation to how light and temperature covary with depth or reflect underlying physiological correlations. Importantly, the maximum achievable growth rate increases with temperature under light saturation, but not under strong light limitation. This implies that light limitation diminishes the temperature sensitivity of bulk phytoplankton growth, even though community structure will be temperature-sensitive. Using a database of primary production incubations, we show that this prediction is consistent with estimates of bulk phytoplankton growth across gradients of temperature and irradiance in the ocean. These results indicate that interactions between temperature and resource limitation will be fundamental for explaining how phytoplankton communities and biogeochemical processes vary across temperature gradients and respond to global change.

The importance of ice algae-produced carbon in the central Arctic Ocean ecosystem: Food web relationships revealed by lipid and stable isotope analyses

Abstract: To better predict ecological consequences of changing Arctic sea ice environments, we aimed to quantify the contribution of ice algae-produced carbon (αIce) to pelagic food webs in the central Arctic Ocean. Eight abundant under-ice fauna species were submitted to fatty acid (FA) analysis, bulk stable isotope analysis (BSIA) of nitrogen (δ15N) and carbon (δ13C) isotopic ratios, and compound-specific stable isotope analysis (CSIA) of δ13C in trophic marker FAs. A high mean contribution αIce was found in Apherusa glacialis and other sympagic (ice-associated) amphipods (BSIA: 87% to 91%, CSIA: 58% to 92%). The pelagic copepods Calanus glacialis and C. hyperboreus, and the pelagic amphipod Themisto libellula showed substantial, but varying αIce values (BSIA: 39% to 55%, CSIA: 23% to 48%). Lowest αIce mean values were found in the pteropod Clione limacina (BSIA: 30%, CSIA: 14% to 18%). Intra-specific differences in FA compositions related to two different environmental regimes were more pronounced in pelagic than in sympagic species. A comparison of mixing models using different isotopic approaches indicated that a model using δ13C signatures from both diatom-specific and dinoflagellate-specific marker FAs provided the most conservative estimate of αIce. Our results imply that ecological key species of the central Arctic Ocean thrive significantly on carbon synthesized by ice algae. Due to the close connectivity between sea ice and the pelagic food web, changes in sea ice coverage and ice algal production will likely have important consequences for food web functioning and carbon dynamics of the pelagic system.

Productivity and depth regulate lake contributions to atmospheric methane

Abstract: Despite significant contributions of inland lakes to the global methane cycle, we lack a process-based understanding of what regulates inter-lake variation in methane emissions. Previous comparative work has identified a potential link between lake primary productivity and methane emissions; also, lab-scale experiments suggest that the addition of algal substrate to anoxic sediments rapidly enhances rates of methanogenesis. This existing work indicates that primary productivity could enhance lake contributions to the global methane cycle. However, a more systematic investigation of the links between lake primary production, methanogenesis, and methane emission to the atmosphere is required to quantify the implications of increased cultural eutrophication for methane evasion from lakes. Using paired measurements of methanogenesis and methane emissions on 16 north temperate lakes, we documented a positive relationship between lake productivity and sediment methanogenesis rates. However, increased methanogenesis rates did not result in an increase in diffusive methane emissions. Rather, they generated greater methane storage during summer stratification and enhanced methane emission to the atmosphere via bubbling (ebullition), dependent on site depth. Ebullition most frequently occurred at sites less than 6 m deep and where methanogenesis rates were high. The relationships between lake productivity, methanogenesis, and depth-dependent ebullition suggests it is likely that shallow, productive lakes contribute significantly more methane to the atmosphere than deep, clear lakes and will continue to do so in light of the growing prevalence of lake eutrophication.

Spatio-temporal variability of lake CH4 fluxes and its influence on annual whole lake emission estimates

Abstract: Lakes are major sources of methane (CH4) to the atmosphere that contribute significantly to the global budget. Recent studies have shown that diffusive fluxes, ebullition and surface water CH4 conc ...

Dissimilatory nitrate reduction to ammonium coupled to Fe(II) oxidation in sediments of a periodically hypoxic estuary

Abstract: Estuarine sediments are critical for the remediation of large amounts of anthropogenic nitrogen (N) loading via production of N2 from nitrate by denitrification. However, nitrate is also recycled within sediments by dissimilatory nitrate reduction to ammonium (DNRA). Understanding the factors that influence the balance between denitrification and DNRA is thus crucial to constraining coastal N budgets. A potentially important factor is the availability of different electron donors (organic carbon, reduced iron and sulfur). Both denitrification and DNRA may be linked to ferrous iron oxidation, however the contribution of Fe(II)-fueled nitrate reduction in natural environments is practically unknown. This study investigated how nitrate-dependent Fe2+ oxidation affects the partitioning between nitrate reduction pathways using 15N-tracing methods in sediments along the salinity gradient of the periodically hypoxic Yarra River estuary, Australia. Increased dissolved Fe2+ availability resulted in significant enhancement of DNRA rates from around 10–20% total nitrate reduction in control incubations to over 40% in those with additional Fe2+, at several sites. Increases in DNRA at some locations were accompanied by reductions in denitrification. Significant correlations were observed between Fe2+ oxidation and DNRA rates, with reaction ratios corresponding to the stoichiometry of Fe2+-dependent DNRA. Our results provide experimental evidence for a direct coupling of DNRA to Fe2+ oxidation across an estuarine gradient, suggesting that Fe2+ availability may exert substantial control on the balance between retention and removal of bioavailable N. Thus, DNRA linked to Fe2+ oxidation may be of general importance to environments with Fe-rich sediments.

Short-term mudflat dynamics drive long-term cyclic salt marsh dynamics

Abstract: 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 (δzTF). The resulting difference in elevation between the tidal flat and adjacent marsh (ΔZ) initiates lateral marsh erosion. Marsh erosion rate was found to depend on sediment type and to increase with increasing ΔZ and hydrodynamic exposure. Laboratory and field experiments revealed that seedling establishment was negatively impacted by an increasing δzTF. As the amplitude of δzTF 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.

Organic carbon in seagrass sediments is influenced by seagrass canopy complexity, turbidity, wave height, and water depth

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

Unbalanced reduction of nutrient loads has created an offshore gradient from phosphorus to nitrogen limitation in the North Sea

Abstract: Measures to reduce eutrophication have often led to a more effective decline of phosphorus (P) than nitrogen (N) concentrations. The resultant changes in riverine nutrient loads can cause an increase in the N : P ratios of coastal waters. During four research cruises along a 450 km transect, we investigated how reductions in nutrient inputs during the past 25 yr have affected nutrient limitation patterns in the North Sea. This revealed a strong offshore gradient of dissolved inorganic N : P ratios in spring, from 375 : 1 nearshore toward 1 : 1 in the central North Sea. This gradient was reflected in high nearshore N : P and C : P ratios of particulate organic matter (mainly phytoplankton), indicative of severe P deficiency of coastal phytoplankton, which may negatively affect higher trophic levels in the food web. Nutrient enrichment bioassays performed on-board showed P and Si limitation of phytoplankton growth nearshore, co-limitation of N and P in a transitional region, and N limitation in the outer-shore waters, confirming the existence of an offshore gradient from P to N limitation. Different species were limited by different nutrients, indicating that further reductions of P loads without concomitant reductions of N loads will suppress colonial Phaeocystis blooms, but will be less effective in diminishing harmful algal blooms by dino- and nanoflagellates. Hence, our results provide evidence that de-eutrophication efforts in northwestern Europe have led to a large imbalance in the N : P stoichiometry of coastal waters of the North Sea, with major consequences for the growth, species composition, and nutritional quality of marine phytoplankton communities.

Climate‐induced warming of lakes can be either amplified or suppressed by trends in water clarity

Abstract: Climate change is rapidly warming aquatic ecosystems including lakes and reservoirs. However, variability in lake characteristics can modulate how lakes respond to climate. Water clarity is especially important both because it influences the depth range over which heat is absorbed, and because it is changing in many lakes. Here, we show that simulated long-term water clarity trends influence how both surface and bottom water temperatures of lakes and reservoirs respond to climate change. Clarity changes can either amplify or suppress climate-induced warming, depending on lake depth and the direction of clarity change. Using a process-based model to simulate 1894 north temperate lakes from 1979 to 2012, we show that a scenario of decreasing clarity at a conservative yet widely observed rate of 0.92% yr−1 warmed surface waters and cooled bottom waters at rates comparable in magnitude to climate-induced warming. For lakes deeper than 6.5 m, decreasing clarity was sufficient to fully offset the effects of climate-induced warming on median whole-lake mean temperatures. Conversely, a scenario increasing clarity at the same rate cooled surface waters and warmed bottom waters relative to baseline warming rates. Furthermore, in 43% of lakes, increasing clarity more than doubled baseline bottom temperature warming rates. Long-term empirical observations of water temperature in lakes with and without clarity trends support these simulation results. Together, these results demonstrate that water clarity trends may be as important as rising air temperatures in determining how waterbodies respond to climate change.

Aerobic gammaproteobacterial methanotrophs mitigate methane emissions from oxic and anoxic lake waters

Abstract: 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.

Hydrology controls dissolved organic matter export and composition in an Alpine stream and its hyporheic zone

Abstract: Streams and rivers transport dissolved organic matter (DOM) from the terrestrial environment to downstream ecosystems. In light of climate and global change it is crucial to understand the temporal dynamics of DOM concentration and composition, and its export fluxes from headwaters to larger downstream ecosystems. We monitored DOM concentration and composition based on a diurnal sampling design for 3 years in an Alpine headwater stream. We found hydrologic variability to control DOM composition and the coupling of DOM dynamics in the streamwater and the hyporheic zone. High-flow events increased DOM inputs from terrestrial sources (as indicated by the contributions of humic- and fulvic-like fluorescence), while summer baseflow enhanced the autochthonous imprint of DOM. Diurnal and seasonal patterns of DOM composition were likely induced by biological processes linked to temperature and photosynthetic active radiation (PAR). Floods frequently interrupted diurnal and seasonal patterns of DOM, which led to a decoupling of streamwater and hyporheic water DOM composition and delivery of aromatic and humic-like DOM to the streamwater. Accordingly, DOM export fluxes were largely of terrigenous origin as indicated by optical properties. Our study highlights the relevance of hydrologic and seasonal dynamics for the origin, composition and fluxes of DOM in an Alpine headwater stream.

Carbon-to-chlorophyll ratio for phytoplankton in temperate coastal waters: Seasonal patterns and relationship to nutrients

Abstract: 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.

Effects of climate change on methane emissions from seafloor sediments in the Arctic Ocean: A review

Abstract: Large quantities of methane are stored in hydrates and permafrost within shallow marine sediments in the Arctic Ocean. These reservoirs are highly sensitive to climate warming, but the fate of methane released from sediments is uncertain. Here, we review the principal physical and biogeochemical processes that regulate methane fluxes across the seabed, the fate of this methane in the water column, and potential for its release to the atmosphere. We find that, at present, fluxes of dissolved methane are significantly moderated by anaerobic and aerobic oxidation of methane. If methane fluxes increase then a greater proportion of methane will be transported by advection or in the gas phase, which reduces the efficiency of the methanotrophic sink. Higher freshwater discharge to Arctic shelf seas may increase stratification and inhibit transfer of methane gas to surface waters, although there is some evidence that increased stratification may lead to warming of sub-pycnocline waters, increasing the potential for hydrate dissociation. Loss of sea-ice is likely to increase wind speeds and sea-air exchange of methane will consequently increase. Studies of the distribution and cycling of methane beneath and within sea ice are limited, but it seems likely that the sea-air methane flux is higher during melting in seasonally ice-covered regions. Our review reveals that increased observations around especially the anaerobic and aerobic oxidation of methane, bubble transport, and the effects of ice cover, are required to fully understand the linkages and feedback pathways between climate warming and release of methane from marine sediments.

The persistence of cyanobacterial (Microcystis spp.) blooms throughout winter in Lake Taihu, China

Abstract: Temperature is generally considered as a key factor controlling algal bloom formation. Previous studies have indicated that the bloom-forming cyanobacteria Microcystis spp. overwinters near the sediment surface and does not actively grow below 15°C. However, satellite images and field collections from Lake Taihu, China have shown that Microcystis spp. blooms persisted when water temperatures were below 10°C during winter, although their magnitudes were smaller than during periods of higher temperature. Winter Microcystis cells maintained low activity and were able to grow again when exposed to elevated temperatures (≥12.5°C). Hence, cyanobacterial blooms may appear year-round in eutrophic lakes. Temperature increases coupled with nutrient enrichment promoted the growth of cyanobacteria, while low temperature decreased the loss rate of Microcystis, allowing winter blooms to persist. High concentrations of overwintering vegetative cells may provide a large inoculum for blooms during warmer seasons. Controlling winter blooms may reduce their magnitude during the warmer seasons.

The role of microbial exopolymers in determining the fate of oil and chemical dispersants in the ocean

Abstract: The production of extracellular polymeric substances (EPS) by planktonic microbes can influence the fate of oil and chemical dispersants in the ocean through emulsification, degradation, dispersion, aggregation, and/or sedimentation. In turn, microbial community structure and function, including the production and character of EPS, is influenced by the concentration and chemical composition of oil and chemical dispersants. For example, the production of marine oil snow and its sedimentation and flocculent accumulation to the seafloor were observed on an expansive scale after the Deepwater Horizon oil spill in the Northern Gulf of Mexico in 2010, but little is known about the underlying control of these processes. Here, we review what we do know about microbially produced EPS, how oil and chemical dispersant can influence the production rate and chemical and physical properties of EPS, and ultimately the fate of oil in the water column. To improve our response to future oil spills, we need a better understanding of the biological and physiochemical controls of EPS production by microbes under a range of environmental conditions, and in this paper, we provide the key knowledge gaps that need to be filled to do so.

Increased variability and sudden ecosystem state change in Lake Winnipeg, Canada, caused by 20th century agriculture

Abstract: 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 N2-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 μg 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%, δ15N enriched 3–4‰, and concentrations of many fossil pigments increased 300–500%. Nearly 75% of 20th 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.

Intertidal salt marshes as an important source of inorganic carbon to the coastal ocean

Abstract: 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 CO2 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 CO2 (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.

Coupled nitrification–denitrification leads to extensive N loss in subtidal permeable sediments

Abstract: We investigated microbial pathways of nitrogen transformation in highly permeable sediments from the German Bight (South-East North Sea) by incubating sediment cores percolated with 15N-labeled substrates under near in situ conditions. In incubations with added math formula, production of math formula occurred while the sediment was oxic, indicating ammonia oxidation. Similarly, math formula production during math formula incubations indicated nitrite oxidation. Taken together these findings provide direct evidence of high nitrification rates within German Bight sands. The production of 15N-N2 on addition of math formula revealed high denitrification rates within the sediment under oxic and anoxic conditions. Denitrification rates were strongly and positively correlated with oxygen consumption rates, suggesting that denitrification is controlled by organic matter availability. Nitrification and denitrification rates were of the same magnitude and the rapid production of 15N-N2 in incubations with added math formula confirmed close coupling of the two processes. Areal rates of N-transformation were estimated taking advective transport of substrates into account and integrating volumetric rates over modeled oxygen and nitrate penetration depths, these ranged between 22 μmol N m−2 h−1 and 94 μmol N m−2 h−1. Furthermore, results from the 15N-labeling experiments show that these subtidal permeable sediments are, in sharp contrast to common belief, a substantial source of N2O. Our combined results show that nitrification fuels denitrification by providing an additional source of nitrate, and as such masks true N-losses from these highly eutrophic sediments. Given the widespread occurrence of anthropogenically influenced permeable sediments, coupled benthic nitrification–denitrification might have an important but so far neglected role in N-loss from shelf sediments.

Recent accelerated warming of the Laurentian Great Lakes: Physical drivers

Abstract: 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.

Long-term alkalinity trends in the Baltic Sea and their implications for CO2-induced acidification

Abstract: Anthropogenic CO2 emissions currently decrease open ocean pH, but on multi-millennial time scales intensified continental weathering is expected to contribute to increasing oceanic alkalinity (AT) and thus mitigate the acidification signal. The Baltic Sea is an ideal study site for such AT dynamics, due to its direct link to terrestrial processes, short water residence time and long history of AT measurements dating back to the early 20th century. We compiled an extensive AT data set that revealed the highest data quality and coverage for the past two decades. Within that period, surface water AT 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 AT 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 CO2-induced acidification by almost 50% and 100%, respectively. Further, the AT trends enhanced the CO2 storage capacity and stabilized the CaCO3 saturation state of the Baltic Sea over the past two decades. We discuss the attribution of the AT trends to potential changes in precipitation patterns, continental weathering driven by acidic rain and increasing atmospheric CO2, agricultural liming and internal AT sources.

Ocean pH time‐series and drivers of variability along the northern Channel Islands, California, USA

Abstract: Eastern boundary current systems (EBCSs) experience dynamic fluctuations in seawater pH due to coastal upwelling and primary production. The lack of high-resolution pH observations in EBCSs limits the ability to relate field pH exposures to performance of coastal marine species under future ocean change (acidification, warming). This 3-yr study describes spatio-temporal pH variability across the northern Channel Islands, along a persistent temperature gradient (1–4°C) within the eastern boundary California Current System. pH and Conductivity, Temperature, Depth, and Oxygen sensors were deployed on island piers in eelgrass and kelp habitat and on a subtidal mooring. Due to event-scale primary production, the temperature gradient across the islands did not manifest in a pH gradient. We resolved spatial pH variability on diel (ΔpHT 0.05–0.2: photosynthesis), event-scale (ΔpHT 7.9. The lowest pH observations (>1 SD below mean pHT) occurred under either warm (respiration during warm nights) or cold (advection of upwelled water) temperatures. We emphasize the importance of incorporating site-specific environmental variability in studies of ocean change biology, particularly in the design of multistressor experiments.

Optical backscattering by particles in Arctic seawater and relationships to particle mass concentration, size distribution, and bulk composition

Abstract: The magnitude and spectral shape of the optical backscattering coefficient of particles, b(bp)(lambda), 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)(lambda) 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)(lambda) and the slope of the particle size distribution in this environment.

Linking large‐scale climate variability with Arctica islandica shell growth and geochemistry in northern Norway

Abstract: 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 (δ18O) 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.

Nonlinear effects of body size and optical attenuation on Diel Vertical Migration by zooplankton

Abstract: We adopt a trait-based approach to explain Diel Vertical Migration (DVM) across a diverse assemblage of planktonic copepods, utilizing body size as a master trait. We find a reproducible pattern of body size-dependence of day and night depths occupied, and of DVM. Both the smallest surface-dwelling and the largest deeper-dwelling copepods refrain from migrations, while intermediate-sized individuals show pronounced DVM. This pattern apparently arises as a consequence of size-dependent predation risk. In the size classes of migratory copepods the amplitude of DVM is further modulated by optical attenuation in the ocean water column because increased turbidity decreases encounter rates with visually hunting predators. Long-term changes in the ocean optical environment are expected to alter the vertical distributions of many copepods and thus to affect predator-prey encounters as well as oceanic carbon export.

Feedback between sediment and light for seagrass: Where is it important?

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

Diazotroph derived nitrogen supports diatom growth in the South West Pacific: A quantitative study using nanoSIMS

Abstract: 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 N-15(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 N-15(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.