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A large part of the anthropogenic emissions of carbon dioxide have been absorbed by the oceans. However, many Swedish coastal areas are currently affected by eutrophication, making them a source of greenhouse gases, mainly in the form of methane. Reducing eutrophication is crucial for limiting methane emissions and thus mitigating climate change.

Coastal ecosystems dominate oceanic methane (CH4) emissions. However, there is limited knowledge about how biotic interactions between infauna and aerobic methanotrophs (i.e. CH4 oxidizing bacteria) drive the spatial–temporal dynamics of these emissions. Here, we investigated the role of meio- and macrofauna in mediating CH4 sediment–water fluxes and aerobic methanotrophic activity that can oxidize significant portions of CH4. We show that macrofauna increases CH4 fluxes by enhancing vertical solute transport through bioturbation, but this effect is somewhat offset by high meiofauna abundance. The increase in CH4 flux reduces CH4 pore-water availability, resulting in lower abundance and activity of aerobic methanotrophs, an effect that counterbalances the potential stimulation of these bacteria by higher oxygen flux to the sediment via bioturbation. These findings indicate that a larger than previously thought portion of CH4 emissions from coastal ecosystems is due to faunal activity and multiple complex interactions with methanotrophs.

Sea-ice dynamics can affect carbon cycling in polar oceans, with sea-ice ikaite acting as a potentially important carbon pump. However, there is no large-scale direct field evidence to support this. Here we used a unique data set that combined continuous measurements of atmospheric and water CO2 concentrations with water chemistry data collected over 1,200 km along the East Siberian Sea, the widest Arctic shelf sea. Our results reveal large spatial heterogeneity of sea-ice ikaite contents, which directly interact with carbonate chemistry in the water column. Our findings demonstrate that the CO2 drawdown by sea-ice ikaite dissolution could be as important as that by primary production. We suggest that the role of ikaite in regulating the seasonal carbon cycle on a regional scale could be more important than we previously thought. Effects of the warmer climate on sea ice loss might also play a role in the ikaite inventory.

Coastal environments are a major source of marine methane in the atmosphere. Eutrophication and deoxygenation have the potential to amplify the coastal methane emissions. Here, we investigate methane dynamics in the eutrophic Stockholm Archipelago. We cover a range of sites with contrasting water column redox conditions and rates of organic matter degradation, with the latter reflected by the depth of the sulfate-methane transition zone (SMTZ) in the sediment. We find the highest benthic release of methane (2.2-8.6 mmol m-2 d-1) at sites where the SMTZ is located close to the sediment-water interface (2-10 cm). A large proportion of methane is removed in the water column via aerobic or anaerobic microbial pathways. At many locations, water column methane is highly depleted in 13C, pointing toward substantial bubble dissolution. Calculated and measured rates of methane release to the atmosphere range from 0.03 to 0.4 mmol m-2 d-1 and from 0.1 to 1.7 mmol m-2 d-1, respectively, with the highest fluxes at locations with a shallow SMTZ and anoxic and sulfidic bottom waters. Taken together, our results show that sites suffering most from both eutrophication and deoxygenation are hotspots of coastal marine methane emissions.

Stricter national regulations on fertilisation and improved use of manure in agriculture, as well as set-aside schemes, do have measurable effects on the riverine loads of nitrogen to the Baltic Sea, although it can take up to a decade to reach full depletion of legacies in agricultural soils. However, reaching the environmental goals in a cost-effective way, requires more targeted measures to areas where the risk for leaching is high.

Stricter national regulations on fertilisation and improved use of manure in agriculture, as well as set-aside schemes, do have measurable effects on the riverine loads of nitrogen to the Baltic Sea, although it can take up to a decade to reach full depletion of legacies in agricultural soils. However, reaching the environmental goals in a cost-effective way, requires more targeted measures to areas where the risk for leaching is high.

Stricter national regulations on fertilisation and improved use of manure in agriculture, as well as set-aside schemes, do have measurable effects on the riverine loads of nitrogen to the Baltic Sea, although it can take up to a decade to reach full depletion of legacies in agricultural soils. However, reaching the environmental goals in a cost-effective way, requires more targeted measures to areas where the risk for leaching is high.

Coastal environments are a major source of marine methane in the atmosphere. Eutrophication and deoxygenation have the potential to amplify the coastal methane emissions. Here, we investigate methane dynamics in the eutrophic Stockholm Archipelago. We cover a range of sites with contrasting water column redox conditions and rates of organic matter degradation, with the latter reflected by the depth of the sulfate–methane transition zone (SMTZ) in the sediment. We find the highest benthic release of methane (2.2–8.6 mmol m–2 d–1) at sites where the SMTZ is located close to the sediment–water interface (2–10 cm). A large proportion of methane is removed in the water column via aerobic or anaerobic microbial pathways. At many locations, water column methane is highly depleted in 13C, pointing toward substantial bubble dissolution. Calculated and measured rates of methane release to the atmosphere range from 0.03 to 0.4 mmol m–2 d–1 and from 0.1 to 1.7 mmol m–2 d–1, respectively, with the highest fluxes at locations with a shallow SMTZ and anoxic and sulfidic bottom waters. Taken together, our results show that sites suffering most from both eutrophication and deoxygenation are hotspots of coastal marine methane emissions.

A large part of the anthropogenic emissions of carbon dioxide have been absorbed by the oceans. However, many Swedish coastal areas are currently affected by eutrophication, making them a source of greenhouse gases, mainly in the form of methane. Reducing eutrophication is crucial for limiting methane emissions and thus mitigating climate change.

A large part of the anthropogenic emissions of carbon dioxide have been absorbed by the oceans. However, many Swedish coastal areas are currently affected by eutrophication, making them a source of greenhouse gases, mainly in the form of methane. Reducing eutrophication is crucial for limiting methane emissions and thus mitigating climate change.

A large part of the anthropogenic emissions of carbon dioxide have been absorbed by the oceans. However, many Swedish coastal areas are currently affected by eutrophication, making them a source of greenhouse gases, mainly in the form of methane. Reducing eutrophication is crucial for limiting methane emissions and thus mitigating climate change.

En stor del av de mänskligt orsakade utsläppen av koldioxid har tagits upp av haven. Men många av de svenska kusterna är idag påverkade av övergödning, vilket gör dem till en källa till växthusgaser, främst i form av metan. Att minska övergödningen är avgörande för att begränsa metanutsläppen och därmed motverka klimatförändringarna.

Intensive agriculture has been linked to increased nitrogen loads and adverse effects on downstream aquatic ecosystems. Sustained large net nitrogen surpluses have been shown in several contexts to form legacies in soil or waters, which delay the effects of reduction measures. In this study, detailed land use and agricultural statistics were used to reconstruct the annual nitrogen surpluses in three agriculture-dominated watersheds of Denmark (600-2700 km2) with well-drained loamy soils. These surpluses and long-term hydrological records were used as inputs to the process model ELEMeNT to quantify the nitrogen stores and fluxes for 1920-2020. A multi-objective calibration using timeseries of river nitrate loads, as well as other non-conventional data sources, allowed to explore the potential of these different data to constrain the nitrogen cycling model. We found the flux-weighted nitrate concentrations in the root zone percolate below croplands, a dataset not commonly used in calibrating watershed models, to be critical in reducing parameter uncertainty. Groundwater nitrate legacies built up in all three studied watersheds during 1950-1990 corresponding to & SIM;2% of the surplus (or & SIM;1 kg N ha yr-1) before they went down at a similar rate during 1990-2015. Over the same periods active soil nitrogen legacies first accumulated by approximately 10% of the surplus (& SIM;5 kg N ha yr-1), before undergoing a commensurate reduction. Both legacies appear to have been the drivers of hysteresis in the diffuse load at the catchments' outlet and hindrances to reaching water quality goals. Results indicate that the low cropland surpluses enforced during 2008-2015 had a larger impact on the diffuse river loads than the European Union's untargeted grass set-aside policy of 1993-2008. Collectively, the measures of 1990-2015 are estimated to have reset the diffuse load regimes of the watersheds back to the situation prevailing in the 1960s.

Coastal ecosystems can efficiently remove carbon dioxide (CO2) from the atmosphere and are thus promoted for nature-based climate change mitigation. Natural methane (CH4) emissions from these ecosystems may counterbalance atmospheric CO2 uptake. Still, knowledge of mechanisms sustaining such CH4 emissions and their contribution to net radiative forcing remains scarce for globally prevalent macroalgae, mixed vegetation, and surrounding depositional sediment habitats. Here we show that these habitats emit CH4 in the range of 0.1 – 2.9 mg CH4 m−2 d−1 to the atmosphere, revealing in situ CH4 emissions from macroalgae that were sustained by divergent methanogenic archaea in anoxic microsites. Over an annual cycle, CO2-equivalent CH4 emissions offset 28 and 35% of the carbon sink capacity attributed to atmospheric CO2 uptake in the macroalgae and mixed vegetation habitats, respectively, and augment net CO2 release of unvegetated sediments by 57%. Accounting for CH4 alongside CO2 sea-air fluxes and identifying the mechanisms controlling these emissions is crucial to constrain the potential of coastal ecosystems as net atmospheric carbon sinks and develop informed climate mitigation strategies.

Coastal ecosystems can efficiently remove carbon dioxide (CO2) from the atmosphere and are thus promoted for nature-based climate change mitigation. Natural methane (CH4) emissions from these ecosystems may counterbalance atmospheric CO2 uptake. Still, knowledge of mechanisms sustaining such CH4 emissions and their contribution to net radiative forcing remains scarce for globally prevalent macroalgae, mixed vegetation, and surrounding depositional sediment habitats. Here we show that these habitats emit CH4 in the range of 0.1 – 2.9 mg CH4 m−2 d−1 to the atmosphere, revealing in situ CH4 emissions from macroalgae that were sustained by divergent methanogenic archaea in anoxic microsites. Over an annual cycle, CO2-equivalent CH4 emissions offset 28 and 35% of the carbon sink capacity attributed to atmospheric CO2 uptake in the macroalgae and mixed vegetation habitats, respectively, and augment net CO2 release of unvegetated sediments by 57%. Accounting for CH4 alongside CO2 sea-air fluxes and identifying the mechanisms controlling these emissions is crucial to constrain the potential of coastal ecosystems as net atmospheric carbon sinks and develop informed climate mitigation strategies.

Coastal ecosystems can efficiently remove carbon dioxide (CO2) from the atmosphere and are thus promoted for nature-based climate change mitigation. Natural methane (CH4) emissions from these ecosystems may counterbalance atmospheric CO2 uptake. Still, knowledge of mechanisms sustaining such CH4 emissions and their contribution to net radiative forcing remains scarce for globally prevalent macroalgae, mixed vegetation, and surrounding depositional sediment habitats. Here we show that these habitats emit CH4 in the range of 0.1 – 2.9 mg CH4 m−2 d−1 to the atmosphere, revealing in situ CH4 emissions from macroalgae that were sustained by divergent methanogenic archaea in anoxic microsites. Over an annual cycle, CO2-equivalent CH4 emissions offset 28 and 35% of the carbon sink capacity attributed to atmospheric CO2 uptake in the macroalgae and mixed vegetation habitats, respectively, and augment net CO2 release of unvegetated sediments by 57%. Accounting for CH4 alongside CO2 sea-air fluxes and identifying the mechanisms controlling these emissions is crucial to constrain the potential of coastal ecosystems as net atmospheric carbon sinks and develop informed climate mitigation strategies.

Diapycnal mixing impacts vertical transport rates of salt, heat, and other dissolved substances, essential for the overturning circulation and ecosystem functioning in marine systems. While most studies have focused on mixing induced by individual obstacles in tidal flows, we investigate the net effect of non-tidal flow over multiple small-scale (<1 km) bathymetric features penetrating a strongly-stratified density interface in a coastal region. We combine high-resolution broadband acoustic observations of turbulence microstructure with traditional shear microstructure profiling, to resolve the variability and intermittency of stratified turbulence related to the rough bathymetry. Scale analysis and acoustic imaging suggest that underlying mixing mechanisms are related to topographic wake eddies and breaking internal waves. Depth averaged dissipation rates (1.1 × 10−7 Wkg−1) and turbulent vertical diffusivities (7 × 10−4 m2s−1) in the halocline exceed reference values by two orders of magnitude. Our study emphasizes the importance of rough small-scale bathymetric features for the vertical transport of salt in coastal areas.

It is estimated that up to half of global methane (CH4) emissions are derived from microbial processes in aquatic ecosystems. However, it is not fully understood which factors explain the spatial and temporal variability of these emissions. For example, light has previously been shown to both inhibit and stimulate aerobic methane-oxidizing bacteria (i.e., methanotrophs) in the water column. These contrasting results indicate that the mechanisms that light has on CH4 oxidation are not yet clearly known, even less so for benthic aerobic methanotrophs. Here, we tested whether light reaching the seafloor can inhibit methanotrophic activity on the sediment surface. We sampled and distributed over 40 intact sediment cores from two coastal sites (illuminated 10 m, and a dark site at 33 m water depth) into 0, 50, and 100 PAR light treatments. After 10 days, we found no difference between treatments for each site in pore-water CH4 concentrations, relative abundance of aerobic methanotrophs, or the number of RNA transcripts related to methane oxidation. Our results suggest that light attenuation in coastal waters does not significantly affect aerobic methanotrophs in coastal sediments.

It is estimated that up to half of global methane (CH4) emissions are derived from microbial processes in aquatic ecosystems. However, it is not fully understood which factors explain the spatial and temporal variability of these emissions. For example, light has previously been shown to both inhibit and stimulate aerobic methane-oxidizing bacteria (i.e., methanotrophs) in the water column. These contrasting results indicate that the mechanisms that light has on CH4 oxidation are not yet clearly known, even less so for benthic aerobic methanotrophs. Here, we tested whether light reaching the seafloor can inhibit methanotrophic activity on the sediment surface. We sampled and distributed over 40 intact sediment cores from two coastal sites (illuminated 10 m, and a dark site at 33 m water depth) into 0, 50, and 100 PAR light treatments. After 10 days, we found no difference between treatments for each site in pore-water CH4 concentrations, relative abundance of aerobic methanotrophs, or the number of RNA transcripts related to methane oxidation. Our results suggest that light attenuation in coastal waters does not significantly affect aerobic methanotrophs in coastal sediments.

It is estimated that up to half of global methane (CH4) emissions are derived from microbial processes in aquatic ecosystems. However, it is not fully understood which factors explain the spatial and temporal variability of these emissions. For example, light has previously been shown to both inhibit and stimulate aerobic methane-oxidizing bacteria (i.e., methanotrophs) in the water column. These contrasting results indicate that the mechanisms that light has on CH4 oxidation are not yet clearly known, even less so for benthic aerobic methanotrophs. Here, we tested whether light reaching the seafloor can inhibit methanotrophic activity on the sediment surface. We sampled and distributed over 40 intact sediment cores from two coastal sites (illuminated 10 m, and a dark site at 33 m water depth) into 0, 50, and 100 PAR light treatments. After 10 days, we found no difference between treatments for each site in pore-water CH4 concentrations, relative abundance of aerobic methanotrophs, or the number of RNA transcripts related to methane oxidation. Our results suggest that light attenuation in coastal waters does not significantly affect aerobic methanotrophs in coastal sediments.

En stor del av de mänskligt orsakade utsläppen av koldioxid har tagits upp av haven. Men många av de svenska kusterna är idag påverkade av övergödning, vilket gör dem till en källa till växthusgaser, främst i form av metan. Att minska övergödningen är avgörande för att begränsa metanutsläppen och därmed motverka klimatförändringarna.

En stor del av de mänskligt orsakade utsläppen av koldioxid har tagits upp av haven. Men många av de svenska kusterna är idag påverkade av övergödning, vilket gör dem till en källa till växthusgaser, främst i form av metan. Att minska övergödningen är avgörande för att begränsa metanutsläppen och därmed motverka klimatförändringarna.

En stor del av de mänskligt orsakade utsläppen av koldioxid har tagits upp av haven. Men många av de svenska kusterna är idag påverkade av övergödning, vilket gör dem till en källa till växthusgaser, främst i form av metan. Att minska övergödningen är avgörande för att begränsa metanutsläppen och därmed motverka klimatförändringarna.

En stor del av de mänskligt orsakade utsläppen av koldioxid har tagits upp av haven. Men många av de svenska kusterna är idag påverkade av övergödning, vilket gör dem till en källa till växthusgaser, främst i form av metan. Att minska övergödningen är avgörande för att begränsa metanutsläppen och därmed motverka klimatförändringarna.

It is estimated that up to half of global methane (CH4) emissions are derived from microbial processes in aquatic ecosystems. However, it is not fully understood which factors explain the spatial and temporal variability of these emissions. For example, light has previously been shown to both inhibit and stimulate aerobic methane-oxidizing bacteria (i.e., methanotrophs) in the water column. These contrasting results indicate that the mechanisms that light has on CH4 oxidation are not yet clearly known, even less so for benthic aerobic methanotrophs. Here, we tested whether light reaching the seafloor can inhibit methanotrophic activity on the sediment surface. We sampled and distributed over 40 intact sediment cores from two coastal sites (illuminated 10 m, and a dark site at 33 m water depth) into 0, 50, and 100 PAR light treatments. After 10 days, we found no difference between treatments for each site in pore-water CH4 concentrations, relative abundance of aerobic methanotrophs, or the number of RNA transcripts related to methane oxidation. Our results suggest that light attenuation in coastal waters does not significantly affect aerobic methanotrophs in coastal sediments.

It is estimated that up to half of global methane (CH4) emissions are derived from microbial processes in aquatic ecosystems. However, it is not fully understood which factors explain the spatial and temporal variability of these emissions. For example, light has previously been shown to both inhibit and stimulate aerobic methane-oxidizing bacteria (i.e., methanotrophs) in the water column. These contrasting results indicate that the mechanisms that light has on CH4 oxidation are not yet clearly known, even less so for benthic aerobic methanotrophs. Here, we tested whether light reaching the seafloor can inhibit methanotrophic activity on the sediment surface. We sampled and distributed over 40 intact sediment cores from two coastal sites (illuminated 10 m, and a dark site at 33 m water depth) into 0, 50, and 100 PAR light treatments. After 10 days, we found no difference between treatments for each site in pore-water CH4 concentrations, relative abundance of aerobic methanotrophs, or the number of RNA transcripts related to methane oxidation. Our results suggest that light attenuation in coastal waters does not significantly affect aerobic methanotrophs in coastal sediments.

Coastal ecosystems can efficiently remove carbon dioxide (CO2) from the atmosphere and are thus promoted for nature-based climate change mitigation. Natural methane (CH4) emissions from these ecosystems may counterbalance atmospheric CO2 uptake. Still, knowledge of mechanisms sustaining such CH4 emissions and their contribution to net radiative forcing remains scarce for globally prevalent macroalgae, mixed vegetation, and surrounding depositional sediment habitats. Here we show that these habitats emit CH4 in the range of 0.1 – 2.9 mg CH4 m−2 d−1 to the atmosphere, revealing in situ CH4 emissions from macroalgae that were sustained by divergent methanogenic archaea in anoxic microsites. Over an annual cycle, CO2-equivalent CH4 emissions offset 28 and 35% of the carbon sink capacity attributed to atmospheric CO2 uptake in the macroalgae and mixed vegetation habitats, respectively, and augment net CO2 release of unvegetated sediments by 57%. Accounting for CH4 alongside CO2 sea-air fluxes and identifying the mechanisms controlling these emissions is crucial to constrain the potential of coastal ecosystems as net atmospheric carbon sinks and develop informed climate mitigation strategies.

Coastal ecosystems can efficiently remove carbon dioxide (CO2) from the atmosphere and are thus promoted for nature-based climate change mitigation. Natural methane (CH4) emissions from these ecosystems may counterbalance atmospheric CO2 uptake. Still, knowledge of mechanisms sustaining such CH4 emissions and their contribution to net radiative forcing remains scarce for globally prevalent macroalgae, mixed vegetation, and surrounding depositional sediment habitats. Here we show that these habitats emit CH4 in the range of 0.1 – 2.9 mg CH4 m−2 d−1 to the atmosphere, revealing in situ CH4 emissions from macroalgae that were sustained by divergent methanogenic archaea in anoxic microsites. Over an annual cycle, CO2-equivalent CH4 emissions offset 28 and 35% of the carbon sink capacity attributed to atmospheric CO2 uptake in the macroalgae and mixed vegetation habitats, respectively, and augment net CO2 release of unvegetated sediments by 57%. Accounting for CH4 alongside CO2 sea-air fluxes and identifying the mechanisms controlling these emissions is crucial to constrain the potential of coastal ecosystems as net atmospheric carbon sinks and develop informed climate mitigation strategies.

Location, specific topography, and hydrographic setting together with climate change and strong anthropogenic pressure are the main factors shaping the biogeochemical functioning and thus also the ecological status of the Baltic Sea. The recent decades have brought significant changes in the Baltic Sea. First, the rising nutrient loads from land in the second half of the 20th century led to eutrophication and spreading of hypoxic and anoxic areas, for which permanent stratification of the water column and limited ventilation of deep-water layers made favourable conditions. Since the 1980s the nutrient loads to the Baltic Sea have been continuously decreasing. This, however, has so far not resulted in significant improvements in oxygen availability in the deep regions, which has revealed a slow response time of the system to the reduction of the land-derived nutrient loads. Responsible for that is the low burial efficiency of phosphorus at anoxic conditions and its remobilization from sediments when conditions change from oxic to anoxic. This results in a stoichiometric excess of phosphorus available for organic-matter production, which promotes the growth of N2-fixing cyanobacteria and in turn supports eutrophication. This assessment reviews the available and published knowledge on the biogeochemical functioning of the Baltic Sea. In its content, the paper covers the aspects related to changes in carbon, nitrogen, and phosphorus (C, N, and P) external loads, their transformations in the coastal zone, changes in organic-matter production (eutrophication) and remineralization (oxygen availability), and the role of sediments in burial and turnover of C, N, and P. In addition to that, this paper focuses also on changes in the marine CO2 system, the structure and functioning of the microbial community, and the role of contaminants for biogeochemical processes. This comprehensive assessment allowed also for identifying knowledge gaps and future research needs in the fi

Location, specific topography, and hydrographic setting together with climate change and strong anthropogenic pressure are the main factors shaping the biogeochemical functioning and thus also the ecological status of the Baltic Sea. The recent decades have brought significant changes in the Baltic Sea. First, the rising nutrient loads from land in the second half of the 20th century led to eutrophication and spreading of hypoxic and anoxic areas, for which permanent stratification of the water column and limited ventilation of deep-water layers made favourable conditions. Since the 1980s the nutrient loads to the Baltic Sea have been continuously decreasing. This, however, has so far not resulted in significant improvements in oxygen availability in the deep regions, which has revealed a slow response time of the system to the reduction of the land-derived nutrient loads. Responsible for that is the low burial efficiency of phosphorus at anoxic conditions and its remobilization from sediments when conditions change from oxic to anoxic. This results in a stoichiometric excess of phosphorus available for organic-matter production, which promotes the growth of N2-fixing cyanobacteria and in turn supports eutrophication. This assessment reviews the available and published knowledge on the biogeochemical functioning of the Baltic Sea. In its content, the paper covers the aspects related to changes in carbon, nitrogen, and phosphorus (C, N, and P) external loads, their transformations in the coastal zone, changes in organic-matter production (eutrophication) and remineralization (oxygen availability), and the role of sediments in burial and turnover of C, N, and P. In addition to that, this paper focuses also on changes in the marine CO2 system, the structure and functioning of the microbial community, and the role of contaminants for biogeochemical processes. This comprehensive assessment allowed also for identifying knowledge gaps and future research needs in the field

The occurrence of regime shifts in marine ecosystems has important implications for environmental legislation that requires setting reference levels and targets of quantitative restoration outcomes. The Baltic Sea ecosystem has undergone large changes in the 20(th) century related to anthropogenic pressures and climate variability, which have caused ecosystem reorganization. Here, we compiled historical information and identified relationships in our dataset using multivariate statistics and modeling across 31 biotic and abiotic variables from 1925 to 2005 in the Central Baltic Sea. We identified a series of ecosystem regime shifts in the 1930s, 1970s, and at the end of the 1980s/beginning of the 1990s. In the long term, the Central Baltic Sea showed a regime shift from a benthic to pelagic-dominated state. Historically, benthic components played a significant role in trophic transfer, while in the more recent productive system pelagic-benthic coupling was weak and pelagic components dominated. Our analysis shows that for the entire time period, productivity, climate, and hydrography mainly affected the functioning of the food web, whereas fishing became important more recently. Eutrophication had far-reaching direct and indirect impacts from a long-term perspective and changed not only the trophic state of the system but also affected higher trophic levels. Our study also suggests a switch in regulatory drivers from salinity to oxygen. The reference ecosystem identified in our analysis may guide the establishment of an ecosystem state baseline and threshold values for ecosystem state indicators of the Central Baltic Sea.

Hypoxia is presently seen as the principal driver behind the decline of the former dominating Eastern Baltic cod stock (EBC; Gadus morhua). It has been proposed that both worsening conditions for reproduction and lower individual growth, condition, and survival are linked to hypoxia. Here, we elucidate the ecological envelope of EBC in terms of salinity stratification, oxygen content, and benthic animal biomasses, and how it has affected EBC productivity over time. The spawning conditions started deteriorating in the Gotland Deep in the 1950s due to oxygen depletion. In contrast, in the Bornholm Basin, hydrographic conditions have remained unchanged over the last 60 years. Indeed, the current extent of both well-oxygenated areas and the frequency of hypoxia events do not differ substantially from periods with high EBC productivity in the 1970s–1980s. Furthermore, oxygenated and therefore potentially suitable feeding areas are abundant in all parts of the Baltic Sea, and our novel analysis provides no evidence of a reduction in benthic food sources for EBC over the last 30 years. We find that while reproduction failure is intricately linked to hydrographic dynamics, a relationship between the spread of hypoxia and the decline in EBC productivity during the last decades cannot be substantiated.

Stratified oceanic turbulence is strongly intermittent in time and space, and therefore generally underresolved by currently available in situ observational approaches. A promising tool to at least partly overcome this constraint are broadband acoustic observations of turbulent microstructure that have the potential to provide mixing parameters at orders of magnitude higher resolution compared to conventional approaches. Here, we discuss the applicability, limitations, and measurement uncertainties of this approach for some prototypical turbulent flows (stratified shear layers, turbulent flow across a sill), based on a comparison of broadband acoustic observations and data from a free-falling turbulence microstructure profiler. We find that broadband acoustics are able to provide a quantitative description of turbulence energy dissipation in stratified shear layers (correlation coefficient r = 0.84) if the stratification parameters required by the method are carefully preprocessed. Essential components of our suggested preprocessing algorithm are 1) a vertical low-pass filtering of temperature and salinity profiles at a scale slightly larger than the Ozmidov length scale of turbulence and 2) an automated elimination of weakly stratified layers according to a gradient threshold criterion. We also show that in weakly stratified conditions, the acoustic approach may yield acceptable results if representative averaged vertical temperature and salinity gradients rather than local gradients are used. Our findings provide a step toward routine turbulence measurements in the upper ocean from moving vessels by combining broadband acoustics with in situ CTD profiles.

Explaining the recent decline of eastern Baltic cod (EBC) remains scientifically challenging. Brander proposes in a comment to Svedäng et al.that the observed trend in oxygen in SD 25 supports the idea that juvenile cod are balancing the physiological cost of living under mild hypoxiaby offsetting the risk of being eaten by diving seals and cormorants in shallower water with more oxygen. There are a number of objections tothis conjecture, besides the fact that supporting observations are missing. Hence, it is difficult to reconcile the long-term development of EBCunder varying oxygen conditions with the hypothesis that a small reduction in oxygen content can explain the current strong and uniform declinein growth observed in the entire southern Baltic Sea.

The occurrence of regime shifts in marine ecosystems has important implications for environmental legislation that requires setting reference levels and targets of quantitative restoration outcomes. The Baltic Sea ecosystem has undergone large changes in the 20th century related to anthropogenic pressures and climate variability, which have caused ecosystem reorganization. Here, we compiled historical information and identified relationships in our dataset using multivariate statistics and modeling across 31 biotic and abiotic variables from 1925 to 2005 in the Central Baltic Sea. We identified a series of ecosystem regime shifts in the 1930s, 1970s, and at the end of the 1980s/beginning of the 1990s. In the long term, the Central Baltic Sea showed a regime shift from a benthic to pelagic-dominated state. Historically, benthic components played a significant role in trophic transfer, while in the more recent productive system pelagic–benthic coupling was weak and pelagic components dominated. Our analysis shows that for the entire time period, productivity, climate, and hydrography mainly affected the functioning of the food web, whereas fishing became important more recently. Eutrophication had far-reaching direct and indirect impacts from a long-term perspective and changed not only the trophic state of the system but also affected higher trophic levels. Our study also suggests a switch in regulatory drivers from salinity to oxygen. The “reference ecosystem” identified in our analysis may guide the establishment of an ecosystem state baseline and threshold values for ecosystem state indicators of the Central Baltic Sea.

This study presents an assessment of benthic nutrient regeneration and its role for the nutrient budget of the outer Laptev and East Siberian shelf sea. Porewater profiles of the major nutrients dissolved silica (DSi), dissolved inorganic nitrogen (DIN), and dissolved inorganic phosphate (DIP) as well as total dissolved iron (DFe) were evaluated with a one-dimensional reaction transport model to derive net reaction rates and benthic nutrient fluxes from shelf and slope 16 stations. Integrated over the shelf area the benthic fluxes of DSi, DIN, and DIP were found to be 7.1, 1.2, and 0.5 Gmol/year in the Laptev Sea and 29.8, 9.5, and 2.8 Gmol/year in East Siberian Sea, respectively. A comparison of the ratios of the benthic nutrient fluxes with marine and riverine inputs and Arctic plankton stoichiometry indicate substantial benthic nitrogen loss likely due to denitrification relative to DIP and DSi. Our benthic flux estimation is likely a low estimate of benthic nutrient fluxes considering potentially higher regeneration rates of nutrients from more productive, bioturbated near-shore sediments. The estimate emphasizes the role of benthic nutrient fluxes by returning nutrients with a fundamentally different stoichiometry to bottom waters from that of Arctic marine phytoplankton, riverine sources, and open water inflow. With a simple box model, we provide a snapshot of today's nutrient budget in the two seas and estimate that about 10%-20% of nutrients required by primary production are derived from sediments. This proportion is expected to increase for a future warmer Arctic continental shelf in response to increasing primary production.

In this paper, we investigate the potential gains in cost-effectiveness from changing the spatial scale at which nutrient reduction targets are set for the Baltic Sea, with particular focus on nutrient loadings from agriculture. The costs of achieving loading reductions are compared across five levels of spatial scale, namely the entire Baltic Sea; the marine basin level; the country level; the watershed level; and the grid square level. A novel highly-disaggregated model, which represents decreases in agricultural profits, changes in root zone N concentrations and transport to the Baltic Sea is used. The model includes 14 Baltic Sea marine basins, 14 countries, 117 watersheds and 19,023 10-by-10 km grid squares. The main result which emerges is that there is a large variation in the total cost of the program depending on the spatial scale of targeting: for example, for a 40% reduction in loads, the costs of a Baltic Sea-wide target is nearly three times lower than targets set at the smallest level of spatial scale (grid square). These results have important implications for both domestic and international policy design for achieving water quality improvements where non-point pollution is a key stressor of water quality.

Food webs are central entities mediating processes and external pressures in marine ecosystems. They are essential to understand and predict ecosystem dynamics and provision of ecosystem services. Paradoxically, utilization of food web knowledge in marine environmental conservation and resource management is limited. To better understand the use of knowledge and barriers to incorporation in management, we assess its application related to the management of eutrophication, chemical contamination, fish stocks, and non-indigenous species. We focus on the Baltic, a severely impacted, but also intensely studied and actively managed semi-enclosed sea. Our assessment shows food web processes playing a central role in all four areas, but application varies strongly, from formalized integration in management decisions, to support in selecting indicators and setting threshold values, to informal knowledge explaining ecosystem dynamics and management performance. Barriers for integration are complexity of involved ecological processes and that management frameworks are not designed to handle such information. We provide a categorization of the multi-faceted uses of food web knowledge and benefits of future incorporation in management, especially moving towards ecosystem-based approaches as guiding principle in present marine policies and directives. We close with perspectives on research needs to support this move considering global and regional change.

Published 9 May. QC 20210517