Hermann Werner Bange

The Baltic Sea, which is surrounded by countries with intensive industrial and agricultural activities, is affected by an excess of nitrogen causing large-scale eutrophication. Thus, the nitrogen cycle of the Baltic Sea has received considerable attention over the past five decades mainly due to the ongoing expansion of oxygen-free zones (anoxia), which was identified as a major threat already in the 1970s. Measures to reduce eutrophication have resulted in a decrease of nitrogen inputs since the 1980s, but the environmental status of the Baltic Sea is still far from good. The nitrogen cycle of the Baltic Sea is driven by various microbial processes, which occur to varying extent in both the water column and sediments under oxic and anoxic conditions. However, the overall nitrogen budget of the Baltic Sea is mainly controlled by microbial nitrogen fixation and denitrification in combination with riverine input, aerosol deposition, burial in sediments, and exchange with the North Sea. Other contributors to the nitrogen budget are minor, such as nitrous oxide emissions, or still uncertain, such as submarine groundwater discharge. In general, nitrogen flux estimates are associated with a high degree of uncertainties largely due to inadequate seasonal and spatial coverage of most measurements. To address the existing knowledge gaps and to decipher the processes and fluxes of the Baltic Sea’s nitrogen cycle in the 21st century, we suggest establishing time-series measurements and joint process studies at representative sites in the open and coastal Baltic Sea, including the sea-ice covered areas. These studies should combine microbial, molecular, ecological and biogeochemical methods across nano- to kilometre scales to obtain a better understanding of the process rates, their controlling factors and the microbial communities involved. Such results will help to improve physical, biogeochemical and ecosystem models and allow us to reduce the uncertainties in projections of the future nitrogen cycle of the Baltic Sea which can advance/help management measures for ecosystem restoration.

Many climate-active trace gases in the atmosphere are closely linked to production and consumption in the ocean, which are, in turn, influenced by the sea surface microlayer (SML). The SML is the upper most layer of the ocean with up to 1 mm thickness, often enriched in organics. Studies of trace gases in the SML aim to identify and quantify potential processes unique to the SML and to understand the SML's influence on the transfer between air and sea. Established sampling techniques of the SML (e.g., glass plate, mesh screen) are associated with high losses for the volatile trace gases. Despite the high losses, in this study we find that meaningful analysis of glass plate samples for the studied trace gases is possible. We experimentally determined the sampling efficiency for the short-lived trace gases dimethyl sulfide (DMS), isoprene, and carbon disulfide (CS₂). Water temperature and trace gas concentration were the main drivers for sampling efficiency variability, while salinity and the number of dips of the glass plate were not significant. The (physicochemical) effect of surface-active substances (surfactants), modelled by the artificial surfactant Triton X-100, could not finally be untangled. Although our results are consistent within our experiments, we do not quantify a sampling efficiency to correct individual measurements, as our experiments did not encompass the full suite of environmental parameters normally encountered in the field, particularly limited to cases of oversaturation. Instead, we discuss using the identified sampling efficiencies of 0.13 ± 0.01 (±standard error) for DMS and isoprene, and 0.12 ± 0.01 for CS₂ as thresholds to identify cases of enrichment in the SML. Future studies should extend to long-lived species (e.g., nitrous oxide, methane), encompass the full range of trace gas saturation, investigate a range of single surfactants as well as mixtures, include the effect of wind, and be repeated for the mesh screen. We hypothesize that a correction of individual measurements requires determining sampling efficiency as a function of environmental parameters, which were limited in the present study.

Nitrous oxide (N2O) is a greenhouse gas with a global warming potential ∼ 300 times that of carbon dioxide. Coastal areas are important sources of N2O to the atmosphere but the biogeochemical pathways of N2O production and consumption are not well understood. We measured the concentrations and nitrogen (N) and oxygen (O) stable isotopes (δ15N and δ18O) of N2O in the Baltic Sea to constrain the sources and sinks of N2O in this system. Further, we used the intramolecular 15N variation or the site preference (SP) as additional tracer. Samples were taken at 7 stations during a cruise with R/V Elisabeth Mann Borgese to the Eastern Gotland Basin (central Baltic Sea) in May/June 2019. The isotope signatures of N2O accumulation in the oxycline reflected production predominantly via ammonia oxidation. In the waters where hydrogen sulfide (H2S) was detected, active N2O consumption was implied by pronounced decrease in N2O levels in tandem with enrichments in δ15Nbulk, δ18O and SP. High site preference values of N2O observed in suboxic waters of the stations where H2S buildup was minimal to non-detectable point to the possibility of non-canonical denitrification pathways mediated by fungi or abiotically. A sedimentary source of N2O was observed in those stations, which resulted in a decoupling of the δ15Nbulk and δ18O of N2O. Our results reveal that transient oxygenation events have the potential to modify the N cycling within the oxic-anoxic interface even if for shorter time scales.

The gas transfer velocity k for the air-sea exchange of CO2 is often parameterized as a function of wind speed alone, as wind speed fundamentally controls turbulence at the air-sea interface and thus the flux across it. However, numerous other processes affect the air-sea gas exchange, such as the presence of surface-active substances (surfactants) directly at the interface, the so-called sea surface microlayer (SML). These processes are not explicitly accounted for in the wind speed-only parameterizations. Surfactants in the SML likely reduce k, potentially due to two effects. Firstly, the surfactants represent a physicochemical barrier at the interface, and secondly, they dampen the turbulence at the interface. Consequently, the presence of surfactants leads to lower gas transfer velocities than estimated from the wind speed-only parameterizations of k, especially since the SML can be stable up to medium high wind speeds. The mechanisms that control how exactly surfactants in the SML affect the air-sea gas exchange are, however, not yet fully understood. Therefore, it is important to measure air-sea gas exchange under various surfactant conditions to potentially include the SML effects in future parameterizations of k. During a research cruise to the Gotland Basin in the early summer of 2025, the direct air-sea flux of CO2 was measured using the eddy covariance method. This method is particularly well-suited to study the influence of surface processes on gas exchange, as it can determine k on timescales of 10 minutes and is therefore likely to resolve the variability in different surfactant states. In addition to the direct CO2 flux measurements, a range of other parameters influencing air-sea flux were also measured. In particular, the surfactants in the SML were sampled and analysed during the research cruise. Consequently, we investigate the behaviour of k under not only varying wind speeds, but now also under various surfactant states, including the presence of a surface slick.

Coastal areas within the Eastern South Atlantic Ocean are known hotspots for production and emissions of climate-relevant trace gases. Local circulation, the occurrence of upward tracer transport events through e.g. coastal upwelling and coastally-trapped waves, and a pronounced oxygen minimum zone are crucial in setting the overall emissions of nitrous oxide (N2O) towards the atmosphere. While previous studies quantified the magnitude of cross- and along-shelf gradients of N2O in the region, its main formation pathways, and its seasonal variability, to date it is unclear at what extent extreme events might affect N2O dynamics. Given the projected increase in frequency and severity of events such as storms and oceanic heat waves, which might temporarily, yet significantly modify environmental conditions under which N2O is produced and transported across the sediment-water-air interfaces, it is therefore critical to assess the role of such events on its distribution and emissions. In this study we combine physical, chemical and microbial observations gathered during two major expeditions in 2018 and 2023 to present evidence of a hitherto unseen enhancement in air-sea fluxes of N2O in association with storm events and mesoscale activity. We show that during periods of sustained winds off Walvis Bay at 23°S, water column mixing down to 100 m depth can lead to a two-fold increase in air-sea N2O fluxes driven by the transport of enriched, near-bottom waters towards the surface, which surpasses by far values observed during typical upwelling events. Observations across a mesoscale cyclonic eddy off Angola centered at 16⎼17°S (a rare feature which is thought to occur in average 2 times per year in the region), show that both extreme warming-driven outgassing at the sea surface and enhanced upward transport of N2O-enriched waters at the eddy core play a role in enhancing the overall emissions from waters otherwise thought to be mostly representative of open ocean conditions (i.e. in near equilibrium with the atmosphere). In this contribution, we discuss the main mechanisms by which these extreme events resulted in enhanced N2O air-sea fluxes and how they might impact current marine N2O emission estimates, which due to the lack of targeted observations, do not capture this source of variability.

Nitrous oxide (N₂O) is an important climate-relevant atmospheric trace gas. The open and coastal oceans are a major source for atmospheric N₂O. However, its production and consumption pathways in the ocean are not well-known and its emission estimates are associated with a high degree of uncertainty. Potential N₂O production pathways in the oxic surface ocean include microbial nitrification, release from phytoplankton and photochemodenitrification. In order to decipher the effect of a phytoplankton bloom on dissolved N₂O concentrations, N₂O was measured – for the first time – in the sea surface microlayer (SML, i.e. the upper 1 mm of the water column) and in the corresponding underlying water (ULW) during a mesocosm study with Jade Bay (coastal water, southern North Sea, Germany) water from 16 May to 16 June 2023. N₂O concentrations were slightly enriched in the SML compared to the ULW although the difference of the mean N₂O concentrations between the ULW and SML was statistically not significant. N₂O was supersaturated (100 %–157 %) in the ULW and SML during the course of the study which indicated an in-situ production of N₂O. N₂O in-situ production under the experimental conditions is most consistent with photochemodenitrification in combination with the release from phytoplankton, whereas microbial production of N₂O via nitrification appeared to be of minor importance. N₂O concentrations in both the ULW and the SML were remarkably constant over time and were apparently not affected by irradiation and a phytoplankton bloom which was triggered by nutrient additions. We therefore conclude that the N₂O in-situ sources were balanced by the release of N₂O to the atmosphere resulting in a steady state of the system. Our results indicate a potential important role of the SML for N₂O cycling in the surface ocean and its emissions to the atmosphere, which has been overlooked so far. Moreover, our results are in line with results from field studies which showed that phytoplankton blooms in the ocean do not result in temporarily enhanced N₂O concentrations in the ocean surface layer.

Many climate-active trace gases in the atmosphere are closely linked to production and consumption in the ocean, which are, in turn, influenced by the sea surface microlayer (SML). The SML is the upper most layer of the ocean with up to 1 mm thickness, often enriched in organics. Studies of trace gases in the SML aim to identify and quantify potential processes unique to the SML and to understand the SML's influence on the transfer between air and sea. Established sampling techniques of the SML (e.g., glass plate, mesh screen) are associated with high losses for the volatile trace gases. Despite the high losses, in this study we find that meaningful analysis of glass plate samples for trace gases is possible. We experimentally determined the sampling efficiency for the short-lived trace gases dimethyl sulphide (DMS), isoprene, and carbon disulphide (CS₂). Water temperature and trace gas concentration were the main drivers for sampling efficiency variability, while salinity and the number of dips of the glass plate were not significant. The effect of surfactants could not finally be untangled. Although our results are consistent, we do not quantify a sampling efficiency to correct individual measurements, as our experiments did not encompass the full suite of environmental parameters normally encountered in the field. Instead, we suggest to use 0.13 ± 0.01 (± standard error) for DMS and isoprene, and 0.12 ± 0.01 for CS₂ as thresholds to identify cases of net production in the SML. Future studies should extend to long-lived species (e.g., nitrous oxide, methane), include the effect of wind, and be repeated for the mesh screen. We hypothesize that a correction of individual measurements requires to determine sampling efficiency as a function of environmental parameters, for which the underlying physicochemical relationships need to be unraveled by increasing the parameter space studied here.

Many climate-active trace gases in the atmosphere are closely linked to production and consumption in the ocean, which are, in turn, influenced by the sea surface microlayer (SML). The SML is the upper most layer of the ocean with up to 1 mm thickness, often enriched in organics. Studies of trace gases in the SML aim to identify and quantify potential processes unique to the SML and to understand the SML's influence on the transfer between air and sea. Established sampling techniques of the SML (e.g., glass plate, mesh screen) are associated with high losses for the volatile trace gases. Despite the high losses, in this study we find that meaningful analysis of glass plate samples for trace gases is possible. We experimentally determined the sampling efficiency for the short-lived trace gases dimethyl sulphide (DMS), isoprene, and carbon disulphide (CS₂). Water temperature and trace gas concentration were the main drivers for sampling efficiency variability, while salinity and the number of dips of the glass plate were not significant. The effect of surfactants could not finally be untangled. Although our results are consistent, we do not quantify a sampling efficiency to correct individual measurements, as our experiments did not encompass the full suite of environmental parameters normally encountered in the field. Instead, we suggest to use 0.13 ± 0.01 (± standard error) for DMS and isoprene, and 0.12 ± 0.01 for CS₂ as thresholds to identify cases of net production in the SML. Future studies should extend to long-lived species (e.g., nitrous oxide, methane), include the effect of wind, and be repeated for the mesh screen. We hypothesize that a correction of individual measurements requires to determine sampling efficiency as a function of environmental parameters, for which the underlying physicochemical relationships need to be unraveled by increasing the parameter space studied here.

Nitrous oxide (N₂O) is an important climate-relevant atmospheric trace gas. The open and coastal oceans are a major source for atmospheric N₂O. However, its production and consumption pathways in the ocean are not well-known and its emissions estimates are associated with a high degree of uncertainty. Potential N₂O production pathways in the oxic surface ocean include microbial nitrification, release from phytoplankton and photochemodenitrification. In order to decipher the effect of a phytoplankton bloom on dissolved N₂O concentrations, N₂O was measured – for the first time – in the sea surface microlayer (SML, i.e. the upper 1 mm of the water column) and in the corresponding underlying water (ULW) during a mesocosm study with Jade Bay (southern North Sea) water from 16 May to 16 June 2023. N₂O concentrations were slightly enriched in the SML compared to the ULW although the difference of the mean N₂O concentrations between the ULW and SML was statistically not significant. However, the enrichment of N₂O in the SML was most probably underestimated due to the loss of N₂O during sampling with the glass plate method. N₂O was supersaturated (100 %–157 %) in the ULW and SML during the course of the study which indicated an in-situ production of N₂O. N₂O in-situ production was most probably driven by photochemodenitrification in combination with the release from phytoplankton whereas microbial production of N₂O via nitrification appeared to be of minor importance. N₂O concentrations in both the ULW and the SML were remarkably constant over time and were apparently not affected by irradiation and a phytoplankton bloom which was triggered by nutrient additions. We therefore conclude that the N₂O in-situ sources were balanced by the release of N₂O to the atmosphere resulting in a steady state of the system. Our results indicate that the role of the SML for N₂O cycling in the surface ocean and its emissions to the atmosphere has been overlooked so far. Moreover, our results are in line with results from field studies which showed that phytoplankton blooms in the ocean do not result in temporarily enhanced N₂O concentrations in the ocean surface layer.

Nitrous oxide (N₂O) is an important climate-relevant atmospheric trace gas. The open and coastal oceans are a major source for atmospheric N₂O. However, its production and consumption pathways in the ocean are not well-known and its emissions estimates are associated with a high degree of uncertainty. Potential N₂O production pathways in the oxic surface ocean include microbial nitrification, release from phytoplankton and photochemodenitrification. In order to decipher the effect of a phytoplankton bloom on dissolved N₂O concentrations, N₂O was measured – for the first time – in the sea surface microlayer (SML, i.e. the upper 1 mm of the water column) and in the corresponding underlying water (ULW) during a mesocosm study with Jade Bay (southern North Sea) water from 16 May to 16 June 2023. N₂O concentrations were slightly enriched in the SML compared to the ULW although the difference of the mean N₂O concentrations between the ULW and SML was statistically not significant. However, the enrichment of N₂O in the SML was most probably underestimated due to the loss of N₂O during sampling with the glass plate method. N₂O was supersaturated (100 %–157 %) in the ULW and SML during the course of the study which indicated an in-situ production of N₂O. N₂O in-situ production was most probably driven by photochemodenitrification in combination with the release from phytoplankton whereas microbial production of N₂O via nitrification appeared to be of minor importance. N₂O concentrations in both the ULW and the SML were remarkably constant over time and were apparently not affected by irradiation and a phytoplankton bloom which was triggered by nutrient additions. We therefore conclude that the N₂O in-situ sources were balanced by the release of N₂O to the atmosphere resulting in a steady state of the system. Our results indicate that the role of the SML for N₂O cycling in the surface ocean and its emissions to the atmosphere has been overlooked so far. Moreover, our results are in line with results from field studies which showed that phytoplankton blooms in the ocean do not result in temporarily enhanced N₂O concentrations in the ocean surface layer.