Dynamics of N2O under coastal hypoxia: the case of Lake Grevelingen (The Netherlands) 

Coastal margins play a crucial role for greenhouse gases (GHG) budgets because biological cycling and sediment-water-air exchanges are more intense than in the open ocean. Given the tight connection between marine GHG cycling (production, consumption, fluxes) and dissolved oxygen dynamics, ongoing deoxygenation is of particular concern in coastal systems that experience seasonal or perennial hypoxia, and could therefore have a large impact on regional GHG budgets. N2O stands out among other long-lived GHG because of its role as ozone-depleting compound and its effectiveness in enhancing Earth’s warming. Based on the vast majority of studies, marine coastal margins are expected to be hotspots of N2O emissions to the atmosphere, with nitrification and partial denitrification as the main sources. However, despite significant advances in constraining the marine budget of N2O over the last decade, the magnitude and seasonal variability of coastal emissions are still highly uncertain. While N2O depletion in marine settings is usually associated to complete denitrification, recent evidence indicates the possibility of consumption at oxic-hypoxic interfaces or under fully oxic conditions. Moreover, a mechanistic understanding on the benthic-pelagic coupling of N2O fluxes and its potential changes with deoxygenation is still rudimentary. To amend this deficit, we conducted a comprehensive study in Lake Grevelingen (Netherlands), a marine coastal reservoir characterized by seasonally hypoxic/anoxic conditions resulting from limited water exchange with the North Sea and eutrophication. Our study combined biogeochemical and microbial analyses and comprised multiyear (2020–2023) shipboard observations. We observed that unlike most coastal systems, surface waters of Lake Grevelingen are a rather weak source of atmospheric N2O, with annual sea-air fluxes that represent <0.1% of the global marine emissions, and are below the regional climatological mean of the adjacent North Sea. Overall, the water column distribution of N2O across the lake showed enhanced concentrations towards the bottom, which intensified during summer (stratified, low-oxygen period) at the deepest part of the basin (~45m). Nevertheless, computed gas saturations ranged between 20 and 100%, suggesting the occurrence of N2O consumption which cannot be explained by solubility changes alone. Quasi-monthly observations in 2021 showed a clear seasonal variability with comparatively enhanced N2O throughout the water column during summer. Although overall low, collocated measurements of amoA (gene marker of ammonia oxidation during nitrification) abundances within the oxycline showed a similar seasonal pattern, explaining part of the temporal N2O variability in the lake. Cross-lake observations showed low spatial variability in the distribution of N2O albeit ubiquitous low-oxygen conditions, suggesting N2O production/accumulation to also occur in the shallowest (~10m) parts of the basin. Benthic micro-profile measurements showed enhanced concentrations within bottom waters and a rapid decline within the sediments. Analysis of sediments from different sites indicated this pattern to be consistent, such that the sediment-water interface of the lake acted as a source of N2O. During this presentation we discuss these results, provide the first N2O budget for the lake and put forward the potential implications of our study for the future representation of coastal fluxes in modelling studies.