Nitrous oxide (N 2 O) in the sea surface microlayer and underlying water during a phytoplankton bloom: a mesocosm study

Nitrous oxide (N 2 O) is an important climate-relevant atmospheric trace gas. The open and coastal oceans are a major source for atmospheric N 2 O. 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 N 2 O 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 N 2 O concentrations, N 2 O 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. N 2 O concentrations were slightly enriched in the SML compared to the ULW although the difference of the mean N 2 O concentrations between the ULW and SML was statistically not significant. N 2 O was supersaturated (100 %–157 %) in the ULW and SML during the course of the study which indicated an in-situ production of N 2 O. N 2 O in-situ production under the experimental conditions is most consistent with photochemodenitrification in combination with the release from phytoplankton, whereas microbial production of N 2 O via nitrification appeared to be of minor importance. N 2 O 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 N 2 O in-situ sources were balanced by the release of N 2 O to the atmosphere resulting in a steady state of the system. Our results indicate a potential important role of the SML for N 2 O 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 N 2 O concentrations in the ocean surface layer.