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

Abstract. 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.