Spatial and temporal dynamics of dissolved organic matter in the sea surface microlayer during a bloom of coccolithophores

The sea surface microlayer (SML) is a microscopic boundary that covers the ocean’s surface, influences CO₂ exchange with the atmosphere, and is often exposed to high levels of UV irradiation. The SML is a unique biome and shelters diverse microbial communities. Bioaggregates, containing carbohydrates, lipids and proteinaceous material accumulate in the SML, affecting gas exchange. Despite its role in the global carbon cycle, the biogeochemical processes controlling the production and turnover of organic matter in the SML are poorly understood. This study is part of the collaborative research unit ’Biogeochemical processes and Air-sea exchange in the Sea-Surface microlayer’ (BASS). Our goal is to decipher the underlaying forces behind the accumulation of dissolved organic matter (DOM) in the SML and its spatial and temporal dynamics. Furthermore, we aim to link the molecular properties of DOM in the SML to the microbial communities living in the SML, to air-sea gas exchange, and to carbonate chemistry. To address these objectives, we conducted a large-scale mesocosm study with coastal seawater from Jade Bay (North Sea, Germany). Following nutrient addition, a bloom of the coccolithophore Emiliania huxleyi occurred. The SML was sampled with a glass plate, and the underlying water (ULW) was sampled with a tube at a depth of 60 cm. Dissolved organic carbon (DOC) was quantified in filtered samples, which were then desalinated and concentrated for molecular analysis of DOM with ultra-high resolution mass spectrometry. In both the SML and ULW, DOC concentrations almost doubled from pre-bloom to post-bloom conditions. Overall, DOC was higher in the SML than in the ULW, and this discrepancy increased after the algal bloom. Furthermore, the ratio of DOC to DON was significantly higher in the SML than in the ULW after the bloom. Molecular indicators of DOM lability increased concurrently with DOC concentrations, reflecting freshly produced DOM in both SML and ULW during the late algal bloom stages. At the same time, the contributions of aromatic fractions in DOM and a photodegradation index decreased, possibly related to UV exposure of the mesocosm. Overall, our results suggest that primary production is likely to drive organic matter accumulation in the SML.