Variabilities of biochemical properties of the sea surface microlayer: Insights to the atmospheric deposition impacts.

Atmospheric deposition (AD) of nutrients and its impact on the sea surface requires consideration of interfacial processes within the sea surface microlayer (SML), the ocean-atmosphere boundary layer of major importance for many global biogeochemical and climate-related processes. This study comprised a comprehensive dataset, including dissolved NO, NH and PO in ambient aerosol particles, wet deposition and sea surface samples collected from February to July 2019 at a central Adriatic coastal site. The aerosol mean concentration of dissolved nitrogen (DIN = NO + NH) and PO were 48.8 ± 82.8 μmol m and 0.8 ± 0.6 μmol m, respectively, while their total fluxes (dry + wet) ranged from 24.2 to 212.3 μmol m d (mean 123.2 ± 53.2 μmol m d) and from 1.2 to 2.1 μmol m d (mean 1.5 ± 0.3 μmol m d), respectively. Intensive local episodes of open biomass burning (BB) significantly increased aerosol DIN concentrations as well as DIN deposition fluxes, particularly altering the molar DIN/PO ratio of atmospheric samples. The DIN temporal patterns showed high variability in the SML (range 0.2-24.6 μmol L, mean 5.0 ± 7.1 μmol L) in contrast to the underlying water samples (range 0.5-4.2 μmol L, mean 1.9 ± 1.2 μmol L), with significant increases during BB periods. Variability in abundance of heterotrophic bacteria and autotrophs in the SML along with concentrations of bulk dissolved and particulate organic carbon as well as dissolved and particulate lipids and carbohydrates, gel particles and surfactants followed DIN enhancements with a two-week delay. This study showed that AD can affect the short-term scale enrichments of organic matter in the SML, especially when accompanied by BB emissions typical of the overall Mediterranean coastal environment. This could have strong implications for global air-sea exchange processes, including those of climate relevant gases, mediated by the SML.