How do open coastal fire episodes' impact sea surface microlayer neuston communities?

Human-induced climate change is expected to increase the frequency and severity of vegetation fires. The Mediterranean region is considered particularly prone to fire episodes in summer. It is well known that pyrogenic particles are an important source of external nutrients for the marine environment, especially in oligotrophic areas. In this study, the plankton components of the sea surface layers were integrated to evaluate, for the first time, their dynamics over six months and their response to fire events in a typical coastal area of the Adriatic Sea. Concentrations of nutrients and organic compounds, together with plankton communities were significantly higher in the sea surface microlayer (SML, < 1 mm thick), than in the underlying water from 1 m depth. The piconeuston community and chlorophyll a responded with extreme abundance and concentration to the most intense fire event that enriched the SML with NH4+. Phytoneuston abundance increased with a delay of 2 weeks, while diversity indices decreased slightly after the fire events. The large abundances of the studied piconeuston parameters could be explained by the high availability of organic compounds and the immediate availability of NH4+, while the phytoneuston community responded to an increased NO3- concentration, triggered by the fire events. We confirmed that fast-acting marine heterotrophs are important members of biogeochemical cycles associated with fire events and that, together with phytoplankton, they are unavoidable parameters to detect environmental changes.

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 NO3-, NH4+ and PO43- 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 = NO3- + NH4+) and PO43- were 48.8 ± 82.8 µmol m-3 and 0.8 ± 0.6 µmol m-3, respectively, while their total fluxes (dry + wet) ranged from 24.2 to 212.3 µmol m-2 d-1 (mean 123.2 ± 53.2 µmol m-2 d-1) and from 1.2 to 2.1 µmol m-2 d-1 (mean 1.5 ± 0.3 µmol m-2 d-1), 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/PO43- ratio of atmospheric samples. The DIN temporal patterns showed high variability in the SML (range 0.2-24.6 µmol L-1, mean 5.0 ± 7.1 µmol L-1) in contrast to the underlying water samples (range 0.5-4.2 µmol L-1, mean 1.9 ± 1.2 µmol L-1), 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.