Insights to the short-term atmospheric deposition impacts on the biology and chemistry of the sea surface microlayer in the Adriatic Sea coastal region

The Mediterranean basin continuously receives anthropogenic aerosols from industrial and domestic activities from the European region, as well as high rates of aeolian material in the form of mineral dust from northern Africa. Moreover, combustion dominates over natural dust, whereas vegetation fires frequently burn throughout the Mediterranean coastal zone, especially during hot and dry summers. Once in the atmosphere, aerosols become an important external source of nutrients but also of toxins to the marine ecosystem through atmospheric deposition (AD), affecting the quality and quantity of organic matter (OM) produced by phytoplankton in the photic zone, and altering the CO₂ uptake. AD onto sea surface cannot be completely understood without considering the interfacial processes within the sea surface microlayer (SML). As the uppermost millimeter of the sea surface, the SML represents the natural interface of the major environmental importance. It could serve as the first indicator of increasing human impact and climate change due to fast response of its biological and physico-chemical properties. However, surprisingly little research assessed the impact of AD on surface plankton communities, distinguishing between the SML and the water column bellow.

This work is designed to assess the magnitude and temporal variability of atmospheric concentrations and deposition fluxes of nutrients and trace metals, and to gain insight into the AD impacts on the nature of enrichments of organic compounds within surface layers in a typical Mediterranean coastal environment. The field campaign was conducted during the period of retrieval of sea surface oligotrophic conditions (February-July 2019) at the Adriatic coastal area. On-line black carbon (BC) concentrations were measured while the aerosol particles (PM₁₀), wet and total deposition samples as well as the SML and underlying water (ULW; 0.5 m depth) samples were collected simultaneously. The first comprehensive insight into concentration levels of macro nutrients (N, P) and trace metals (e.g. Cu, Pb, Cd, Ni, Zn, Co) in atmospheric samples, their transport history, source apportionment and deposition fluxes to the coastal Adriatic area will be presented. The temporal dynamics of SML biology as well as concentrations of inorganic and organic constituents enabled the assessment of their sources and the nature of the enrichments taking place within the SML. Due to their significance throughout the Mediterranean coastal area, open-fire episodes and Saharan dust inputs were especially considered. In order to better understand the impacts of ambient AD from diverse sources on the physiology and biomass of the natural plankton population and consequently on the chemistry of the surface layers (SML and ULW), we further conducted the first in situ bioassay incubation experiment of its kind at the Adriatic Sea. We experimentally examined the impact of locally collected anthropogenic aerosols, that had different levels of biologically important nutrients, trace metals and organic pollutants, in contrast to the material mimicking biomass burning events.

Acknowledgment: This work has been supported by the Croatian Science Foundation under the IP-2018-01-3105 project: Biochemical responses of oligotrophic Adriatic surface ecosystems to atmospheric deposition inputs.