Impact of a river flood on marine water quality and planktonic microbial communities

Coastal systems represent primary receptors of land-derived nutrients and pollutants, thus playing a crucial role in carbon burial and marine productivity processes. A closer look into potential effects of river floods and following seawater quality modifications is fundamental to specifically explore the links between the marine contamination patterns and the planktonic microbial processes involved in carbon fluxes. Here we investigated the effects of an extreme flood event (8603m3 s-1) from a large river (Po River, Italy) on the physical and chemical seawater properties, along with the responses of phytoplanktonic and heterotrophic microbial communities to riverine inputs. Following a multidisciplinary oceanographic survey conducted across three sampling transects perpendicular to the Italian coastline, marine waters differently impacted by the river flood could be discriminated according to the variation patterns of relevant seawater properties (i.e., temperature, salinity, dissolved oxygen, total suspended matter). The concentrations of major nutrients and the organic matter composition (i.e., particulate and dissolved organic carbon, stable carbon isotopic composition, chromophoric dissolved organic matter) were significantly higher in waters at high river flood impact levels. The total dissolved organic carbon and the concentrations of selected inorganic and organic pollutants (i.e., heavy metals and polycyclic aromatic hydrocarbons) were unaltered by riverine inputs, showing values similar or lower than those reported at regular flow conditions. Moreover, the phytoplanktonic and heterotrophic microbial communities showed significant changes linked to river flood, as revealed by a net increase of Chlorophyll-b concentrations (related to the occurrence of freshwater algal taxa), higher prokaryotic C production rates, and shorter prokaryotic cell turnover times in highly impacted waters. In conclusion, our results showed that an extreme river flood event could represent a source of energy for the microbial metabolism involved in OM transformation processes, with consequences on the microbially-driven C-flux and to the overall productivity of coastal marine systems.