Phytoplankton and environmental drivers at a long-term offshore station in the northern Adriatic Sea (1988–2018)

A long term (1988-2018) data set of phytoplankton and physico-chemical parameters was analyzed for the first time in an offshore station of the LTER Senigallia-Susak transect (northern Adriatic Sea) not directly affected by the coastal nutrient input. The mean annual cycle of phytoplankton markedly differed from that observed in the coastal areas, showing the maximum in June and the minimum in November. The main component of phyto-plankton community was represented by phytoflagellates, whose trend paralleled that of total phytoplankton. On average, diatoms peaked in July, dinoflagellates in June and coccolithophores in April. The phytoplankton maximum in summer is explained mainly by the allochthonous input of DIN and PO4 carried by the low salinity waters expanding eastward, during the stratification of water column. Resuspension processes seem to be less effective for PO4 than for DIN. The most representative phytoplankton taxa for each season as indicated by the IndVal analysis were identified and were only in part similar to those observed in the coastal area. The inter-annual anomaly trend showed a significant increase of temperature, DIN and phosphates. Although no significant changes were found for the total phytoplankton, a reduction of winter dinoflagellate and coccolithophore abundances was observed.