Long-term data series (1971–2015) of physical and biogeochemical parameters were analyzed in order to assess trends and variability of oceanographic conditions in the northern Adriatic Sea (NAS), a mid-latitude shallow continental shelf strongly impacted by river discharges, human activities and climate changes. Interpolation maps and statistical models were applied to investigate seasonal and spatial variability, as well as decadal trends of temperature, salinity, chlorophyll-a and nutrients. This analysis shows that sea surface temperature increased by +0.36% year−1 over four decades. Annual mean flow of the Po River markedly changed due to the occurrence of periods of persistent drought, whereas the frequency of flow rates higher than 3000 m3 s−1 decreased between 2006 and 2015. Moreover, we observed a long-term decrease in surface phosphate concentrations in Po River water (−1.34% year−1) and in seawater (in summer −2.56% year−1) coupled, however, to a significant increase in nitrate concentration in seawater (+3.80% year−1) in almost all seasons. These changes indicate that the nutrient concentrations in the NAS have been largely modulated, in the last forty years, by the evolution of environmental management practices and of the runoff. This implies that further alteration of the marine environment must be expected as a consequence of the climate changes.

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The Drini and ManfredoniaGulfswere investigated in May 2008 and April 2009, respectively. The gulfs are located in the South Eastern (GulfofDrini) and South Western (GulfofManfredonia)Adriatic Sea. The areas are partially influenced by two main Adriatic surface currents: the Eastern Adriatic Current-EAC that flows north-westward along the eastern side, and the Western Adriatic Current—WAC that flows south-eastward along the western side of the basin. The spatial variations of temperature, salinity, fluorescence, dissolved oxygen concentration, Coloured Dissolved Organic Matter—CDOM, nutrients, chlorophyll a and phytoplankton composition parameters in the two areas were observed and compared. CDOM regulates the penetration of UV light into the sea and plays an important role in many hydrological and biogeochemical processes on the sea surface layer including primary productivity. The phytoplankton specific diversity of the Gulf of Manfredonia showed a spring community with dinoflagellates (21 taxa) as the main important fraction, coccolithophorales (6 taxa) and diatoms with 10 identified taxa. The phytoplankton distribution along the eastern coast showed a different biodiversity: a prevalence of dinoflagellates (58 taxa) included harmful microalgae such as Alexandrium, Dinophysis and Lingulodinium genus. Diatoms were less abundant, among them Pseudo-nitzschia was also reported which could include some potentially toxic species. Nanoplankton are ever abundant in offshore waters and an exceptional bloom of cyanobacteria was registered in Buna-Boyana estuary due to strong industrial impact. The gulfs showed similar physical and biochemical characteristics despite the WAC carried out along the westernAdriatic Seathe water rich in nutrients from the major northern Italian rivers. No correlations were found between CDOM and chlorophyll