Long-term changes in the Adriatic Sea (1971–2023): river influence, climate impacts, and biogeochemical shifts in coastal bottom waters

This study analyses long-term (1971-2023) trends in physical (temperature and salinity) and biogeochemical (chlorophyll-a and nitrate concentration) properties of bottom coastal waters in the Western Adriatic Sea. Results reveal complex patterns driven by a combination of regional hydrological shifts and broader climate change impacts. The basin was divided into North and Central-South sub-basins, focusing on winter and summer data. Over the last 53 years, Po River flow has progressively decreased especially during summer, altering hydrography and nutrient dynamics. Marked regional and seasonal divergences were observed. The North Adriatic showed summer warming (+1.05 % yr- 1), slight winter cooling (-0.27 % yr- 1), and decreased salinity in winter with a slight increase in summer. In contrast, the Central-South Adriatic exhibited stronger winter warming (+0.43 % yr- 1), weaker summer warming (+0.08 % yr- 1), increased salinity in winter and a decline in summer. Biogeochemical trends revealed strong regional and seasonal contrasts in chlorophyll-a and nitrate concentrations, with declining summer chlorophyll-a across the basin and decreases in both chlorophyll-a and nitrate in the Central-South Adriatic, consistent with a long-term, phosphorus-driven reduction in trophic conditions. This research helps to fill a critical knowledge gap concerning increasingly vulnerable and impacted coastal and bottom environments, underscoring the importance of sustained basin-wide monitoring to ensure uniform spatial and temporal coverage.