Long-term dynamics of annual and seasonal physical and biogeochemical properties: Role of minor river discharges in the North-western Adriatic coast

Dynamics of the physical and biogeochemical properties in a temperate coastal area in the north-western Adriatic Sea were analyzed. Multi-year (1997-2019) continuous observations allowed assessing their trends at two sites directly influenced by the discharges of two minor rivers as well as by human activities and climate change. Statistical models were applied to investigate the temporal variability and trends of seawater temperature, salinity, chlorophyll a, nutrients, river discharges and precipitations. The analysis highlighted a role for the minor river inputs and for ecological processes on interannual and seasonal biogeochemical dynamics. We found a significant trend toward more oligotrophic conditions; in particular, chlorophyll a exhibited a long-term decline (-1.38% year-1 and - 1.5% year-1 at the two Foglia and Metauro river transects, respectively) that was largely determined by low phosphate and nitrate seawater concentrations as a result of a significant reduction in the phosphate and nitrate loadings of the two minor rivers (respectively mean values of -4.65% year-1 and -2.65% year-1). In contrast, salinity showed a long-term decrease of -0.24% year-1 and -0.19% year-1 at Foglia and Metauro, respectively, corresponding to a significant increase of the freshwater discharges of the two minor rivers (+1.86% and +1.57% year-1 at Foglia and Metauro, respectively) possibly due to precipitations. Data analysis highlighted the conditions of temperate coastal areas affected by freshwater discharges. Nutrient load management and climate conditions such as precipitation regimes appear to be the main factors driving physical and biogeochemical dynamics in the north-western Adriatic Sea.