Climatological biogeochemical characteristics of the Adriatic Sea

A biogeochemical historical data set relative to the Adriatic Sea is analysed to deduce the climatological characteristics of biogeochemical properties (dissolved oxygen, AOU, nitrate, phosphate, silicate and chlorophyll-a) on a seasonal basis. The Adriatic Sea is divided into four regions, the northern Adriatic, in which a region shallower than 40 m and a region deeper than 40 m are distinguished, the middle Adriatic and the southern Adriatic. The basin exhibits a generally decreasing trend of nutrient concentrations from North to South, due to the nutrient input by rivers, occurring particularly in the northern Adriatic, enabling intense phytoplankton developments in winter and autumn. In the northern basin the dominant cyclonic circulation determines a southward nutrient flow along the western coast; however, the resulting horizontal nutrients distribution can be strongly affected by phytoplankton uptake, as in the case of the winter bloom. Strong bacterial regeneration of the organic matter occurs in spring–summer, with a sensible oxygen depletion and nutrient increase at depth. In the middle Adriatic the phytoplankton biomass is much lower than in the northern Adriatic, but its role in controlling the nutrients distribution is relatively more important, because of the reduced influence of river input. In the southern Adriatic the Modified Levantine Intermediate Water (MLIW) is characterized by high nitrate concentrations. However, a detailed picture of the southern basin is prevented by the lack of data. As a tentative result, the analysis of the Redfield ratios seems to confirm that, generally, the Adriatic Sea is a phosphorus limited basin, but also seems to indicate that the surface water in the middle and, particularly, the southern Adriatic might be characterized by nitrogen limitation.