THE MECHANISM CONTROLLING PLANT NUTRIENT CONCENTRATIONS IN THE NORTHERN ADRIATIC SEA

A 20 year data set for the northern Adriatic was analyzed and the factors establishing the nutrient environment identified. Concentrations ranged widely (TIN 0.0-78, PO2 0.01-1.1, and SiO4 0.0-59 mmol m-3). In early winter remineralization increased concentrations. Characteristic winter, late spring and fall phytoplankton blooms alternately decreased and increased concentrations, as modified by river input. In summer nutrients were minimal under a semi-closed circulation pattern and high vertical stability, due to closely coupled nitrogen and phosphorus assimilation-regeneration processes and biogenic silica sedimentation. "New" primary production supported mainly by river input of "new" nutrients approximated "regenerated" primary production supported by regenerated nutrients, making the ecosystem especially sensitive to eutrophication pressure from anthropogenic increases in the Po River nutrient load.