WHEN DO PHYTOPLANKTON BLOOMS CAUSE THE MOST INTENSE HYPOXIA IN THE NORTHERN ADRIATIC SEA

Eutrophication of the northern Adriatic Sea was investigated using dat a on dissolved oxygen (DO) from 1911 to 1982. Beginning in the period 1955-1965, DO concentrations increased in the surface layer and decrea sed close to the bottom in all seasons except winter. These changes ar e attributed to an increase of the anthropogenic nutrient inflow. The occurrence of hypoxia near the bottom and its disappearance were inves tigated using the vertical distribution of DO on the transect from the Po delta to Rovinj during 1973. Horizontal distributions of DO close to the surface and near the bottom show prominent gradients from the P o delta towards the middle of the northern Adriatic Sea. Application o f a simple box model indicates that phytoplankton blooms which occur i n July cause the lowest DO concentrations near the bottom. For DO conc entrations near the bottom to increase, anthropogenic nutrient inflow into the northern Adriatic Sea must be significantly reduced.