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Recent changes of the oceanographic conditions and eutrophication degree in the northern Adriatic Sea

Significant seasonal and long-term fluctuations of oceanographic, chemical and biological properties in the northern Adriatic were observed in the period 1972-1992 due to variable (primarily climatic) and anthoropogenic factors. In fact, the shallow northern Adriatic pelagic ecosystem is very sensitive to changes of the nutrient load, primarily due to the Po River contribution that accounts for about 75% of the total nutrient inputs in the region. Data related to oceanographic parameters (temperature, salinity and dissolved oxygen), nutrients and chlorophyll a were collected approximately monthly in the period 1993-2000 at 5 stations at the Rovinj-Po Delta profile. These data, as well as the Po River flow rates, were analysed and compared to the previously observed long-term changes for periods since 1972. It was established that in the nineties significant changes in the hydrological regime of the Po River occurred, characterized by frequently higher flow rates in October and November compared to previous periods. As a consequence, nutrient and chlorophyll a concentrations were maximal in autumn, while the spring values were lower than in the seventies and eighties. Changes in concentrations of nitrate and orthosilicate in the surface layers was primarily related to the distribution of freshned waters, while for orthophosphate, ammonia and nitrite different biological processes have played a more important role (assimilation, regeneration and nitrification). Eutrophication degree appeared to be higher in the period 1993-2000 than in the late eighties, when significant decreases of orthophosphate concentrations in the Po River occcurred, but still lower than in the early eighties. Interestingly, in previous periods, near anoxia events appeared more frequently than in the period 1993-2000. The main reason is that the higher nutrient load by the Po River in the autumn has minor effect on the ecosystem than in the spring because of the vertical and horizontal exchanges of the water masses. Excessively produced organic matter is therefore less likely to accumulate near bottom and less oxygen is consumed for the remineralization processes.

Oceanographic conditions; Nutrients; Chlorophyll a; Northern Adriatic

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