Long-term changes in the northern Adriatic ecosystem related to anthropogenic eutrophication

Knowledge of the mechanisms driving long-term changes in the nutrient and p rimary production cycles represents a basic step to verify real eutrophicat ion trends in shallow marine ecosystems, such as the northern Adriatic open waters. In fact, this ecosystem appears to be especially sensitive to seas onal and long-term variations of the anthropogenic nutrient load (primarily the Po River discharges), which impact can be significantly modified by ch anges in the oceanographic conditions, due to climatic fluctuations. To ver ify this assumption, the data for several parameters related to eutrophicat ion (salinity, temperature, chlorophyll a, primary production rate, nutrien ts, water transparency, dissolved oxygen), collected in the period 1966-199 5 in open waters, were analysed and compared with changes of the Po River f low-rate. The data were grouped in typical seasons, periods, and subareas, characterized by different dominant relevant processes. The changes of mean values for salinity and temperature were well correlated with the Po flow- rates in all periods, except during the late 1980s. In these years the surf ace salinity was generally lower and the temperature was higher than expect ed from the flowrate values. This departure was explained by unusual hydrom eteorological conditions, characterized by a calm sea and sunny weather, du e to long periods of high barometric pressure. In these conditions the fres hwater mixing was limited to a thinner surface layer, in which heat accumul ation was favoured. The long-term changes of the nutrient concentrations we re even less correlated with the Po flow-rates. The mean orthophosphate con centration of the surface layer in the season February-April of the late 19 80s appeared to be lower, and that of the total inorganic nitrogen (TIN) hi gher than expected from the pattern of the Po flow-rate. These deviations c an be explained by long-term changes of the river nutrient composition. In relation to this, when compared at the same salinity (i.e. same dilution de gree, independently on the freshwater discharge rate), the chlorophyll a co ncentrations and the photosynthetic activities (estimated by seawater incub ation at constant light with C-14-bicarbonate) were higher in periods of hi gher river orthophosphate concentrations, but not of TIN concentrations. Fu rthermore, the data analysis suggested that more favourable hydrometeorolog ical conditions for phytoplankton growth in spring and summer occurred duri ng the late 1980s than other periods with similar phosphorus availabilities (i.e. the 1970s). Intense surface phytoplankton blooms, localized off the Po Delta, were induced in unusually long periods of calm sea and sunny weat her. Long-term changes of the primary production parameters in the autumn a nd winter were not related to the nutrient composition of the Po waters. Pr obably, the primary production in these seasons is mainly controlled by nut rient recycling within the marine ecosystem. The bottom oxygen saturation d uring summer and autumn of the late 1980s was lower than expected from the Po flow-rate, especially if the decrease of the phosphorus load is consider ed. In fact, the near-anoxic events (and the autumn 1989 anoxia) in the bot tom layer of large areas probably occurred as a consequence of a delayed re activation of the autumn vertical and horizontal water circulation, due to unusual meteorological conditions. This is also confirmed by the fact that similar events were not observed in the next period (1990-95), which was ch aracterized by different meteorological and hydrological cycles. The analys is of the long-term change of the Po flow-rate was expanded to the entire m easurement period (since 1917). It was concluded that during the period 1969-1992 the flow-rate in spring w as on average higher than in the previous period, and that the autumn peak shifted from November to October. These changes might have had some influen ce on the northern Adriatic ecosystem. Moreover, particularly frequent was the occurrence of a secondary peak flow in March during the 1970s. It was c oncluded that significant fluctuations of primary production in the open no rthern Adriatic have occurred since the 1970s, caused by variations of the nutrient load and oceanographic conditions.