Keywords: Adriatic Sea, eutrophication, Po River, primary production
Long-term changes in the northern Adriatic ecosystem related to anthropogenic eutrophication
Knowledge of the mechanisms driving long-term changes in the nutrient and primary production cycles represents a basic step to verify real eutrophication trends in shallow marine ecosystems, such as the northern Adriatic open waters. In fact, this ecosystem appears to be especially sensitive to seasonal and long-term variations of the anthropogenic nutrient load (primarily the Po River discharges), which impact can be significantly modified by changes in the oceanographic conditions, due to climatic fluctuations. To verify this assumption, the data for several parameters related to eutrophication (salinity, temperature, chlorophyll a, primary production rate, nutrients, water transparency, dissolved oxygen), collected in the period 1966–1995 in open waters, were analysed and compared with changes of the Po River flow-rate. The data were grouped in typical seasons, periods, and subareas, characterised 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 surface salinity was generally lower and the temperature was higher than expected from the flow-rate values. This departure was explained by unusual hydrometeorological conditions, characterised by a calm sea and sunny weather, due to long periods of high barometric pressure. In these conditions, the freshwater mixing was limited to a thinner surface layer, in which heat accumulation was favoured. The long-term changes of the nutrient concentrations were even less correlated with the Po flow-rates. The mean orthophosphate concentration of the surface layer in the season February–April of the late 1980s appeared to be lower, and that of the total inorganic nitrogen (TIN) higher than expected from the pattern of the Po flow-rate. These deviations can 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 degree, independently on the freshwater discharge rate), the chlorophyll a concentrations and the photosynthetic activities (estimated by seawater incubation at constant light with 14C-bicarbonate) were higher in periods of higher river orthophosphate concentrations, but not of TIN concentrations. Furthermore, the data analysis suggested that more favourable hydrometeorological conditions for phytoplankton growth in spring and summer occurred during the late 1980s than other periods with similar phosphorus availabilities (i.e. the 1970s). Intense surface phytoplankton blooms, localised off the Po Delta, were induced in unusually long periods of calm sea and sunny weather. Long-term changes of the primary production parameters in the autumn and winter were not related to the nutrient composition of the Po waters. Probably, the primary production in these seasons is mainly controlled by nutrient recycling within the marine ecosystem. The bottom oxygen saturation during 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 considered. In fact, the near-anoxic events (and the autumn 1989 anoxia) in the bottom layer of large areas probably occurred as a consequence of a delayed reactivation 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 characterised by different meteorological and hydrological cycles. The analysis of the long-term change of the Po flow-rate was expanded to the entire measurement period (since 1917). It was concluded that during the period 1969–1992 the flow-rate inspring was on average higher than in the previous period, and that the autumn peak shifted from November to October. These changes might have had some influence on the northern Adriatic ecosystem. Moreover, particularly frequent was the occurrence of a secondary peak flow in March during the 1970s. It was concluded that significant fluctuations of primary production in the open northern Adriatic have occurred since the 1970s, caused by variations of the nutrient load and oceanographic conditions.