NUTRIENT ENRICHMENT AND PHYTOPLANKTON RESPONSE IN AN ADRIATIC KARSTIC ESTUARY

Nutrients, chlorophyll a, primary production (C-14), and standard oceanographic parameters were measured seasonally from 1983 to 1988 along the axis of a karstic estuary of the central Adriatic Sea (the Krka River estuary). Because of anthropogenic phosphorus discharges, the surface-layer orthophosphate concentrations (up to 1.7 mmol m-3), phytoplankton biomass (chlorophyll a up to 23 mg m-3) and primary production (up to 108 mg C m-3 h-1) were significantly higher in Sibenik Bay (lower estuary) than in the other estuarine subregions, and the coastal sea in particular. In contrast, nitrate and orthosilicate (up to 59 and 65 mmol m-3, respectively) distributions during autumn and winter were ascribed to dilution of Krka River nutrients along the estuary. As a consequence, the surface-layer inorganic N/P ratio was extremely high in the upper estuary (averages up to 180), but this ratio was reduced up to three times in Sibenik Bay and the coastal sea. In spring and summer, nitrate and orthosilicate, but not orthophosphate, were almost exhausted from the water because of biological utilization. In the saline layer below the halocline (depth 2-5 m) oxygen saturation varied over a large range, particularly in the upper estuary (16-176%), and nutrient concentration ratios differed from those in the surface layer. A nutrient regeneration stoichiometric model was derived, based on a linear regression analysis: AOU:Si:N:P = -276:16:6:0.4. Anthropogenic nutrient inputs should be urgently reduced to re-establish a natural nutrient environment.