Influence of nutrient inputs from a wetland dominated by agriculture on the phytoplankton community in a shallow harbour at the Spanish Mediterranean coast

Abstract

The Safor Wetland (Western Mediterranean) is a protected ecosystem declared Site of Community Importance under the Habitats Directive. Agricultural practices have been part of this ecosystem throughout history, and its hydrology is anthropogenically manipulated to satisfy cultivation needs. Freshwater from the wetland is discharged through surface channels to Gandia Harbour, a shallow water body with high water residence time. This study evaluated the linear eutrophication gradient downstream from the freshwater inflow locations. The role of the main nutrients in determining the phytoplankton community is discussed. The predominance of agricultural practices, 48% of the watershed soil, caused an excess of nitrogen and an imbalance in the nutrient ratios at all the sampling points. Phosphorus concentrations were particularly low, and did not exceed 1.0μM. Chlorophyll-a concentration was of the order of that found in other eutrophic estuarine waters. In general, flagellates dominated over diatoms at all the harbour sampling points and depths. Potentially blooming species of both phytoplankton groups were detected. The correct implementation of the existing agricultural best management practices should continue to reduce nitrogen and phosphorus loading to the estuary. It seems reasonable that for effective control of the eutrophication effects in this area, strict control over wastewater point sources should be also exercised.