The response of planktonic and microbenthic algal assemblages to nutrient enrichment in shallow coastal waters, southwest Sweden

Abstract

Field and laboratory nutrient (nitrogen and phosphorus) enrichment experiments were performed using natural phytoplankton and microphytobenthic assemblages from the brackish water Öresund, S.W. Sweden. The response of algae from a low-nutrient area (Falsterbo Canal) was compared to that of algae from a polluted, nutrient-rich area (Lomma Bay). The biomass (measured as chlorophyll a) of both phytoplankton and microphytobenthos from the Falsterbo Canal increased after the addition of nitrogen. Phytoplankton growth was stimulated by the addition of phosphorus to the nitrogen-rich water of the polluted Lomma Bay. Sediment chlorophyll a showed no significant increase after the addition of nutrients in the Lomma Bay. In containers without sediment, phytoplankton uptake was calculated to account for ≈ 90% of the disappearance of inorganic fixed nitrogen from the water. In the sediment containers the microphytobenthos was estimated to account for ≈20% of the nitrogen uptake. The rest was presumably lost mainly through denitrification. When containers with microphytobenthos from Lomma Bay were kept in the dark, phosphorus was released at a rate of up to ≈ 180 μM · m −2 · day −1. We suggest that by producing oxygen microbenthic algae keep the sediment surface oxygenated thereby decreasing phosphorus transport from the sediment to the overlying water.