Nutrient reductions in the Gothenburg waste water treatment plant and their effects on nutrient concentrations and chlorophyll in the estuary of river Göta älv
Response to variations in nutrient inputs to the southern estuary of river Göta älv on the Swedish west-coast is investigated by means of estuarine nutrient and chlorophyll (Chl-a) data from 1990–05. The estuary receives nutrients from the river, and from the Gothenburg Waste Water Treatment Plant (WWTP). Since 1972, when the WWTP was built, the estuary has experienced more than 50 % decrease of phosphorus inputs, and about 20% decrease of nitrogen inputs. Estuarine concentrations of phosphate and total phosphorus (TP) and, more recently ammonium and total nitrogen (TN) have decreased substantially. Early reductions of phosphorus also gave a downward trend in Chl-a, while effects of recent reductions of nitrogen in the WWTP have been obscured by the larger and more variable water and nitrogen fluxes from the river. Today, annual inputs from the WWTP are 1200 tons TN and 50 tons TP, compared to river inputs of 5000 tons TN (now decreasing) and 100 tons TP. The estuarine phosphorus budget is dominated by the inflow of Kattegat waters (≈300 tons TP). Estuarine Chl-a decreased during the latest 4-y period (2002–05), compared to preceding periods, mainly because of a substantial decrease in river discharge and river nitrogen load. Lower discharge makes nutrient poor Kattegat waters (with lower Chl-a) more frequent in the estuary. However, lower nitrogen inputs, including those from the WWTP also contribute to this decrease. The importance of initial mixing of waste water is highlighted.