Long-term Nutrient Balances in the Eutrophication of the Himmerfjärden Estuary

The Himmerfjärden estuary interfaces to the Baltic proper and has served as a receiving area of a sewage treatment plant since 1974. Based on water turnover estimates for 12 consecutive years (1978–89), long-term material balances have been computed for the total mass of the nutrients, phosphorus and nitrogen. In these computations, the estuary was partitioned into four basins with an upper and a lower section. The estimated water exchange ranges from 50 m3s−1between the two innermost basins to 750 m3s−1between the outermost basin and the Baltic Sea. Together with the measured nutrient profiles, horizontal transports are produced that, combined with the loadings [total yearly averages are 1•3•103metric tonnes total nitrogen (TN) and 44 metric tonnes total phosphorus (TP)], give nutrient balances for TN and TP that are consistent with material conservation in all four basins. These budgets entail rates of measured net sedimentation and computed denitrification and remobilization of phosphorus under anoxic conditions. For the primary receiving basin of the sewage treatment plant discharge, the denitrification rate was found to be 16 gN m−2year−1.