EFFECTS OF NUTRIENT DISCHARGE BY RIVER WATER AND WASTE-WATER ON THE NITROGEN DYNAMICS IN THE ARCHIPELAGO OF GOTEBORG, SWEDEN

Nutrient dynamics within the strongly stratified River Gota alv estuary were investigated during 1988 to 1990 by measuring salinity and nutrient concentrations as well as uptake, mineralization and denitrification processes (using N-15 technique and the acetylene inhibition method). The river supply was the most dominating contribution to the inorganic nitrogen budget with an addition of nearly 6000 mmol s-1, mainly in the form of nitrate. Further addition to the estuary from the waste water supply and deep water supply amounted to ca 3800 and 2900 mmol s-1, respectively. Ammonium was the main component of the waste water supply. Phosphate supply was dominated by the deep water while the contribution of river and waste water was low. Generally, the estuary is phosphate-limited. Indirect calculations, based on salinity and nutrient measurements, indicated considerable sinks for nitrate and ammonium within the surface water of the estuary. Excluding the winter period with low primary production, the sink for nitrate was 2600 mmol s-1 and for ammonium 3200 mmol s-1, corresponding to a total sink of 250 nmol l-1 h-1. Measured uptake rates, on the other hand, were much lower. The nitrate uptake was 10 to 20 nmol l-1 h-1 and the ammonium uptake averaged 60 nmol l-1 h-1. Neither phytoplankton biomass, chlorophyll a nor oxygen concentrations indicated high primary production which could explain the nitrogen sink. This apparent discrepancy is discussed.