TRACKING WASTEWATER EMISSIONS IN RIVERS ENTERING GULF OF BOTHNIA COAST

Sammanfattning: The Gulf of Bothnia consists of two sub-basins in the northern Baltic Sea: the Bothnian Sea (salinity 4-5‰) and Bothnian Bay (salinity 2-3‰). Changing nutrient concentrations and signs of eutrophication has recently been observed in the Gulf of Bothnia. Many rivers enter this sea area, and potentially river inflows constitute a source of nutrient pollution via waste-water emissions. The aim of this study was to elucidate effects of waste-water emissions in four rivers in northern Sweden, Luleå, Skellefteå, Umeå and Söderhamn. My approach was to compare nutrient concentrations at upstream and downstream sampling stations related to the position of waste-water treatment plants. Temporal data from 2006 to 2021 were used and statistically analyzed using non-parametric tests to establish spatial and temporal patterns for nutrient discharged to the coast. The results showed that there are statistically differences in dissolved inorganic phosphorus (DIP) in the form of phosphate (PO4), ammonia (NH4) and total nitrogen (TotN) between the upstream and downstream of Luleå and Umeå wastewater treatment plants. No statistically significant differences were observed in the upstream and downstream data for Söderhamn and Skellefteå. This suggest that better management and mitigation of nutrient loading from wastewater treatment plants that serve higher populations is paramount to achieve the zero-eutrophication goal in the Gulf of Bothnia.