Hydrologisk och hydrokemisk studie om Trehörningen : En igenväxande sjö i östra Uppland

Sammanfattning: Lake overgrowth occurs in many areas in Sweden, and it is often explained by elevated amounts of nutrients. The overgrowth results in a changed local environment for both communities and ecosystems. In this project the lake Trehörningen, near Spersboda in Norrtälje municipality, has been examined regarding an ongoing overgrowth process. The study contains hydrological and chemical analyses and an inquiry to those living in the area. The purpose has been to survey the area and identify factors that may contribute to elevated nutrient levels, and to give suggestions to measures that may reduce the nutrient transport. The results from the period February–April 2020 indicate a moderate nutrient supply, but that certain areas are more exposed than others. The entire lake can not be seen as nutrient rich, and there is a net loss of nutrients from the lake over the period studied. The analysis of mass transports shows that the primary production in the lake is phosphorus limited, and that the largest addition of phosphorus comes from three streams that all supply the Spersboda bay. This bay is also the shallowest area of the lake and has the most visible emergent plant growth. Analyses of the lake sediments show a potential increase in phosphorus over time, and that there may be some internal loading, but it is unlikely to be a large source of phosphorus to the lake. The streams with the highest nutrient levels have their watershed mostly in human-impacted areas. The study has taken place before the plant growth season, and as such the absolute nutrient numbers may be elevated, but their relative amounts are worth discussing. To reduce the overgrowth process, measures which reduce the nutrient supply should be taken, especially in the watercourses around the Spersboda bay.