Fokus på Mälaren 2022: sammanfattande resultat från miljöövervakning och fokusprojekt knutna till samarbetet mellan SLU och Mälarens vattenvårdsförbund

The Swedish University of Agricultural Sciences and Mälaren’s Water Conservation Association (MVVF) continue their collaboration with focus on Lake Mälaren. During 2022, we have had three webinars. The yearly Lake Mälaren Seminar gathered around 60 participants over two days to get information about monitoring and research results related to the lake and to discuss challenges improving the environmental status of the lake.The year was characterized by a mild start and a warm summer with August standing out as both warm and rainy, which contributed to high levels of cyanobacteria at many sampling stations in August. The status assessment shows, as before, that it is stations with worse status than good that dominate which shows the eutrophication impact on the lake. Some large deep basins start to have low oxygen levels in the cold bottom water at the end of summer, which needs to be taken seriously because the lake has several cold water species that need to stay there in summer.Two doctoral students have partially done their studies in the catchment of Lake Mälaren and they have defended their theses in 2022. One shows that tree borders around watercourses protect biodiversity to a great extent and close the loop returning some nutrients from the water to land. The second showed that it is important to reduce erosion from agricultural land because suspended soil particles in water contribute strongly to phosphorus transport from land to water.Other scientific publications of interest from a perspective of Lake Mälaren are e.g. one about wetlands that shows the need to have a landscape perspective and create groups of nearby wetlands to get the best overall effect. Two studies show that it is important to know the hydrology in order to plan measures and purification to reduce PFAS levels next to known point sources.This year's student projects showed, among other things, that the transport of phosphorus from land to water will increase in a warmer climate, that methane emissions from constructed wetlands can be large during winter, and that the community willingness to pay could be sufficient to cover cost of an aluminum treatment to relieve a lake from effects of eutrophication.