Brownification of lake Mälaren : Effects of land use and climate change

Organic carbon concentrations are increasing in freshwaters in the northern hemisphere. A significant brownification is observed in Lake Mälaren in south-east Sweden since 1965. This process has an impact on drinking water production, ecological functions and might also increase the loss of greenhouse gases to the atmosphere. Drivers behind the brownification have been attributed to the reduced sulphur deposition, warmer and wetter climate and changing land use. This study aims at examining how growing degree days, water discharge and volume of pine and spruce in Mälaren catchment have contributed to the increased water colour since 1965. Based on that analysis, some predictions are made to what can be expected in the future. Data over climate variables have been collected from SMHI and water colour from SLU and Norrvatten. All variables were found to be correlated to water colour, with the strongest correlations found between water colour and pine and spruce volume. Growing degree days increased 30 days since 1968-1972 and water discharge revealed an overall decline for the period 1981-2018. Climate scenarios predict warmer temperatures and higher yearly precipitation for this region. Overall change in discharge is uncertain since a higher evaporation is also expected. An increase would likely lead to an increase in water colour. This could especially affect the eastern basins where increase in water colour currently is slowed down by higher water retention times. The findings in this study indicate that water colour in Mälaren will increase in the future, mainly due to warmer temperatures and increased volume of pine and spruce