Patterns and factors affecting brownification in a boreal river

Abstract: During the last decades a brownification of the lakes and streams in the northern hemisphere has been observed. The change has been attributed to an increase of organic content in the water, leached from the top soil layers, called humic substances. Several theories have been proposed to explain the drivers behind the brownification process, among them the decrease of sulfur deposition from reduced industrial emissions, changes in land use through increased industrial forestry and changing patterns of precipitation, stream discharge and water temperature. More recently, the relationship between iron concentration and color has also been highlighted. The aim of this study was to examine these potential drivers and their relationship with color in River Storån catchment, a forested catchment in Southern Sweden located near the important drinking water source Lake Bolmen. Color values were retrieved from samples taken at four locations in the catchment between 1978 and 2018 by the river basin organization Lagans Vattenråd, and compared to data on precipitation, temperature and land use collected for the same period from various sources. The color in River Storån was found to be increasing, both when examining the average color and the extreme color values in the 95th percentile, which showed a positive linear trend. The precipitation in the catchment was also found to be increasing, as was the frequency of extreme precipitation events. A moderate to high correlation was established between precipitation and color, the highest being 0.74 near the precipitation measuring station in the catchment. A moderate negative correlation between sulfate concentration and color was found in River Storån, in several sampling points. A significant positive correlation was found between iron and color in River Storån, the highest being 0.81. A high correlation was also found between iron concentration and yearly precipitation, which seem to support the theory that the increase of iron is the result of wetter conditions in the upper soil layers due to increasing precipitation. The differences in land use between the sub catchments around Lake Bolmen seem to be related to freshwater color, as the catchments having the highest forest and wetland coverage also have the highest color values. An interesting observation of an increase in color from upstream to downstream in River Storån suggest that the wetlands in the catchment might contribute significantly to the color, however further investigations are needed to confirm this theory. Brownification seem to be the product of several different factors, and as the color of Lake Bolmen continues to change it is important to take the right steps to more accurately monitor the parameters related to this process.