The influence of lime and brownification in surface waters in Västra Götaland county, Sweden : Driving factors and trends

For more than 45 years an extensive liming measure has taken place in the southwest of Sweden in Västra Götaland county. The goal with the liming is to improve the aquatic ecosystems and counteract the acidification that has occurred in a large part of catchments in this area. The acidification problem was discovered in the late seventies and had a global impact but was distinctive in northern Europe. To mitigate the problem, it was necessary to find what was causing the acidification and, in this case, it was the emissions of acidic substances. A legislation about decreasing the emissions of sulfuric acid was established by several countries in Europe and after that a change occurred. Most of the aquatic ecosystems responded well to the decreased amount of sulfuric deposition and the liming. An unexpected response to the improved health of the aquatic ecosystems was the increased water color (mgPt·l-1) in surface waters. The change in water color, often called brownification, can be a natural process and an indication that the surface waters are approaching a more natural state. In this study a total of 120 surface waters in Västra Götaland county were studied under a 40-year period. An increasing browning of surface waters and significant differences in water color between limed lakes and reference lakes was discovered. The area with the highest yearly precipitation had the largest percentual increase in water color particularly for limed lakes. The results showed significant differences in water color in limed lakes between two time periods, 1981-1985 and 2015-2020, that indicate an increasing trend in the study area. The results could not answer why the surface waters had this increase in browning and there was not possible to determine all the driving factors. Other studies have shown that increasing water color relates to a decrease in sulfuric deposition and increasing concentration of dissolved organic carbon (DOC) in the surface waters. Climate change can be a part of the explanation due to warmer temperatures, longer vegetation periods and wetter climate. Further, it is necessary to continue investigating this problem to determine the driving factors and detect possible trends