Increasingly ice-free winters and their effects on water quality in Sweden’s largest lakes
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Mean global air temperatures have steadily increased during recent decades, resulting in an earlier timing of lake ice breakup. In Sweden’s largest lakes, Vänern and Vättern, the breakup of ice has occurred considerably earlier since 1979 and ice-free winters have become more frequent. Comparison between the years when the lakes were ice covered with those when they remained ice-free in terms of 37 lake variables revealed significant differences in water temperatures, sulphate concentrations and the biomass of diatoms in May after ice breakup (P < 0.01). In particular, the biomass of the genus Aulacoseira increased significantly, which may explain increasing complaints about algae that clog fishing-nets, filter-beds and micro-strainers in waterworks in Vänern and Vättern. We assume that Aulacoseira is mainly affected by changes in climate-driven water circulation patterns. In contrast, other observed water quality changes such as changes in sulphate concentration might rather be attributed to changes in atmospheric deposition. To explain water quality changes in Sweden’s largest lakes it is important to consider changes in both climate and atmospheric deposition as well as catchment measures.
KeywordsClimate Atmospheric deposition Ice Water Chemistry Phytoplankton
This work was partly funded by the European Union within the framework of the European Commission projects CLIME (“Climate and Lake Impacts in Europe”, EVK1-CT-2002-00121) and Euro-limpacs (“Integrated Project to Evaluate the Impacts of Global Change on European Freshwater Ecosystems”, GOCE-CT-2003-505540). The first author is a research fellow of the Royal Swedish Academy of Sciences supported by a grant from the Knut and Alice Wallenberg Foundation. Additional funding was received from the Swedish Research Council (621-2005-4335). We thank the Swedish Environmental Protection Agency and the IMA laboratory for financing, sampling and analysing numerous water samples.
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