| Summary: | Mechanistic understanding of the impacts of changing climate on the thermal stratification and mixing dynamics of oxygen in lake ecosystems is hindered by limited evidence on how functioning of individual lakes is affected by interannual variability in meteorological conditions. We studied two boreal lakes (Kuivajärvi and Vendyurskoe) with different water clarity during three years representing a gradient of meteorological forcing. The lakes are located in the same climatic region and have a similar depth, but, because of the difference in water transparency, they responded differently to the weather fronts. Long spring overturn and relatively cold summer resulted in weak stratification and well-oxygenated water column in both lakes. However, in dark-water Kuivajärvi, the hypolimnion was unusually warm leading to high production rate of CO2. Warm summer had the most dramatic impact on clear-water lake Vendyurskoe changing its mixing regime from typical polymixis to dimixis, leading to strong oxygen depletion and ensuing anoxia. In dimictic dark-water Kuivajärvi, hot spring resulted in early onset of stratification with ensuing low hypolimnetic temperatures and persistent oxygen depletion. These results indicate that climate warming will increase the risk of occurrence of deep water anoxia. Peer reviewed
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