Polar lakes act as key early detectors of global change effects. Duration and thickness of snow and ice cover determines albedo, underwater light availability, thermal and chemical regimes and mixing properties. However, quantitative data on responses by aquatic systems to regional-scale change remain scant. Here, by utilizing 48 year (1947-1995) air temperature and 33 year (1963-1996) lake environmental datasets obtained at Signy Island, South Orkney Islands, we report one of the fastest responses to regional climate change so far documented in the southern hemisphere. Between 1980-1995 lake water temperatures increased by 2-3 times the local air temperature rise. Autumn freeze and spring break up dates of lake ice have changed, extending the open water period by up to 4 weeks. Winter total extractable phytopigments, alkalinity and orthophosphate show 2- to 10-fold increases, all linked with an areal reduction of the island's ice cover of similar to 45% in total in the last 50 years. These results indicate that its geographical position allows Signy Island to be a sensitive indicator of rapid ecological change in Antarctic freshwater environments. Such rapid change will be widespread in polar regions if current global change predictions are correct
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