Increased nutrient loading and rapid changes in phytoplankton expected with climate change in stratified South European lakes: sensitivity of lakes with different trophic state and catchment properties
We hypothesised that increasing winter affluence and summer temperatures, anticipated in southern Europe with climate change, will deteriorate the ecological status of lakes, especially in those with shorter retention time. We tested these hypotheses analysing weekly phytoplankton and chemistry data collected over 2 years of contrasting weather from two adjacent stratified lakes in North Italy, differing from each other by trophic state and water retention time. Dissolved oxygen concentrations were higher in colder hypolimnia of both lakes in the second year following the cold winter, despite the second summer was warmer and the lakes more strongly stratified. Higher loading during the rainy winter and spring increased nutrient (N, P, Si) concentrations, and a phytoplankton based trophic state index, whilst the N/P ratio decreased in both lakes. The weakened Si limitation in the second year enabled an increase of diatom biovolumes in spring in both lakes. Chlorophyll a concentration increased in the oligo-mesotrophic lake, but dropped markedly in the eutrophic lake where the series of commonly occurring cyanobacteria blooms was interrupted. The projected increase of winter precipitation in southern Europe is likely to increase the nutrient loadings to lakes and contribute to their eutrophication. The impact is proportional to the runoff/in-lake concentration ratio of nutrients rather than to the retention time, and is more pronounced in lakes with lower trophy.
KeywordsClimate change Anoxia Phosphorus release Silicon limitation Change of dominant species Flushing
The study was supported by the JRC institutional exploratory project of the Action EEWAI and the EU grants WISER and REFRESH. WISER (Water bodies in Europe: Integrative Systems to assess Ecological status and Recovery, Contract No.: 226273) and REFRESH (Adaptive strategies to Mitigate the Impacts of Climate Change on European Freshwater Ecosystems, Contract No.: 244121) are being funded under the 7th EU Framework Programme, Theme 6 (Environment including Climate Change). The authors are grateful for the meteorological data kindly provided by the JRC Ispra Atmospheric Research Station and the RAMAN Meteorological Observatory of the Joint Research Centre. Special thanks to Dr. Anna-Stiina Heiskanen for initiating the lake research programme, to Dr. Ana Cristina Cardoso, Dr. Gary Free, Bruno Paracchini, and Joaquin Pinto Grande for keeping it running and to Dr. Ute Mischke for her help and advice in using the PhytoSee software.
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