Prolongation, deepening and warming of the metalimnion change habitat conditions of the harmful filamentous cyanobacterium Planktothrix rubescens in a prealpine lake
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The most prominent responses of Lake Zurich to climate warming include the increase of surface water temperatures, a reduced depth of spring mixing, and the persistent thriving of the harmful cyanobacterium Planktothrix rubescens, a low-light adapted species concentrating in the metalimnion during summer. To study changes of its habitat, we assessed the spatio-temporal metalimnetic boundaries by applying low-pass filtering and binary thresholding to temperature profiles of long-term data (1978–2013, weekly measurements). Due to increasing temperatures over the last 3.5 decades, the onset and duration of metalimnion formation changed significantly (stratification increased by 33 days). Moreover, the upper metalimnetic boundary has undergone a significant drawdown of 2.3 m, accounting for an overall decrease in metalimnion thickness. Being the most abundant species in this zone, P. rubescens was not negatively affected by altered temporal or spatial stratification patterns during its phase of net growth (July–September), as the zone of its optimal light-dependent buoyancy was still located within the metalimnion. The biomass of P. rubescens in August was even significantly related to increasing temperatures. Nevertheless, a further depression of the metalimnetic top boundary may eventually restrict P. rubescens by forcing it into layers of unfavourable light conditions or into the turbulent epilimnetic zone.
KeywordsLake warming Lake Zurich Long-term data Metalimnion Neutral buoyancy Planktothrix rubescens
We would like to thank Eugen Loher and everyone who helped sampling over the years. The Water Supply Zurich is also acknowledged for providing long-term data on P. rubescens. We thank two anonymous reviewers for their constructive comments. This study is funded by the University Research Priority Program (URPP) ‘Global Change and Biodiversity’ of the University of Zurich and by the Swiss National Fund (SNF 310030E-160603).
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Conflicts of interest
All authors have declared no conflicts of interest.
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