Phosphorus (P) precipitation is among the most effective treatments to mitigate lake eutrophication. However, after a period of high effectiveness, studies have shown possible re-eutrophication and the return of harmful algal blooms. While such abrupt ecological changes were attributed to the internal P loading, the role of lake warming and its potential synergistic effects with internal loading, thus far, has been understudied. Here, in a eutrophic lake in central Germany, we quantified the driving mechanisms of the abrupt re-eutrophication and cyanobacterial blooms in 2016 (30 years after the first P precipitation). A process-based lake ecosystem model (GOTM-WET) was established using a high-frequency monitoring data set covering contrasting trophic states. Model analyses suggested that the internal P release accounted for 68% of the cyanobacterial biomass proliferation, while lake warming contributed to 32%, including direct effects via promoting growth (18%) and synergistic effects via intensifying internal P loading (14%). The model further showed that the synergy was attributed to prolonged lake hypolimnion warming and oxygen depletion. Our study unravels the substantial role of lake warming in promoting cyanobacterial blooms in re-eutrophicated lakes. The warming effects on cyanobacteria via promoting internal loading need more attention in lake management, particularly for urban lakes.

Department of Ecoscience Aarhus University 8000 Aarhus Denmark

Department of Lake Research Helmholtz Centre for Environmental Research UFZ 39114 Magdeburg Germany

Institute of Hydrobiology Biology Centre Czech Academy of Sciences Na Sádkách 7 České Budějovice 37005 Czech Republic

Laboratório de Limnologia Ecotoxicologia e Ecologia Aquática Instituto de Ciências Biológicas Universidade Federal de Minas Gerais Avenida Antônio Carlos 6627 Cep 31270 901 Belo Horizonte Minas Gerais Brazil

State Key Laboratory of Lake Science and Environment Nanjing Institute of Geography and Limnology Chinese Academy of Sciences 210008 Nanjing China

Water Systems and Global Change Group Wageningen University and Research Droevendaalsesteeg 3 6708 PB Wageningen The Netherlands

Zoology and Physiology Department University of Wyoming Laramie Wyoming 82071 United States

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