Global warming and continued external loading increase nitrogen limitation in a temperate reservoir system

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Global warming and continued external loading increase nitrogen limitation in a temperate reservoir system.

Climate change is impacting the hydrology, geochemistry, and biology of lakes and reservoirs worldwide. Warming also affects the important nutrients nitrogen (N) and phosphorus (P), which determine the trophic state and TN:TP-ratio, crucial for algae species composition. Few studies illustrate climate change’s impact on nutrient processing due to compounded effects of warming and catchment changes. The Franconian Lake District (FLD) consists of four deep- and shallow reservoirs; it is well-monitored and catchment variations are buffered downstream by the cascade configuration. Mass balances for N and P and statistical analyses of trends in hypolimnetic anoxia and nutrient species from 2000 to 2019 were conducted. Warming significantly increased water temperature, stratified periods, and hypolimnetic anoxia. Significant increases in TP-concentrations were found in deep reservoirs and across all reservoirs TN-concentration decreased. This was not attributable to external input. Warming had opposite effects on N and P but predominantly influenced N-concentrations by boosting denitrification rates and advancing algal nitrate uptake. Sediment phosphorus release, enhanced by warming-induced stratification and hypolimnetic anoxia, accounted for the TP increase. This study demonstrated how warming differentially affects N and P, showing that warming’s impact on nitrogen primarily drove the TN:TP-decrease. Identified drivers are likely relevant for most temperate reservoirs and will intensify in the coming century. The study underscores the necessity of considering climate change’s disruptive effects in the long-term development and restoration of eutrophic lakes and reservoirs. More case studies, modeling, and meta-analyses explicitly incorporating warming effects are needed to further understand climate change’s impact on lakes and reservoirs.