Global warming enhances nitrogen-limitation in a temperate reservoir system under consistent external load.

Climate change impacts hydrology and biogeochemistry of reservoirs. Thereby, processing of the essential nutrients nitrogen (N) and phosphorus (P) is affected. Clarifying the compounded long-term impact of changed nutrient inputs and effects of climate change on internal nutrient processing requires long-term datasets with sufficient detail. This study evaluates monitoring data from 2000 to 2019 in the Franconian Lake District in Germany, which consists of one shallow (hypertrophic) and three deep reservoirs (meso- to eutrophic), interconnected by a transfer canal. The cascade configuration and continued external load buffer catchment variations, making nutrient trends attributable to internal processing. Mass balances were set up and statistical trends analyses performed for nutrient concentrations, duration of stratification and hypolimnetic anoxia. Across reservoirs, mean water temperature (range: +0.35 to +1.0°C decade⁻¹), stratification (+7 to +18 days decade⁻¹) and hypolimnetic anoxia (+15 to +35 days decade⁻¹) increased significantly. TP increased in deep reservoirs (+0.006 to +0.01 mg P L⁻¹ decade⁻¹) and TN decreased in all reservoirs (−0.2 to −0.4 mg N L⁻¹ decade⁻¹). Increased rates of nitrate loss could be attributed to enhanced denitrification and earlier algal uptake. Increased TP concentrations were attributable to increased sediment P-release, induced by prolonged stratification and hypolimnetic anoxia. Primarily, the decrease in TN drove a strong decrease in TN:TP ratio (−4 to −15 mol:mol decade⁻¹), triggering a biogeochemical regime shift towards N-limitation, associated with proliferation of harmful algae blooms. Identified impacts emphasize the need to consider the potential disruptive effects of intensifying climate change on health and restoration efforts for temperate, eutrophic lakes worldwide.