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

Data belonging to AGU:Water Resources Research submission: Global warming and continued external loading increase nitrogen limitation in a temperate reservoir system. Climate change impacts hydrology, geochemistry, and biology of lakes and reservoirs worldwide, also affecting surface water concentrations of the essential nutrients nitrogen (N) and phosphorus (P). Few studies have illustrated climate change’s impact on nutrient processing due to compounded effects of both climate change and catchment inputs. In this study, we have evaluated monitoring data from the years 2000 to 2019 in the Franconian Lake District (FLD), which consists of one shallow (hypertrophic) and three deep reservoirs (meso- to eutrophic), interconnected by a transfer canal. The cascade configuration and consistent external load buffer catchment variations, making nutrient trends attributable to internal processing. Mass balances were set up and statistical analyses were conducted for trends in stratification, hypolimnetic anoxia and nutrient concentrations. Warming significantly increased water temperature (+0.35-1.0°C decade−1 ), stratification (+7-18 days decade−1), and hypolimnetic anoxia (+15-35 days decade−1). TP increased in deep reservoirs (+0.006-0.01 mg-P L−1 decade−1 ) and TN decreased in all reservoirs (−0.2-0.4 mg-N L−1 decade−1 ). The increased rates of NO3-loss could be related to enhanced denitrification rates 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. The identified impacts emphasizes the need to consider the potential disruptive effects of intensifying climate change on the health and restoration efforts for temperate, eutrophic lakes worldwide.