Global Warming and Continued External Loading Alters Nitrogen and Phosphorus Balance in the Franconian Lake District

Data belonging to Limnology and Oceanography upload: Global Warming and Continued External Loading Alters Nitrogen and Phosphorus Balance in a Temperate Reservoir System. Global warming will intensify in the coming century. Consequently cycling of phosphorus and nitrogen in reservoirs will be affected which could lead to aggravated phytoplankton blooms. For now few case studies have simultaneously investigated effects of warming on cycling of both nitrogen and phosphorus. In this context, the study focused on the Franconian Lake District’s four reservoirs, covering the period from 2000 to 2019. Changes in water temperatures (which increased by 0.3 to 1°C annually), the extension of thermal stratification (9 to 19 days per decade) and hypolimnetic anoxia (14 to 36 days per decade) were determined. These shifts led to decreases in nitrogen concentrations but increases in phosphorus concentrations. Increased denitrification rates reduced nitrogen levels early in the year and ammonification in the anoxic hypolimnia showed only marginal increases, not compensating for denitrification losses. Conversely, phosphorus concentrations increased particularly in the increasingly anoxic summer months. This was ascribed to release of ortho-phosphate from mineralization and redox-sensitive phosphorus fractions in the sediment. Opposing trends led to a sharp decrease in the total nitrogen to total phosphorus (TN:TP) ratios. The study also revealed an increase in bio-available ortho-phosphate and ammonium. Combined the altered nutrient composition could lead to proliferation of bloom-forming, nitrogen-fixing diazotrophic cyanobacteria. This study suggests that most temperate reservoirs and lakes, suffering from ongoing external and internal nutrient loads, will probably experience further reductions in TN:TP-ratio in a warmer world. To mitigate indirect effects of warming, new strategies are required to decrease phosphorus inflows and manage legacy phosphorus release from sediments.