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

Data belonging to AGU:Water Resources Research upload: 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.本数据集为美国地球物理联合会（American Geophysical Union, AGU）旗下《水资源研究》（Water Resources Research）的配套上传数据。 全球变暖与持续外源负荷加剧温带水库系统的氮限制效应。 气候变化会对全球湖泊与水库的水文过程、地球化学循环及生态系统功能产生影响，同时也会改变地表水中必需营养元素氮（N）与磷（P）的浓度水平。由于气候变化与流域输入的复合效应，目前鲜有研究阐明气候变化对营养盐循环过程的具体影响。 本研究针对弗兰科尼亚湖区（Franconian Lake District, FLD）2000-2019年的监测数据展开分析，该湖区包含1座浅型（超富营养化）水库与3座深型（中营养至富营养化）水库，四座水库通过输水渠相互连通。该梯级水库配置与恒定的外源负荷缓冲了流域输入的年际差异，使得营养盐浓度的变化趋势可直接归因于水库内部的生物地球化学循环过程。 本研究构建了物质平衡模型，并针对水库热分层、湖下层缺氧及营养盐浓度的变化趋势开展了统计分析。结果显示，气候变暖显著提升了水体温度（增速为0.35~1.0℃·十年⁻¹）、水库分层时长（增速为7~18天·十年⁻¹）以及湖下层缺氧持续时长（增速为15~35天·十年⁻¹）。 深型水库的总磷（TP）浓度呈上升趋势（增速为0.006~0.01 mg-P·L⁻¹·十年⁻¹），而所有水库的总氮（TN）浓度均有所下降（降幅为0.2~0.4 mg-N·L⁻¹·十年⁻¹）。硝态氮（NO₃⁻）流失速率的提升可能与反硝化作用增强以及藻类提前吸收营养盐有关。 总磷浓度的上升可归因于长期分层与湖下层缺氧引发的沉积物磷释放量增加。总体而言，总氮浓度的下降主导了总氮总磷比（TN:TP）的显著降低（降幅为4~15 mol·mol⁻¹·十年⁻¹），进而引发了生物地球化学系统向氮限制状态的转型，这一过程与有害藻华的爆发密切相关。 本研究揭示的影响表明，亟需关注不断加剧的气候变化对全球温带富营养化湖泊的生态健康与修复工作可能带来的破坏性影响。