鱼类放养和气候变化而非营养富集可能是导致中国亚热带淡水湖泊水质恶化的主要驱动因素
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在过去的几十年里，全球淡水湖泊的物种组成和水质都发生了巨大变化。以往的研究主要集中在流域内养分富集对水质影响。然而，气候变化和鱼类放养对水质影响的相对重要性性尚不清楚。通过对155个湖泊数据进行4期实地调查，发现近20年来水体全氮和全磷变化不显著，然而浮游植物丰度显著增加，水体透明度下降44.1%。研究发现鲢鳙放养和气候变化可能比营养物质对水质影响更重要，这在两个具有长期监测历史的湖泊（东湖湖和千岛湖）中也得到了证明。鱼类放养密度与甲壳类浮游动物负相关，与浮游植物密度和叶绿素含量正相关，进而导致水体透明度下降。总体而言，我们的研究结果对湖泊的恢复和管理具有重要意义，除了削减营养盐的排放，降低鱼类种群的放养密度可能是今后长江中下游湖泊水质改善的工作重点，这为中国亚热带湖泊水质管理保护策略提供了新的见解。
Over the past decades, dramatic changes have occurred in both species composition and water quality of global freshwater lakes. Previous studies have primarily focused on the impacts of watershed-scale nutrient enrichment on water quality. However, the relative importance of climate change and fish stocking in driving water quality variations remains unclear. Based on four-phase field surveys of data from 155 lakes, we found that over the past 20 years, total nitrogen and total phosphorus concentrations in the water showed no significant trends, while phytoplankton abundance increased significantly, and water transparency decreased by 44.1%. Our analyses revealed that silver carp and bighead carp stocking and climate change may exert more substantial impacts on water quality than nutrient loading, a finding that was also validated in two long-term monitored lakes (Lake Donghu and Lake Qiandao). Fish stocking density exhibited a negative correlation with crustacean zooplankton, but positive correlations with phytoplankton density and chlorophyll content, thereby contributing to the decline in water transparency. Overall, our results carry important implications for lake restoration and management. Beyond reducing nutrient emissions, lowering fish stocking densities may represent a key priority for improving water quality in lakes of the middle and lower reaches of the Yangtze River in the future, providing novel insights into water quality management and conservation strategies for China's subtropical lakes.
