Size and temperature drive nutrient retention potential across water bodies in China.

Lentic water bodies, including lakes, reservoirs, and wetlands, retain excess nutrients in runoff from agricultural and urban activities, and protect downstream water bodies from eutrophication. To develop effective nutrient mitigation strategies, it is important to understand the controls on nutrient retention in lentic systems and what drives variability between different systems and geographical regions. Efforts to synthesize water body nutrient retention at the global scale are biased toward studies from North America and Europe. Numerous studies published in Chinese Language journals exist in the extensive China National Knowledge Infrastructure (CNKI), but are missing from global synthesis due to their absence in English language journal databases. We address this gap by synthesizing data from 417 waterbodies in China to assess hydrologic and biogeochemical drivers of nutrient retention. In this study, we found median retention of 46 and 51% for nitrogen and phosphorus, respectively, across all water bodies in our national synthesis, and on average, wetlands retain more nutrients than lakes or reservoirs. The analysis of this dataset highlights the influence of water body size on first-order nutrient removal rate constants, as well as how regional temperature variations affect nutrient retention in water bodies. The dataset was used to calibrate the HydroBio-k model, which explicitly considers the effect of residence times and temperature on nutrient retention. Application of the HydroBio-k model across China reveals patterns of nutrient removal potential, where regions with a higher density of small water bodies retain more nutrients than others, such that regions like the Yangtze River Basin with a greater proportion of smaller water bodies have greater retention rates. Our results emphasize the importance of lentic systems and their function in nutrient removal and water quality improvement, as well as the drivers and variability of these functions at the landscape scale.