Impoundment-induced stoichiometric imbalance exacerbated phosphorus limitation in a deep subtropical reservoir: implications for eutrophication management

Impoundment reservoirs play a vital role as nutrient sinks, capable of retaining and exporting (N) and phosphorus (P) at different rates. The imbalance in P and N stoichiometry relative to phytoplankton demand often determines the limiting nutrient of algal growth for reservoirs. This critical factor has a substantial impact on the management of eutrophication, encompassing the formulation of nutrient control strategies and the setting of regulatory thresholds. Nonetheless, research remains relatively limited on algal limiting factors and nutrient stoichiometry interactions in subtropical impoundment reservoirs. This study fills a critical gap in the current research by providing a comprehensive assessment of the influences of N and P on phytoplankton biomass in Lake Qiandaohu, China. Utilizing field monitoring, nutrient addition experiments, and novel constraint line regression model, we provide new insights into the nutrient dynamics within the lake. Both bioassays experiment and statistics indicated predominant potentially P-limitation in Lake Qiandaohu owing to dam-induced deep-water conditions, characterized by a nearly 1:1 linear relationship between chlorophyll a (Chla) and total phosphorus concentrations. This underscores the pivotal role of P management in curbing algal blooms. Utilizing the constraint line equation that relates total P (TP) to Chla, we have proposed TP thresholds designed to keep Chla within the specified target ranges, specifically below 10, 12, 20, 24, 40, and 60 μg/L. Furthermore, leveraging Vollenweider's models with these TP concentration thresholds, the study has established TP loading targets that accommodate a range of hydrological conditions, from normal to wet and dry years. Furthermore, both nutrient addition experiment and constraint line regression model highlights potentially N and P co-limitation in specific regions, particularly the riverine zone, where the ratios of nitrogen to phosphorus are influenced by unsettled particulate matter resulting in relatively lower ratios. To address this, the study introduces TN thresholds and suggests localized control measures, including ecological floating macrophytes beds, as effective alternatives. Considering the uniform nutrient management policy currently applied across Chinese lakes and reservoirs, which may lead to under- or over-protection for individual water bodies, our research provides a cost-effective eutrophication management framework tailored for the subtropical Eastern Plains ecoregions.