Quantify downstream delivery of bioavailable nutrients in cascade reservoirs by developing a watershed-reservoir...

It is widely perceived that reservoirs retain riverine nutrients, such as nitrogen (N) and phosphorus (P), hence impact downstream primary production and food chains. However, growing evidence indicates that reservoirs could enhance the downstream transport of bioavailable N and P, a phenomenon that has not been adequately quantified. In this study, we developed an integrated watershed-reservoir modeling system that combines a watershed nutrient load model with an in-reservoir mass balance model. Comparing to previous in-reservoir model, the one in this study accounted for detailed transformations of dissolved organic, inorganic, and particulate nutrients. The modelling system was employed to evaluate the effects of cascade reservoirs on riverine bioavailable N and P fluxes in the Lancang River (LCR) and Jinsha River (JSR) on an annual scale. It successfully captured increasing trends in ammonium (NH₄N) and soluble reactive phosphorus (SRP) concentrations. The net NH₄N and SRP yield relative to TN and TP influx increased by 0.2-10.6 % and 0.7-8.3 %, respectively. The bioavailable nutrient pools from the watersheds contributed minimally to the cumulatively enhanced outfluxes of NH₄N and SRP, highlighting the importance of incorporating detailed in-reservoir nutrient cycling into the modeling system. Furthermore, cascade reservoir systems with longer hydraulic residence times (HRT) experience greater enrichment capacity of bioavailable nutrients. Our modeling system provided a viable method to quantify the accumulated impacts of cascade reservoirs on downstream aquatic ecosystems.