The cumulative effects of cascade reservoirs control nitrogen and phosphorus flux: Base on biogeochemical processes.

The reservoir serves as a water source, a flood control structure, a navigational aid, and also impacts the downstream ecosystem as well as the reservoir zone. However, debate exists about effectiveness of cascade reservoirs in controlling the transportation of nutrients, particularly in the Yangtze River basin, which has been significantly affected by reservoir development. This research develops a new model X-NPSEM (X with Nitrogen and Phosphorus Steady-state Reservoir Model) based on biogeochemical processes of nitrogen and phosphorus reaction for investigating the dynamic storage capacity of cascade reservoirs at both reservoir- and watershed scales. Then the cumulative effects of cascade reservoirs and the related mechanism were investigated in Fujiang watershed, China. Based on the results, cascade reservoirs retained 16.3 % of nitrogen fluxes and 37.6 % of phosphorus fluxes annually. Downstream reservoirs have higher retention rates of phosphorus (0.48/d) compared to upstream reservoirs (0.10/d), mainly due to inflow sediment. Nitrogen retention rates show seasonal variations: wet season (0.21/d) and dry season (0.17/d). These fluctuations in nitrogen retention are primarily influenced by changes in temperature rather than other factors such as operation period, nitrogen and phosphorus concentration, or the nitrogen/phosphorus ratio. In upstream, the concentration of sediment entering the reservoir plays a decisive role in the transformation of P retention from sink to source. The X-NPSRM coupler model could be used for global reservoir operation and watershed management.

[Pollution Characteristics and Health Risk Assessment of Microorganism Pollutions in the Beiyun River].

Microorganism pollution in rivers is of great importance to the protection of watershed water quality and public health management. As a typical watershed of the Haihe River watershed, the Beiyun River was chosen as the study area, and the characteristics and health risk of microorganism pollution were assessed from a comprehensive perspective. The results showed that the microbial contamination of the fecal sources was serious during the wet season, and the microbial amount at most river sections was more than 105 MPN·L-1. During the normal season and dry season, the microbial amount was approximately 103-105 MPN·L-1. Therefore, no obvious change could be observed. The fecal pollution in the agricultural river sections was the most severe, and the water quality of over 60% of these river sections was below the state Grade V level. The fecal microbial biomass of some urban river sections increased from 103 MPN·L-1 to 106MPN·L-1 after the rainfall event, indicating an obvious change of fecal microbial pollution during the rainfall process. For the Beiyun River, the exposure risk of the fecal microbial biomass was mainly between 0.015-0.035, while the Lianhua River, Macao River, lower reaches of Qinghe River, and lower reaches of Wenyu River were hotspots for contamination. Greater attention should be paid to these areas.