Impact of a large sub-tropical reservoir on the cycling of nutrients in a river

The quantity and composition of nutrients carried by rivers play an important role in maintaining the ecosystem of downstream rivers and marginal seas. To reveal the impact of damming on the composi-tion and flux changes of nutrients in rivers, this study conducted a detailed survey of a large sub-tropical reservoir (Xinanjiang Reservoir, XAJR) in eastern China from August 2013 to June 2014 obtaining samples at bi-monthly intervals. The thermal stratification, water quality in situ parameters, and the contents of nutrients in the water column of the river inflow, transition area, central reservoir area, and discharge water of the XAJR were analyzed in detail along the fluvial direction. The results show that the thermal stratification of the XAJR had seasonal and spatial heterogeneity. Accordingly, the pH and dissolved oxygen saturation degree in water also showed a similar stratification phenomenon. The analysis of nutrient limitation for primary productivity indicated that in different seasons, varying limiting degrees of the silicon or phosphorus were developed in different locations along the XAJRriver system. Among them, the river area and transition area were more susceptible to silicon restriction in winter, whereas phosphorus restriction mainly occurred during the warm seasons from April to October. XAJR had a retention effect on nitrogen, phosphorus, and silicon, among which phosphorus was more easily retained by the reservoir. In addition, nitrogen underwent transformation processes between different forms inside the reservoir. In the reservoir, the different degrees of retention effect of nutrients also led to a significant increase in the ratio of silicon to phosphorus and nitrogen to phosphorus in the discharged water. This study confirmed that thermal stratification has important control over the contents and forms of nutrients in water. This finding provided an idea for the restoration of the fluxes and stoichiometric ratios of the nutrients in the downstream river using the reservoir's capacity for artificial regulation. (c) 2020 Elsevier Ltd. All rights reserved.