Spatial-temporal Distribution of Nutrients in Hanfeng Lake After Official Operation

Abstract

The hydrograph of Hanfeng Lake, which is the largest pre-dam of theThree Gorges Reservoir, varied between the fluvial stage and lake stage after the lake was officially operated, resulting in large shifts in the aquatic biogeochemical processes. To explore the spatial-temporal distribution of nitrogen and phosphorus concentrations and identify their influencing factors in Hanfeng Lake, seven sampling sites were set up to monitor the changes of nutrients and other water indicators at different water depths monthly from January to December 2018. The results showed that completely vertical mixing across water profiles was observed. The nutrient concentrations were not significantly different between the top, middle, and bottom water depths (P>0.05). Total nitrogen concentration decreased from January to September but gradually increased from October to December, with a monthly average concentration of 1.52 mg·L-1. NO2--N concentration decreased in the first four months, increased sharply from May to June, and decreased from July to December with a monthly average concentration of 0.05 mg·L-1. NO3--N concentration gradually decreased from January to June, and gradually increased from July to December. NH4+-N concentration was the highest in July, with a concentration of 0.44 mg·L-1, and the change in other months was not notable, with a monthly average concentration of 0.09 mg·L-1. The concentrations of total phosphorus (TP), dissolved phosphorus, and soluble reactive phosphorus showed insignificant changes in trends throughout the year, with monthly average concentrations of 0.17 mg·L-1, 0.11 mg·L-1, and 0.05 mg·L-1, respectively. The phosphate concentration was mainly sourced from the upstream Nan River and Taoxi River, and gradually decreased from upper Zhendong to the downstream regulating dam. Of these nutrients, TP was the key factor in the growth of algae in Hanfeng Lake.