RESUMO
Based on the seasonal changes in the nitrogen and phosphorus concentrations in Hanfeng Lake from March 2017 to February 2018, the nutrient limitation status was evaluated by the stoichiometric molar ratio of nitrogen and phosphorus. The results showed that the average concentrations of TN, DN, and NO3--N were 1.60, 1.25, and 0.91 mg·L-1 in Hanfeng Lake, respectively. The seasonal changes of those indicators were similar, showing the highest concentration in winter and lowest in summer. NO3--N accounted for TN significantly in the water body, and the concentrations of NH4+-N and NO2--N remained at low levels and changed steadily. The average concentrations of TP, DP, and PO43--P were 0.13, 0.09, and 0.06 mg·L-1, respectively. The changes in the concentrations of TP and DP were similar, showing a trend of increasing in spring and summer, and then decreasing in autumn and winter, while the PO43--P concentration showed the trend of fluctuated decrease. TN/TP varied from 11.07 to 56.02, with an average value of 29.23. TN/TP changed seasonally, with the highest value occurring in winter and the lowest value in summer. The conditions of the water body were conducive to growth and reproduction of algae for most of the time during sampling months. The water body was occasionally nitrogen limited and rarely phosphorus limited. The seasonal variation in TN/TP ratio was affected by several factors such as rainfall runoff, fertilizer use, sewage discharge, and aquatic biological activities. Further, protection strategies were proposed for the improvement of the water body in terms of present water quality characteristics in Hanfeng Lake.
RESUMO
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.