[Seasonal Variations in Nitrogen and Phosphorus Concentration and Stoichiometry of Hanfeng Lake in the Three Gorges...

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 PO₄^3--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 PO₄^3--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.