Temporal and spatial distribution of nitrogen and phosphorus and evaluation of trophic status of a water supply lake in Taihu basin

Total nitrogen(TN),total phosphorus(TP),nitrate-nitrogen(NO-3-N),ammonium-nitrogen(NH+4-N),nitrite-nitrogen(NO-2-N) and phosphate(PO3-4) of water samples collected at 20 points of a water supply lake in Taihu basin were measured monthly from November 2009 to October 2010.Temporal and spatial distributions of nitrogen,phosphorus and their different forms were discussed and the trophic status was evaluated.The results showed that annual average concentrations of TN and TP were 1.50,0.05 mg/L,respectively.TN concentration in winter and spring was higher than that in summer and autumn,while the highest TP concentration was found in winter.The overall spatial distribution pattern of TN and TP was similar;they both higher in Southwest than in Northeast,higher in inlet of lake than in outlet and center of lake.The concentration of NO-3-N presented a double-peak profile with annual average concentration was 0.68 mg/L,the peak value was found in March and September.NO-3-N in winter and spring was higher than in summer and autumn,it was also higher in inlet of lake than in outlet and center of the lake,which showed the contribution of denitrification to NO-3-N.The annual average concentration of NH+4-N was 0.23 mg/L,the concentration of NH+4-N was gradually increased from spring and reach maximum value at July.The concentrations of NO-2-N and PO3-4 were relatively low(0.01 mg/L),and their temporal and spatial distribution was not obvious.According to Carlson trophic state index,the lake in winter and spring was mesotrophic,and eutrophic in summer and autumn,and phosphorus was the limiting factor of eutrophication.