Characteristics of nutrient export from a typical polder around Chaohu Lake
For the prevention and control of pollution in the polder area around Chaohu Lake with the goal of eutrophication control and ecological restoration of the lake,the discharge,nutrient emissions,water quality and spatiotemporal change characteristics of the polder area should be observed and in-vestigated extensively.The quantity and quality of water from each discharge events of the nine drain-age pumping stations in the Tongdawei Polder,a typical polder around Chaohu Lake,were monitored from June 2020 to June 2021.Meanwhile,water samples were taken and tested every 7-10 days from the pond of each station.Altogether 968 water samples and 5 171 nitrogen(N)and phosphorus(P)concentration data were obtained.The results showed that during the year,1.06X 108 m3 of water was discharged from the polder into Chaohu Lake.The annual mean concentrations of total nitrogen(TN)and total phosphorus(TP)were(3.988±4.748)mg/L and(0.236±0.184)mg/L,respective-ly.Concentrations of ammonia nitrogen and TP in nearly 50%of discharge samples were below the limits for Class Ⅲ surface water quality standards.But still 16%of samples had TP over 0.4 mg/L,9%of samples had ammonia nitrogen over 2.0 mg/L,and more than 60%of samples had TN over 2.0 mg/L.Part of the discharge events had very high NP concentrations,which caused dark color and negative impact on water quality of the recipient river.NP concentrations of discharges were clearly related to precipitation and fertilization.Soil erosion during heavy rainfall resulted in increment of par-ticulate phosphorus(PP)in discharges,whereas rainfall after fertilization might cause significant N loss.The annual emission loads of ammonia nitrogen,TN,NO3-N and TP of Tongdawei Polder were 57.39 t,348.76 t,173.84 t and 32.46 t,respectively,and 66%of TN and 90%of TP were released during June to October,i.e.,the rice season.Discharge volume is the main factor affecting NP emis-sion loads.Reducing discharge volume and NP concentrations are the keys to tackle pollutions from the polder.Suggestions on the engineering and management strategies are proposed accordingly.