[Impact on nitrogen and phosphorous export of wetlands in Tianmu Lake watershed].

Focused on understanding the function of wetland in improving water quality, Pingqiao watershed and Zhongtian watershed in Tianmu Lake drinking water sources area were selected as the research region. We integrated remote sensing, GIS techniques with field investigation and chemical analysis to analyze the relationship between wetland and water quality in watershed scale. Results show: (1) There are many wetland patches in Pingqiao and Zhongtian watershed, wetlands patch densities were respectively 7.5 km(-2) and 7.1 km(-2). Wetlands widely distributed in the Pingqiao watershed with mostly located away from the river of 500 m, whereas wetlands relatively concentrated in the lower reach within 500 meters of riverside in Zhongtian watershed. (2) Nitrogen and phosphorus nutrient retention of wetland in watershed scale was significant. The annual mean TN and DTN concentration had a strong relationship with percent area of wetlands in Zhongtian watershed while the weakest relationship was found with TP and DTP concentrations, especially, the mean TN and DTN concentrations in spring and winter had the significantly negative relationship with wetland areas of watershed. The negative relationship was existed for nitrogen in autumn of Pingqiao watershed, which suggested that watersheds varying in area of wetlands have the different nutrient reducing efficiency in seasonal periods. (3) A certain number and area of wetland will improve river water quality in watershed scale, which can instruct water environment treatment. However, considering the complexity of nutrient transport processes in watershed, wetland-related factors such as area, location, density, ecosystem structure and watershed-related factors such as temporal interval, spatial scales, slope and land use will impact on the transport processes, and related theoretical and practical problems need further research.