Abstract: Dynamics of water purification services function and their driving forces are of great importance to aquatic environment protection and land use planning. Therefore the dynamics of the total nitrogen and phosphorus output combined with the InVEST-Nutrient Delivery Ratio (NDR) Model and ArcGIS software based on the land use data of Nansihu Lake Basin during 1975-2020 was analyzed, and the output loads of nitrogen and phosphorus in the basin were the proxies of water purification service function. The driving forces of water purification service function, including land use, landscape pattern indices and other physical and social factors, were analyzed by ArcGIS 10.2 and Fragstats 4.2. The results show that the total nitrogen and total phosphorus output in Nansihu Lake Basin increased first, then decreased, and increased again, which suggested an increasing trend during the period of 1975-2020. In terms of spatial distribution pattern, the output of nitrogen and phosphorus was high in the western plain, followed by the northern part, and lowest in the southeastern part. The correlation analysis suggests that forest and grassland intercepted the output of nitrogen and phosphorus while construction land promoted their output. Some landscape pattern indices, included SHAPE_MN, SIMI_MN, COHESION, CONTAG and LSI could accelerate the output of nitrogen and phosphorus, while other indices including CONTIG_MN, CAI_MN and IJI could prevent the output of nitrogen and phosphorus. The analysis of Geodetector indicates that the proportion of construction land in the basin is the main controlling factor to the output of nitrogen and phosphorus.

Key words: InVEST-NDR model, land use, landscape pattern, Nansihu Lake Basin, Geodetector, water purification services function