Assessing the Impact of Land Use and Landscape Patterns on Water Quality in Yilong Lake Basin (1993–2023)

To investigate the influence of land use landscape pattern on lake water quality in the basin, the land use and water quality data of Yilong Lake Basin from 1993 to 2023 were analyzed with GIS, RS and landscape ecology methods in this research. The results showed that: 1) the land use landscape pattern and water quality of Yilong Lake Basin had significant changes: the water surface of Yilong Lake decreased, the area of farmland and shrubs decreased, the area of grassland decreased dramatically, which were the main sources of growth of other land types, the area of trees increased significantly, and the area of building land increased by five times. The landscape pattern analysis showed that the aggregation degree of core habitat in the basin increased and the landscape had decreased patch density and increased heterogeneity. In terms of water quality, the concentrations of TN, TP, NH₄⁺-N, I_Mn and BOD₅ decreased, the concentration of DO increased, and the concentration of Chl-a fluctuated for a long time but did not decrease dramatically at the end of the period compared with the beginning. In general, the eutrophication degree of Yilong Lake slightly reduced. 2) There was a significant correlation between landscape pattern indexes and water quality indexes: ED, LSI, LPI and PD were negatively correlated with eutrophication of Yilong Lake, CONTAG was positively correlated, SHDI was strongly positively correlated with TN and I_Mn, and COHESION had a low interpretation rate for water quality changes. Among them, LPI and COHESION were significantly positively correlated with the concentration of DO, PD was significantly positively correlated with I_Mn, and SHDI was significantly negatively correlated with the concentration of DO, indicating that the "sink" landscape in the basin gradually dominated and the restoration of key ecological patches and the enhancement of landscape cohesion helped to improve water quality, while the increase of patch density and landscape heterogeneity possibly had negative influence on water quality. 3) In the past 30 years, the ecological management and protection work of Yilong Lake, such as returning farmland to forests and lake, and wetland and sewage pipe network construction, achieved remarkable results which were reflected in the change of the relationship between land use landscape pattern and water quality in the basin. However, human activities still affected the dynamic evolution of water quality: the expansion of building land increased patch density, the reduction of shrubs and grassland weakened natural filtration, and the rapid urbanization process introduced more pollution sources. Although the increase of trees helped to improve water quality, the effect was not fully developed.