Evolution patterns, driving mechanisms, and ecological indicative effects of 730 lakes water color in the Yangtze River Basin (1984–2023)

Abstract

Water color, as derived from the Forel-Ule Index (FUI) plays a critical role in evaluating large-scale aquatic ecological environments. However, previous research has not yet applied the FUI to the entire basin. This study generates 730 lakes FUI data for the Yangtze River Basin from 1984 to 2023, divided into ten periods of four years each, using Google Earth Engine (GEE). Pixel-scale calculations and classifications were employed to objectively determine the spatial–temporal evolution patterns of lakes FUI. Various primary indicators, including hydrology, climate, economy, and human activity, were analyzed to explore the driving mechanisms of FUI variations. Lakes in the Yangtze River Basin are categorized into two groups: those located in the source region on the Qinghai-Tibet Plateau and the remaining non-source region lakes. The findings revealed that (1) over 80 % of lake water in the source region was blue (FUI < 10), with 83.06 % of lakes showing a decreasing trend from 1984 to 2023; (2) lakes in non-source areas exhibited seasonal variations. Among these, 80 % of lakes were green or yellow (FUI > 10), and 67.51 % showed a decreasing trend from 1984 to 2023; (3) the increase in rainfall and air temperature was identified as the primary cause of the bluing of all lakes in the source region, also leading to increased erosion intensity and the formation of small thermokarst lakes (<5 km²) that appeared yellow (FUI > 10); and (4) The enhanced protection of the Yangtze River after 2008 has reduced pollution emissions, leading to the increase of total nitrogen (TN), total phosphorus (TP) and chemical oxygen demand (COD), and significantly promoted the reduction of lakes FUI in the non-source region. Besides, the massive reduction of human activities during the COVID-19 epidemic directly led to the alleviation of the lake’s trophic status index (TSI) and the reduction of the urban lake’s FUI. This study suggests that more efficient lake management and protection strategies are needed to address the challenges facing the Yangtze River Basin due to climate change and human activities.