The decreasing Chlorophyll-a in Tibet Plateau lakes during 1986–2021 based on Landsat image inversion and their impact causes

Lake chlorophyll-a (Chl-a) is one of the important components of the lake ecosystem. The Chl-a concentration of global water has generally increased in recent decades due to climate change and intensified anthropogenic activity. However, few researches have been done on the lake Chl-a variations in remote areas with less disturbance by human activities such as the Tibet Plateau (TP). Here, we combined 95 in situ measured lake Chl-a concentration data and Landsat reflection spectrum to establish an inversion model of Chl-a concentration through the backpropagation (BP) neural network prediction method, by which the mean annual Chl-a concentration in the past 35 years (1986–2021) of 318 lakes with an area of > 10 km² in the TP have been retrieved. Meteorological and hydrological data, measured water quality parameters and glacier change in the lake basin were used to elucidate the driving factors of the Chl-a concentration changes in the TP lakes, with the help of geographic information system (GIS) technology and by spatial statistical analysis. The results showed that the mean annual Chl-a in the 318 lakes performed overall decrease during 1986-2021, but 63%, 32% and 5% of the total number exhibited no significant change, significant decrease and significant increase, respectively. After a slight increase during 1986–1995 (0.05 μg/L/y), the mean annual lake Chl-a significantly decreased during 1995–2004 (–0.18 μg/L/y). Further, after a slight increase during 2004–2011 (0.07 μg/L/y), it decreased slightly during 2011–2021 (–0.04 μg/L/y). The mean annual lake Chl-a concentration was significantly negatively correlated with precipitation (R² = 0.48, P < 0.01), air temperature (R²= 0.31, P < 0.01), lake surface water temperature (LSWT) (R² = 0.51, P < 0.01), lake area (R²= 0.42, P < 0.01) and lake water volume change (R² = 0.77, P < 0.01). The decrease in mean annual Chl-a was in consistant to the decrease in that of salinity (R²= 0.69, P < 0.01) and increase in that of transparency (R²= 0.55, P < 0.01). The Chl-a concentrations of non-glacial meltwater-fed lakes were higher than those of glacial meltwater-fed lakes, except during higher precipitation period. Our result of lake Chl-a inversion and their variation reason analyses is able to further deeply understand the climate change impacts on Chl-a changes in the TP lakes.