Impact of climatic factors on eutrophication in the World’s largest lake

• We explore impact of climatic factors on eutrophication in the world’s largest lake. • Data from MODIS-Aqua and the ERA5 model, spanning 2003 to 2021, were used. • We used the GAM to understand dynamics of Chl- a in response to the changing climate. • Photosynthetically active radiation dominantly impacted Chl- a changes in the lake. Climatic and anthropogenic factors both contribute to lake eutrophication. However, the influence of climatic factors, particularly in large, deep, and transboundary lakes, remains poorly understood due to technical challenges, data scarcity, and geopolitical constraints. This is especially true for the Caspian Sea, the world’s largest lake, where its unique continental climate further complicates efforts to quantify the climate contribution to eutrophication. This study leverages extensive datasets from MODIS-Aqua and the ERA5, spanning 2003 to 2021, to develop a generalized additive model (GAM) aimed at investigating the impact of climatic factors on chlorophyll- a (Chl- a ) concentrations in the Caspian Sea. Given the sea’s distinct continental climate, complex morphometric characteristics, and significant spatial variability in Chl- a , the basin was divided into 14 subzones to better capture regional responses of Chl- a to climatic changes. The GAM, trained to predict Chl- a , demonstrated acceptable performance (correlation coefficient > 0.5) in 12 of the 14 subzones. Results indicate the predominant influence of photosynthetically active radiation on Chl- a changes in nine subzones, particularly in the southern Caspian Sea. This parameter is critical for regulating light availability for phytoplankton productivity. Sea surface temperature emerged as the second most influential driver of Chl- a levels, likely due to its role in controlling thermal stratification and upwelling, which stimulate phytoplankton growth. Precipitation, by contrast, was found to be the least significant driver of Chl- a levels during the study period. By elucidating the relationships between climatic drivers and Chl- a levels, this study provides a comprehensive understanding of the complex dynamics of eutrophication under changing climate conditions in the Caspian Sea.