Satellite‐Based Monitoring of Eutrophication in the Earth's Largest Transboundary Lake

The world's large lakes and their life‐supporting services are rapidly threatened by eutrophication in the warming climate during the Anthropocene. Here, MODIS‐Aqua level 3 chlorophyll‐a data (2018–2021) were used to monitor trophic state in our planet's largest lake, that is, the Caspian Sea that accounts for approximately 40% of the total lacustrine waters on Earth. We also used the in situ measurements of chlorophyll‐a data (2009–2019) to further verify the accuracy of the data derived from the MODIS‐Aqua and to explore the deep chlorophyll‐a maxima (DCMs) in the south Caspian Sea. Our findings show an acceptable agreement between the chlorophyll‐a data derived from the MODIS‐Aqua and those measured in situ in the coast of Iran (coefficient of determination = 0.71). The oligotrophic, mesotrophic, and eutrophic states cover 66%, 20%, and 13% of the sea surface area, respectively. The DCMs are dominantly regulated by water transparency and they generally observe at depths of less than 20 and 30 m during the cold (autumn and winter) and warm (spring and summer) seasons, respectively. Our results suggest an ever‐increasing chlorophyll‐a in the shallow zones (i.e., coasts) and even in deep regions of the sea, mainly due to nutrient inputs from the Volga river delta. Alarming increase of chlorophyll‐a in this transboundary lake can amplify eutrophication under the lens of global warming and further threaten the lake ecosystem's health, where almost all legal agreements have not yet been implemented to protect the lake environment and its rich resources. Plain Language Summary Following the fall of the Soviet Union, the “Tehran Convention” was signed by the post‐Soviet countries to protect the Caspian Sea environment. However, the development of urbanization and the increase of industrial and agricultural activities in the Caspian Sea region introduce large nutrient loads, mostly from the north end, leading to ever‐increasing symptoms of eutrophication and repeatedly algal bloom events with acute implications for the positive functions of the sea ecosystem. Here, we investigated the recent spatial‐temporal changes in chlorophyll‐a concentration and the sea eutrophication using the MODIS‐Aqua level 3 satellite data in the Caspian Sea (2018–2021), as the Earth's largest lake, to highlight alarming increase of chlorophyll‐a in the sea, which can further threaten the lake ecosystem's health. Given the projected decline in the Caspian Sea's water level and large nu