Rapid shallowing can trigger dramatic ecological changes and challenges within and beyond arid, nutrient-rich lakes

The Salton Sea, California's largest lake, faces severe environmental degradation from agricultural runoff and diminishing water supply. Here we show the lake's shifted biogeochemical regime from these anthropogenic pressures. Field, historical and satellite data over the past 20 years reveal a breakdown of the lake's thermal stratification. As the lake level drops, the thermal barrier between oxygen-rich surface waters and anoxic bottom waters weakens, leading to a more mixed water column. Unlike its historical pattern of extreme anoxic events during late summer overturn, the lake now maintains poorly oxygenated, low-sulfidic waters, which alters seasonal algal production patterns. These conditions create a consistently unfavorable environment for wildlife and have regional implications for air quality. The lake's water quality evolution, categorized into three stages (stratified oxygenated-deoxygenated layers, expanded low-oxygen zones, and potential full oxygenation in shallow, nutrient-rich waters), highlights the utmost importance of targeted nutrient input reduction for effective management of shallowing arid lakes worldwide.