Global drivers of lake dead zone formation at the mouths of major rivers

Dead zones are commonly associated with marine coastal areas where rivers deposit excessive nutrients leading to local anoxia sometimes stretching for hundreds of kilometers. Marine dead zones are well-recognized for their adverse effects on ecosystems, fisheries, and coastal communities. But in contrast, the global extent and drivers of dead zone formation near inflowing rivers to the world’s lakes remains uncertain despite the importance of lakes for drinking water supplies, recreation, and biodiversity. Here, I used 742 million bias-corrected chlorophyll-a (chl-a) estimates merged over 6 satellite sensors (daily, 1 to 4 km resolution) to map dead zones at the mouths of major inflowing rivers in more than 100 large lakes and asses their changes from 1997 to 2020. Dead zones were present in lakes across geographic and climatic gradients and were associated with a combination of urban and agricultural activities in lake watersheds. Dead zones expanded in some lakes even as water quality offshore improved. This spatiotemporal complexity demonstrates the value of moderate resolution mapping of lake dead zones to inform water management decision-making and to determine the local ecological consequences of human activity.