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The concentration of dissolved oxygen in aquatic systems helps to regulate biodiversity^,, nutrient biogeochemistry, greenhouse gas emissions, and the quality of drinking water. The long-term declines in dissolved oxygen concentrations in coastal and ocean waters have been linked to climate warming and human activity^,, but little is known about the changes in dissolved oxygen concentrations in lakes. Although the solubility of dissolved oxygen decreases with increasing water temperatures, long-term lake trajectories are difficult to predict. Oxygen losses in warming lakes may be amplified by enhanced decomposition and stronger thermal stratification^, or oxygen may increase as a result of enhanced primary production. Here we analyse a combined total of 45,148 dissolved oxygen and temperature profiles and calculate trends for 393 temperate lakes that span 1941 to 2017. We find that a decline in dissolved …