Monitoring has shown that summer levels of dissolved oxygen in lakes are declining. New analysis reveals that this is probably due to effects of global warming, and will be detrimental for aquatic species and potentially methane emissions.

The level of dissolved oxygen is a determining factor in various aspects of aquatic ecosystems, influencing its quality as biodiversity habitat – low oxygen levels make water unsuitable for aerobic organisms (which need oxygen to breathe). In temperate lakes across the world, data indicating that average summer oxygen levels are declining are, therefore, of concern. Warmer temperatures lead to less oxygen solubility in lake water closer to the surface, and this has been well studied in relation to climate change. However, much remains unknown about the factors contributing to deoxygenation in deep water, where temperatures have not consistently warmed in recent decades.

Despite the importance of this part of lake ecology, there has been no large-scale investigation into the amount of deep lake habitat suffering critically low oxygen levels. To address this knowledge gap, researchers looked at data on more than 400 temperate lakes, in widespread locations, to identify drivers of this phenomenon and quantify the volume of lake water reaching levels of depletion that could be damaging for ecology.