Environment: Temperate lake oxygen levels in decline

Widespread long-term declines in temperate lake oxygen levels are reported in Nature this week. This trend, calculated for nearly 400 lakes within an 80-year period, may be linked to warming temperatures and decreasing water clarity. The declines could threaten essential lake ecosystems.

The concentration of dissolved oxygen in aquatic systems can affect the balance of nutrients, biodiversity, the quality of drinking water and greenhouse gas emissions. While oxygen loss in oceans has been documented, the changes in dissolved oxygen concentrations in lakes are less well understood, in part owing to a lack of long-term and large-scale studies.

To address this issue, Kevin Rose and authors compiled temperature and dissolved oxygen data measurements for 393 temperate lakes (mostly in Europe and the United States) collected by government, university and not-for-profit sources between 1941 and 2017. Declines in dissolved oxygen in these freshwater environments are around two to nine times greater than those observed in the oceans. Increased water temperatures are associated with reduced oxygen concentration in surface waters, whereas lower oxygen levels in deeper waters are linked to stronger thermal stratification (the formation of distinct thermal layers at different depths) and reduced water clarity, the authors report. They note some exceptions to these trends; for example, a large subset of lakes (87) exhibited increases in both water temperature and dissolved oxygen concentration. However, this anomaly could be attributed to algal blooms, which may increase concentrations at the surface, but reduce oxygen solubility lower down.

Human activity and warming temperatures are expected to continue to drive future losses in lake dissolved oxygen, and ongoing rigorous efforts to manage lake systems will be needed to counter these effects, the authors conclude./p>