Under normal conditions, plenty of oxygen can be found below the surface of lakes, rivers, and oceans. However, human activity since the 1950s appears to be wreaking havoc on underwater oxygen supply.
New research from Jenny et al. examined the oxygen levels in three lakes in the French Alps over the course of the past 11,500 years. Using sediment cores, which preserve a layer-by-layer history of the lakes’ environments, the team observed a sharp drop off in oxygen levels during the early 1950s. They attribute the decline to the phosphorus-containing compounds that gained popularity in agriculture around the same time. As rainwater washed phosphorus fertilizers and urban wastewaters into tributaries, the overall nutrient level in the lakes rose to unnaturally high levels, allowing bacteria and algae to flourish. As these creatures grew in number, they consumed the lake’s oxygen, depleting its supply.
The authors also observed something curious: The lakes remained hypoxic even after phosphorus usage was curtailed later in the 1960s and 1970s, suggesting that the balance had been permanently shifted, likely because of the change in the internal loads of organic matter and phosphorus. Temperature was not shown to have caused the regime shift to hypoxia in the lake but did change how much of the lake was affected by hypoxia: Increased temperatures increased the affected volume, whereas decreased temperatures and increased river flow into the lake shrank it.
Accordingly, the authors suggest that restoring the natural winter river flood regime now limited by dams could help correct the oxygen depletion and counteract the effects of global warming and phosphorus pollution. (Global Biogeochemical Cycles, doi:10.1002/2014GB004932, 2014)
—David Shultz, Freelance Writer
Citation: Shultz, D. (2015), Past phosphorus runoff causes present oxygen depletion in lakes, Eos, 96, doi:10.1029/2015EO032999. Published on 24 July 2015.