Benthic microbial production of oxygen supersaturates the bottom water of a stratified hypersaline lake.

Lake Hayward is a hypersaline lake that stratifies seasonally and maintains oxygen supersaturation in its bottom water for about 6 months each year. This phenomenon was found to be the result of photosynthesis by the benthic microbial communities, composed primarily of the cyanobacteria Cyanothece spp., Spirulina sp., and Oscillatoria sp. When these communities were present and the lake was stratified, the bottom water was supersaturated with oxygen (up to 370%). During illumination, the benthic microbial communities rapidly developed very high concentrations (e.g., >500 μM) of oxygen, which then diffused into the overlying water. However, while the overlying water became supersaturated, the concentration in the water was lower than in the benthic microbial communities because (1) transport across the sediment-water interface was limited by diffusion, and (2) turbulence rapidly mixed the oxygen throughout the much larger volume of the bottom water (approximately 1.5 m deep). Thus, import of oxygen by the benthic microbial communities at night proceeded more slowly than daytime export, allowing supersaturation of the bottom water to develop.