Metalimnetic oxygen minima alter the vertical profiles of carbon dioxide and methane in a managed freshwater reservoir.

Metalimnetic oxygen minimum zones (MOMs) commonly develop during the summer stratified period in freshwater reservoirs because of both natural processes and water quality management. While several previous studies have examined the causes of MOMs, much less is known about their effects, especially on reservoir biogeochemistry. MOMs create distinct redox gradients in the water column which may alter the magnitude and vertical distribution of dissolved methane (CH) and carbon dioxide (CO). The vertical distribution and diffusive efflux of CH and CO was monitored for two consecutive open-water seasons in a eutrophic reservoir that develops MOMs as a result of the operation of water quality engineering systems. During both summers, elevated concentrations of CH accumulated within the anoxic MOM, reaching a maximum of 120 μM, and elevated concentrations of CO accumulated in the oxic hypolimnion, reaching a maximum of 780 μM. Interestingly, the largest observed diffusive CH effluxes occurred before fall turnover in both years, while peak diffusive CO effluxes occurred both before and during turnover. Our data indicate that MOMs can substantially change the vertical distribution of CH and CO in the water column in reservoirs, resulting in the accumulation of CH in the metalimnion (vs. at the sediments) and CO in the hypolimnion.