Diffusion of greenhouse gases (GHGs) from freshwater ecosystems into the atmosphere can contribute a large fraction of total GHG efflux. In temperate waterbodies, annual peak diffusive flux generally occurs during fall turnover when methane (CH4) and carbon dioxide (CO2) in the anoxic bottom layer (the hypolimnion) mix to the surface. However, when thermal stratification develops, some waterbodies become anoxic in the middle of the water (the metalimnion), not the hypolimnion. This can change the distribution of pCH4 and pCO2 in the water column and alter diffusive CH4 and CO2 efflux phenology. Metalimnetic oxygen minimum zones (OMZs) may increase in waterbodies due to global change, but their effects on GHG dynamics remain unknown. We experimentally created metalimnetic OMZs in a eutrophic drinking water reservoir using a hypolimnetic oxygenation system (HOx). The HOx mixes oxygen throughout the hypolimnion, and increases the strength of the thermal gradient between the epilimnion and the hypolimnion. This allows interflowing low oxygen water from the sediments and settling organic matter from the surface to develop a metalimnetic OMZ. In summers 2015 and 2016, we used the HOx to develop metalimnetic OMZs, and then monitored physical and chemical conditions weekly to determine how pCH4 and pCO2 dynamics and diffusive effluxes responded.
Results/Conclusions
In both summers, we successfully generated metalimnetic OMZs and observed substantial accumulation of pCH4 within the metalimnion after the OMZ developed. Simultaneously, pCO2 accumulated in the well-oxygenated hypolimnion and the metalimnetic OMZ. The reservoir was consistently a source of CH4 to the atmosphere via diffusive flux, while CO2 varied between a source and a sink throughout the stratified period of both years. In both years, we observed that the peak diffusive flux for both CO2 and CH4 occurred multiple weeks before seasonal turnover as a result of summer storms mixing the metalimnion and epilimnion. Consequently, our study suggests metalimnetic OMZs may have substantial implications for reservoir GHG dynamics by changing the vertical distribution of pCH4 and pCO2 in the water column and the resulting timing of their peak GHG diffusive flux. Our study suggests that metalimnetic OMZs – which may be increasing in waterbodies due to global change – could substantially modify annual diffusive GHG efflux phenology and waterbody carbon cycling in freshwater ecosystems.