The effects of intermittent oxic-anoxic conditions on reservoir ecosystem services: A whole-ecosystem experiment

Background/Question/Methods

Anthropogenic change scenarios predict increasing hypolimnetic anoxia in many lakes and reservoirs due to warmer temperatures and stronger thermal stratification in the summer. However, it is also predicted that an increase in strong storms will result in short-term mixing events that disrupt anoxic periods and temporarily alleviate anoxia. To examine the effects of episodic anoxic/oxic conditions on water quality, we conducted a whole-ecosystem experiment by intermittently oxygenating a thermally-stratified eutrophic reservoir in Virginia, USA during two summers. We used an oxygenation system to inject supersaturated oxygenated water into the reservoir’s hypolimnion, which substantially mixed the bottom waters and increased oxygen concentrations in the hypolimnion by ~1 mg/L/week. Throughout the experiment, we examined the effects of the oxic-anoxic conditions on two important reservoir ecosystem services: water for consumption and carbon storage. We assessed water quality by monitoring the water column concentrations of metals (iron and manganese) and estimated carbon balance by measurements of hypolimnetic respiration and CO₂ fluxes with flux chambers.

Results/Conclusions

We were able to successfully manipulate the hypolimnetic oxygen concentration of the reservoir to create alternate oxic and anoxic conditions during sequential 4-5 week periods throughout the experiment. During both summers, we observed higher rates of internal loading of iron and manganese during anoxic than oxic conditions. Interestingly, oxygenation significantly altered ecosystem metabolism and increased respiration, as demonstrated by high-frequency oxygen data and greater CO₂ fluxes from the reservoir to the atmosphere.  Consequently, increasing anoxia may reduce water quality by promoting internal metal and nutrient loading from the sediments, but may also stimulate carbon burial in small eutrophic reservoirs. Our results emphasize the importance of taking multiple ecosystem services into account as we try to adaptively manage reservoirs in the face of global change.