Water level drawdown boosts greenhouse gas production in a small eutrophic reservoir

Background/Question/Methods

As atmospheric concentrations of greenhouse gases (GHGs) continue to rise, it is important to quantify both natural and anthropogenic sources.  Recent work suggests that freshwater systems may be globally significant sources of both nitrous oxide (N₂O) and methane (CH₄).  Little is known about how dams affect GHG fluxes, but there is reason to think that their contribution may not be trivial.  While reservoirs cover only a small portion of global surface area, they are particularly active with regards to biogeochemical transformations.  This may, in part, be due to the flooding of large amounts of terrestrial organic matter that occurs during reservoir formation.  This research aims to quantify the effect of a controlled dam spill on the evolution of dissolved CH₄ and N₂O within a small eutrophic reservoir. We collected airtight water samples along a vertical profile in the deepest part of the reservoir on 9 dates in 2010 and on 14 dates in 2011 and analyzed them on a gas chromatograph.  Hypolimnetic fluxes were estimated by quantifying total change in volume-weighted GHG masses over time.   Diffusive fluxes were estimated using surface GHG concentrations and published relationships between wind speed and gas transfer velocity.

Results/Conclusions

In both 2010 and 2011, water level drawdown resulted in order of magnitude increases in hypolimnetic CH₄ fluxes.  Summer fluxes averaged 0.88 mmol CH₄ m^-2 d^-1 whereas dam spill fluxes averaged 17.8 mmol CH₄ m^-2 d^-1.  Hypolimnetic CH₄ concentrations remained high following the spill (1.6 mg/L), and did not drop off until lake turnover. This pattern suggests that over 50% of the CH₄⁺ turnover flux is generated during a one-week dam spill event. While there was no observable pattern in N₂O flux during 2010; in 2011 N₂O flux increased by over two orders of magnitude during early dam spill, from 7.5 μmol N₂O-N m^-2 d^-1 to 1,582 μmol N₂O-N m^-2 d^-1.  Unlike CH₄, hypolimnetic N₂O concentrations did not remain elevated for the remainder of the spill, but rather dropped off, suggesting microbial uptake. Surface concentrations of CH₄ and N₂O also increased during the dam spill, by approximately 400% and 340% respectively. Diffusive fluxes were extremely small relative to turnover and ebullition-based fluxes.   Overall, this study demonstrates that anthropogenic manipulations to reservoir water level can dramatically affect reservoir to atmosphere GHG fluxes, particularly in the case of CH₄.