Eddy covariance data reveal that a small freshwater reservoir emits a
substantial amount of carbon dioxide and methane
Abstract
Small freshwater reservoirs are ubiquitous and likely play an important
role in global greenhouse gas (GHG) budgets relative to their limited
water surface area. However, constraining annual GHG fluxes in small
freshwater reservoirs is challenging given their footprint area and
spatially and temporally variable emissions. To quantify the GHG budget
of a small reservoir, we deployed an eddy covariance system in a small
(0.1 km2) reservoir located in southwestern Virginia,
USA for a full year to measure carbon dioxide (CO2) and
methane (CH4) fluxes near-continuously. Fluxes were
coupled with in situ sensors measuring multiple environmental
parameters. Throughout the year, we found the reservoir to be a
substantial source of CO2 (~600 g
CO2-C m-2 yr-1) and
CH4 (~1.0 g CH4-C
m-2 yr-1) to the atmosphere, with
significant sub-daily, daily, weekly, and approximately monthly
timescales of variability. Importantly, we found annual GHG emissions
estimated using eddy covariance were over an order of magnitude greater
than diffusive GHG fluxes measured weekly to biweekly. During the
winter, we found GHG fluxes during partial ice-on and open-water
conditions were not statistically different, suggesting reservoirs may
play an important role in freshwater GHG budgets throughout the year,
not just during the open-water period. Finally, we identified several
key environmental variables that may be driving GHG fluxes,
specifically, surface water temperature and dissolved oxygen
concentrations. Overall, our novel year-round eddy covariance data from
a small reservoir indicate that these freshwater ecosystems likely
contribute a substantial amount of CO2 and
CH4 to global GHG budgets.