The relationship between sediment temperature and methane ebullition in
a small eutrophic reservoir: insights from two years of intensive
monitoring
Abstract
Reservoirs are a globally important source of methane (CH4) to the
atmosphere, but measuring CH4 emission rates from reservoirs is
difficult due to the spatial and temporal variability in emissions via
the emission pathways of ebullition (bubbling) and diffusion. The
dominant source of CH4 in reservoirs is production by methanogens in the
reservoir sediment, a process that has been widely shown to have a
positive correlation with temperature. However, oxidation of CH4 to
carbon dioxide by methanotrophs, an important sink for CH4 within lakes,
also scales with temperature. Understanding the relationship between
reservoir CH4 emission (i.e. production – consumption) and temperature
is made more complex by this dual feedback. This study presents results
from multiple in-situ monitoring efforts at a small eutrophic reservoir
in the Midwest US that look at how CH4 emissions vary with temperature
across space and time. Using data sets from eddy covariance monitoring
as well as inverted funnels, we found strong log relationships between
daily average CH4 fluxes and daily average sediment temperature, with R2
values of 0.58, 0.45, and 0.7 for the eddy covariance data, the inverted
funnel deployed at the 1.3-m site (“shallow”), and the inverted funnel
deployed at the 8-m site (“deep”), respectively. The Q10 values for
the shallow and deep site were 32 and 20, respectively, indicating a
stronger dependence on temperature at the shallow site. However, both
the shallow and deep sites had similar emission rates, scaling with
relative maximum sediment temperature at each respective site. Sediment
temperature was also found to be the second most important variable
input to the artificial neural network used for gap-filling the eddy
covariance CH4 fluxes (after wind speed). Improving our understanding of
the temperature – methane emission feedback in freshwaters will enhance
our ability to predict future global methane emissions.