258
methanogens). These anaerobic microorganisms are encoun-
tered in the profound and epilimnetic sediments as well as in
anoxic water columns. As discussed later in the paper, their
activities and the related methane production rates (MPRs)
depend on temperature, organic matter availability and isolation
from oxygen. These factors are influenced by climate, lake size
and depth, and ecosystem productivity (EPA 2010 ). For exam-
ple, CH 4 fluxes measured in tropical lakes (2023 mmol.m−2.
yr−1) are higher than those of boreal (331 mmol.m−2.yr−1) and
temperate (1110 mmol.m−2.yr−1) lakes (Bastviken 2009 ).
Moreover, CH 4 production is not uniform over the entire lake
surface. For example, substantially higher CH 4 production can
occur in littoral relative to profound sediments (Michmerhuizen
et al. 1996 ; Murase et al. 2005 ).
Considering that anaerobic carbon mineralization pro-
cesses account for as much as 20–60 % of the overall carbon
mineralization in freshwater environments, and that metha-
nogenesis is responsible for 30–80 % of anaerobic carbon
mineralization in waters and sediments, methanogenesis
likely accounts for 10–50 % of the overall carbon mineraliza-
tion (Bastviken et al. 2008 ). Several studies also indicate that
20–59 % of the sestonic carbon imputs to lake sediments is
converted into CH 4 (Wetzel 2001 ).
16.2.3.2 Methane Transport
There are four pathways for CH4 transport from lakes: bub-
bling, diffusion, seasonal overturning and plant-mediated
transport. Wind speed is an important control on gas
exchange between a lake and the atmosphere. Flux rates by
all pathways generally increase with increasing wind speed.
- The bubbling process has been determined to be the
dominant pathway for the release of CH4 accounting for
more than 50 % of CH4 emissions from lakes (Bastviken
et al. 2004 ). Bubbling occurs when CH 4 concentrations
exceed the limit of solubility which relies on the hydro-
static pressure and temperature (at 20 °C and atmospheric
pressure, CH 4 solubility is 0.023 g.kg−1 of water com-
pared to 1.7 g.kg−1 of water for CO 2 ). Sediments situated
under a shallow water column are the major contributors
of this process, due to lower hydrostatic pressure and
wave-induced perturbations which favor the formation
and release of the CH 4 bubbles (Bastviken et al. 2004 ). - Most of the CH 4 that reaches the upper mixed layer of the
water column is transported by diffusive flux. Diffusion
rates are lower in areas of physical and chemical gradients
(e.g., thermocline, chemocline) and at the water-sediment
interface. In epilimnetic areas, CH 4 transport can be
accelerated by the turbulence associated with wind.
Several studies suggest that epilimnetic dissolved CH 4
may be derived from epilimnetic sediments rather than
from hypolimnetic waters (e.g., Murase et al. 2005 ).
Diffusion and turbulent transport may be responsible for
26–48 % of CH 4 emissions from lakes.- CH 4 accumulating in anoxic hypolimnia during a period
of stratification can be rapidly released to the atmosphere
upon lake overturning. For example, temperate dimictic
lakes mix from top to bottom during spring and fall,
resulting in significant emissions of CH 4 to the atmo-
sphere (accounting for 24–86 % of total CH 4 emissions
from lakes). - CH 4 produced in sediment along lake margins can also be
transported by the aerenchyma of emergent vegetation
(plant-mediated transport). This flux component
depends on CH 4 production and oxidation in the sedi-
ments, and on vegetation characteristics (Bastviken et al.
2004 ).
16.2.3.3 Methane Oxidation
As soon as CH 4 reaches oxic sediment or water, a large pro-
portion (30–99 %) is likely oxidized by CH 4 -oxidizing bacte-
ria (Bastviken et al. 2002 ). It is noteworthy that, in the last
decades, consortia of anaerobic microbes have been
described that convert CH 4 to CO 2 while reducing sulfate,
nitrate, manganese or iron. These processes are discussed
further in following sections.16.2.4 Methane Profile in Lake Pavin16.2.4.1 Origin of Methane
The average carbon (δ^13 C) and hydrogen (δD) isotopic compo-
sition of CH 4 in the monimolimnion of Lake Pavin are −60 ‰
and −276 ‰, respectively (Agrinier P. Personal communica-
tion). These values are indicative of a biogenic origin of CH 4
rather than of a geothermal or magmatic origin (Whiticar
1999 ).16.2.4.2 Methane in the Profundal Sediment
(Fig. 16.1a)
CH 4 concentrations in the first 40 cm deep sediment, col-
lected at 92 m water depth, range from 4 to 9 mM (Borrel
et al. 2012a). Modelisation of CH 4 production in Lake Pavin
suggests that 90 % of CH 4 is formed in lake sediments (Lopes
et al. 2011 ).16.2.4.3 Methane in the Water Column
(Fig. 16.1a)
CH 4 concentrations extend from few micromolars at 60 m to
4 mM at 92 m water depths. It has been estimated that the
monimolimnion of Lake Pavin contains 260.10^3 kg of CH 4
(Jezequel et al. 2010 ). CH 4 concentrations in the oxic part of
the water column are near the boundary of the detection
threshold of 3 nM.A.-C. Lehours et al.