LIMNOLOGY
November 1996
AND
Volume 41
OCEANOGRAPHY
Number 7
Lwnnol. Oceanogr., 41(7), 1996, 1375-1383
0 1996, by the American Society of Limnology and Oceanography, Inc
Trapped methane volume and potential effects on methane
ebullition in a northern peatland
Elizabeth J. Fechner-Levy’ and Harold F. Hemond2
Department
of Civil and Environmental
Engineering,
Massachusetts Institute
of Technology,
Cambridge 02 139
Abstract
A novel way of estimating the gas bubble volume in the floating mat sediment of a peatland was developed
at Thoreau’s Bog in Concord, Massachusetts. Statistically significant relationships between the buoyancy of
the floating Sphagnum mat and atmospheric pressure were observed, and these relationships were used to
estimate the gas bubble volume. The mass of CH, stored in gas bubbles is estimated to be as much as 3
times the mass of dissolved CH,, depending on the time of year. The gas bubble volume is frequently large
enough to serve as a significant buffer between microbial production of CH, and the release of CH, to the
atmosphere. Changes in atmospheric pressure, temperature, and water-table elevation may result in modulation of the ebullitive CH, flux. Periods of rapidly rising atmospheric pressure or equivalent pressure
changes due to water-table elevation are capable of arresting bubble volume growth, thereby halting CH,
ebullition. Periods of rapid cooling of the bog could also temporarily halt ebullition, as thermally induced
contraction of bubbles and dissolution of CH, offset bubble volume growth due to methanogenesis.
Methane is a greenhouse gas whose tropospheric concentration is increasing at an annual rate of - 1% (Khalil
and Rasmussen 1990). A significant source of tropospheric CH4 is releases from northern peatlands (Aselmann and Crutzen 1989; Fung et al. 199 1). Published
estimates of the magnitude of CH, fluxes from northern
peatlands vary considerably, most likely owing to the high
spatial and temporal variability
of peatland flux measurements (e.g. Roulet et al. 1994).
Methane enters the atmosphere through several processes: diffusion, ebullition (Chanton and Martens 1988;
Lansdown et al. 1992), and transport through plants
(Morrissey and Livingston 1992). Ebullition, or bubbling,
has been proposed as the dominant CH, transport process
in many environments,
including a temperate bog in
Washington (Lansdown et al. 1992) beaver ponds in
northern Ontario (Bubier et al. 1993) wetlands on the
Amazon floodplain (Devol et al. 1SSS), and floating grass
* Present address: Abt Associates Inc., 4800 Montgomery
Bethesda, Maryland 208 14.
2 Corresponding author.
Lane,
Acknowledgments
We thank B. Levy for statistical assistance. We also appreciate
the comments provided by Nigel Roulet and anonymous reviewers.
We acknowledge the partial support of NSF grants BSR 9 l11427 and EAR 93-04471.
mats in the Amazon (Bartlett et al. 1988, 1990). At one
site in a tidal freshwater estuary, the ebullitive flux was
estimated to equal the diffusive flux (Chanton et al. 1989).
Ebullition could account for the high spatial and temporal variability
of chamber-based CH4 flux measurements. For example, ebullition may occur through discrete channels (bubble tubes) in the sediment (e.g. the
work of Martens et al. 1980 in subtidal sediments), and
individual
chambers may sample only a small number
of wetland bubble tubes. However, the ability of ebullition to introduce temporal variability in CH, fluxes must
be related to the ability of the peatland sediment to store
or accumulate gas-phase CH, between episodic releases.
Thus, estimation of the magnitude of CH4 gas storage in
peatlands, as discussed herein, is important in determining the extent to which ebullition causes temporal variability in CH, release from peatlands.
Initial CH, bubble formation in shallow water or sediments requires a CH, partial pressure of only -0.2 atm,
with the remaining 0.8 atm partial pressure being provided by NZ. As CH, continues to be produced and gases
escape by ebullition, N, is stripped from the system. Partial pressures of N2 ~0.8 atm in sediment pore waters
have been observed (e.g. Army 1987; Chanton et al. 1989).
The partial pressure of CH4 in the sediment pore waters
may thus approach absolute hydrostatic pressure. In some
locations, supersaturation of dissolved CH, in peat pore
waters has been measured (e.g. Romanowicz et al. 1993;
Army 1987).
1375
1376
Fechner-Levy and Hemond
Changes in absolute hydrostatic pressure clearly should
affect the amount of ebullition that occurs in submerged,
organic-rich sediments. Moore et al: (1990) found that
lowering the water table in fens in subarctic Quebec by
5-l 0 cm led to degassing of CH,. Similarly, Chanton et
al. (1989) found that ebullition from subtidal freshwater
sediments in an estuary was triggered by changes in hydrostatic pressure due to tides. Maximum bubble release
occurred when hydrostatic pressure was at a minimum.
The influence of atmospheric pressure on ebullition has
also been noted. Ebullition
from sediments in a New
Hampshire lake was correlated with changes in local air
pressure (Mattson and Likens 1990), and episodic emissions of CH4 from beaver ponds may be associated with
low atmospheric pressures (Bubier et al. 1993). Episodic
emissions of CH4 from a Minnesota peatland were associated with decreases in atmospheric pressure as well
as a declining water table (Shurpali et al. 1993).
Although stored gas volume and ebullition are closely
linked, we found only one in situ measurement of bubble
volume in the literature: Chanton et al. (1989) measured
the volume of CH4 in bubbles in subtidal freshwater sediments by corralling pyramid samplers with planks and
stirring the sediment to a depth of 1 m beneath the samplers to release the gas bubbles. After measuring the dissolved CH, inventory, Chanton et al. concluded that the
sedimentary gas bubble reservoir comprised lo-30% of
the total CH4 present in the sediments. In the present
study, we explore an indirect approach to the measurement of gas bubble volume based on the buoyancy of gas
bubbles. We test the hypothesis that variations in the
buoyancy of peatland sediments are caused by variations
in the gas bubble volume due to changes in atmospheric
pressure. This pressure-volume
effect is the same effect
that causes a toy Cartesian diver to float or sink in response to changes in pressure imposed, via a flexible
membrane and the water column, on the trapped gas of
the diver. We then use the magnitude of the buoyancy
response to changes in atmospheric pressure to calculate
the entrapped CH4 bubble volume and the variation of
this volume over time. Knowledge of the entrapped CH,
bubble volume, in turn, allows us to quantitatively
estimate the effects of changes in atmospheric pressure and
temperature on CH4 ebullition from a northern peatland.
Materials and methods
Study site-Our
research was done at Thoreau’s Bog
in Concord, Massachusetts (Fig. 1). This ombrotrophic
bog is dominated by Sphagnum spp. and low woody
shrubs, particularly Chamaedaphnc calyculata. The pore
water is acidic and dominated by humic substances. This
site has been previously characterized with respect to vegetation, hydrology, and several element budgets (Hemond
1980), nitrogen cycling (Hemond 1983), dissolved humic
substances (McKnight et al. 1985), dissolved CH, profiles
(Hemond et al. 1987), and CH4 emissions (Fechner and
Hemond 1992). It is of particular relevance to this study
that the Sphagnum and peat sediment of the bog forms
a floating
(Hemond
mat th;st rises and falls with the water table
1980).
Measurement Gf mat and water levels-Thoreau’s
Bog
is instrumented with a mat-level recorder and a waterstage recorder (Fig. 2). Both recorders are Stevens Type
F recorders run on 32-d cycles and geared for a 1 : 1
relationship between change in stage and pen displacement on the chars. The mat-level recorder is located on
a fixed platform made of open-mesh expanded metal on
a rigid steel frame supported by polyvinyl chloride-encased iron pipes sunk into the mineral soil beneath the
bog. A stake made of steel angle iron (h 0.5 m long) is
driven into the mat beneath the recorder so that the stake
moves up and down with the mat. The water-stage recorder uses a float and stilling well (Hemond 1980).
Elevations of the floating sediment mat and water table
at Thoreau’s Bog were measured during the ice-free periods from 24 Jute 1988 through 26 July 199 1. Data were
digitized with a Calcomp 9000 digitizer board that has
resolution better than 1 mm. Data were digitized at 8-h
intervals, and time was recorded to the nearest half hour.
Visual inspection of the mat- and water-level recorder
charts shows int#:rvals of stage decrease due to evapotranspiration during daylight (Hemond 19 80). These tluctuations, on the order of a millimeter, are superimposed
on changes on th,= order of tens of centimeters in the mat
and water elevations associated with seasonal rise and fall
of the water table.
Atmospheric pressure -Atmospheric
pressure is recorded hourly at Hanscom Field in Bedford, Massachusetts (IV 2.4 km tc, the east of Thoreau’s Bog). We obtained
these atmospheric pressure data from the National Center
for Climatic Data. When our analyses of data from Thoreau’s Bog required pressure readings on the half hour,
we calculated the arithmetic mean of the previous and
subsequent hourly pressure readings.
Relationship of buoyant force to floating mat elevation -Changes in elevation of the floating sediment mat
relative to the water table (referred to as relative mat level
or relative mat elevation) result from changes in buoyant
force, which we attribute to changes in the volume of gas
bubbles trapped within the floating sediment mat. The
relationship between changes in the floating mat elevation
with respect to the water table and changes in the net
buoyant force acting on the mat was previously determined with an apparatus described by Fechner and Hemond (1992). 13riefly, we carefully removed cores of
Sphagnum and peat from the upper 30-40 cm of the
floating mat with a 12.7-cm-diameter,
thin-walled steel
coring tube with a sharpened lower edge. This device was
worked into the peat and the peat core then freed from
the bog by reaching down the outside of the core tube
and pulling up both the tube and its contained peat. In
the laboratory, each peat core was hung freely from a toploading balance. with part of the core submerged in a tank
of water. The core was allowed to equilibrate so that the
water table in the core was close to its natural level and
1377
CH, volume and ebullition
N
Thoreau’s
Concord
p21
wetland
Bog
y
I
I
0
Fig. 1. Location
was the same as the water level in the tank. The load on
the balance at that water level was recorded. The water
level was then lowered incrementally,
the core allowed
to reequilibrate, and a new weight recorded. This process
was repeated until the water table in the peat core was
below the range of interest. These experiments gave a
relationship between relative mat level and buoyant force
of 740 (dyn cm-l) cm-2.
Calculation of gas bubble volume-Several
steps were
required to calculate the gas bubble volume in the peat
(Fig. 3). In the first step, we identified all mat-level and
water-level data from intervals at least 3 d in length in
which absolute mat and water levels changed monotonically (i.e. mat and water levels were continuously rising
or continuously
falling). The purpose of excluding data
outside of these intervals, or blocks, was to eliminate error
due to hysteresis in the recording systems. Thirty-three
blocks of data were identified, and all subsequent buoyancy analyses were based on thcsc blocks. In all 33 blocks,
absolute mat and water levels were continuously falling.
In the second step, absolute mat level within each block
of data was regressed against absolute water level, resulting in an equation for mat level as a function of water
level. This regression was performed for each block of
data because the slope of the relationship between mat
level and water table in each block was not exactly 1 : 1.
(A 1 : 1 relationship may not exist, at least in part because
the mat may not be entirely free of connection with the
underlying sediments.) Then, within each data block, the
relative mat levels were determined as the departure of
each mat-level datum from the regression line (i.e. the
residuals).
50m
of Thoreau’s
Bog.
In the third step, we plotted the relative mat levels
(residuals) in each block of data as a function of atmospheric pressure and then performed a second regression,
between relative mat levels and atmospheric pressure. In
the fourth step, we used the laboratory-derived
relationship between peat buoyancy and relative mat level, together with the relationships calculated in the previous
step between relative mat levels and pressure, to calculate
changes in buoyancy as a function of changes in atmospheric pressure.
Finally, in the fifth step, WC calculated gas bubble volume in the peat for each block of data by using the observed atmospheric pressure dependency of buoyancy for
Water
Level
Recorder
Mat
Level
Recorder
1
I--
Pulley
1
~,
w iig ht fl
P”“ey
-.----------
----__
St&e
-
---
Open
Water
__
Stilling
Well
-\
\~
u
A
Fig. 2.
m
Schema ic of recorders at Thoreau’s
Bog.
Fech ner- Levy and Hemond
1378
Step
2
P
Step 3
Extract
residual
c
Pressure
pressure
Relative
relationship
mat
level
vs.
buovancv
step
Peat buoyancy
experiments
(Fkhner
& Hemond
Fig. 3.
Diagram
1992)
of the steps required to estimate the gas bubble volume in the floating mat.
the block (as determined in the fourth step), the ideal gas
law, and the Henry’s law constant for CH4.
Calculations in the fifth step can be made with either
of two assumptions. One assumption is that changes in
atmospheric pressure are more rapid than the establishment of equilibrium
between gaseous and aq ueous CH4
phases. The volume change in an arbitrary volume of
water and gas bubbles that results from an -atmospheric
pressure change can then be computed by d ifferentiation
bf the ideal ias law (Eq. la) with respect to pressure,
yielding Eq. lb:
PV, = nR7
(la)
$J- -- -b
dP
5
P’
P is the absolute pressure of the gas bubbles (taken as
- 1.1 atm for a mean depth of 1 m below the water table),
Vg is volume of gas bubbles, n is moles of gas bubbles,
R is the universal gas constant, and Tis temperature. For
the component of buoyancy due to gas bubbles, the derivative dVgldP is directly proportional
to the rate of
change of buoyancy with change in atmospheric pressure.
The alternative assumption we use in the fifth step is
that CH4 and N2 dissolved in the aqueous phase rapidly
equilibrate with gas bubbles as atmospheric pressure
changes. In this situation, Henry’s law must be used along
with the ideal gas law. The result is a larger change in
gas-phase volume than would otherwise occur for a given
pressure change (i.e. an increase in pressure not only compresses the bubbles but causes some gas to dissolve). Under this assumption, the pressure-volume relationship is
shown to be
dVg_
-_--- -J’s
dP
P
V/w
(24
PH
or
,3 dWg/ Ku) =---- Vg
VW
dP
1
H’
CW
VWis the volum? of water in which Vg occurs as bubbles,
and H is the dirnensionless Henry’s law constant for the
gas. In this analysis, we use Henry’s law constants for
CH4 calculated from Wiesenburg and Guinasso (1979).
Because of the somewhat lower solubility of N2, a small
decrease in magnitude of gas bubble volume change per
unit pressure change results if the bubbles are partly N2.
In our subsequent calculations, we use the second assumption of equilibrium,
thus arriving at somewhat
smaller estimates of Vg than would be calculated otherwise. Changes in Vg from one data block to another can
result from gas loss by ebullition or from CH4 transfer
between aqueous and gas bubble phases. The latter is
influenced both by microbial CH, production, which increases the aqueous CH4 concentration and thus tends to
result in an incl-ease in gas bubble volume, and by temperature, which affects the Henry’s law constant.
Results
Data on floating mat levels, water-table levels, and
pressure were obtained over a 4-yr period during ice-free
intervals (i.e. w nen the lagg was not frozen: June-December 1988, Marc+December
1989, June-December 1990,
and April-July
199 1). The floating nature of the mat is
1379
CH, volume and ebullition
39
70
65
37
g
5
4
5 35
E
P
‘Z
d
LL
Sinking
33
50
45
31
15
20
25
30
Water-table level (cm)
35
40
’
Fig. 4. Relationship between the absolute floating mat level
and the absolute water-table level at Thoreau’s Bog, 24 June10 December 1988. The regression has a slope of 0.9 and an T*
of 0.97. Note that levels are measured as distances below a fixed
datum.
clearly shown by plotting the absolute floating mat level
vs. the absolute water-table level (Fig. 4). The regression
between mat level and water level for June-December
1988 has a slope of 0.9 and a coefficient of determination
(r2) of 0.97.
One of the 33 blocks of data, comprising 26 d in June
1990, is plotted in Fig. 5. During this period, absolute
elevations of both the water table and the floating mat
decreased monotonically.
The straight line represents the
regression between absolute mat level and absolute watertable level over the block; the regression has a slope of
0.8 and an r2 of 0.99. As hypothesized, departures of the
mat elevation from the regression lint (residuals) correspond systematically to episodes of increasing or decreasing atmospheric pressure.
We performed a two-tailed t-test on the 33 blocks of
data to determine whether the slope of the regression
between relative mat elevations (residuals) and atmospheric pressure was significantly different from 0 at an
cy level of 0.05. For 14 of the blocks, the slope was significantly different from 0; for 12 of these blocks, the slope
was negative, as hypothesized. For these 14 blocks, we
calculated the 95% confidcncc intervals around the slopes
(Table 1). Of the remaining 19 blocks, 17 also had negative slopes, although they were not significantly different
from 0 at an cylevel of 0.05 (Table 1). We think that errors
associated with recording and digitizing data, as well as
the simplifications
of our buoyancy model, may have
weakened the statistical significance of these regressions.
We also conducted an analysis to determine whether
the relative mat levels had a diurnal signal that could
1
3
5
7
Water-table
level (cm)
9
11
Fig. 5. The absolute floating mat level and the absolute water
table level at Thoreau’s Bog, l-26 June 1990. The regression
has a slope of 0.8 and an r* of 0.99. Arrows indicate episodes
of generally increasing or generally decreasing atmospheric pressure. Note that levels are measured as distances below a fixed
datum. Thus, data below the regression line indicate that the
mat is floating higher relative to the water table.
partially explain the observed correlations. For each block,
we applied a complex fast Fourier transform function to
transform data to a frequency domain. We then examined
the frequencies of the seven largest signals to determine
if 24-h frequencies (i.e. diurnal fluctuations) were important. Of the 33 blocks, only six had a frequency between 23 and 24.5 h. Of these six blocks, two had significant correlations between residuals and pressure; the
diurnal frequencies in these two blocks were the fifth and
seventh largest signals, respectively.
Based on the results of this analysis, as well as visual
inspection of the records, we do not think that diurnal
fluctuations of relative mat level due to evapotranspiration can explain the correlations between the relative mat
level and atmospheric pressure. Diurnal fluctuations are
typically < 1 mm in amplitude, and their major effect is
probably the introduction
of noise (as is the case with
recording and digitizing errors). We noted that in many
of the blocks that did not exhibit a significant correlation
between relative mat level and pressure, the ranges of the
residuals were smaller than the ranges present in blocks
with significant correlations (Table 1).
In Fig. 6, the 12 negative slopes that are significantly
different from 0 are plotted as a function of the time of
year; approximate average peat temperature at a depth
of 60 cm, as measured in 1990, is also plotted for reference. The average regression slope for the 12 blocks having negative slopes is -0.016 cm mb-l. The greatest
negative slopes ( - -0.03 cm mb-l) occurred in three
blocks, all during the warmest months of the year.
1380
E
25
Fechner- Levy and Hemond
-0.020
-0.015
:
0
E
-0.010
!
J
F
M
A
M
J
J
A
S
0
N
D
Month
Fig. 6. Plot of 12 regression slopes between relative mat
levels and atmospheric pressure that were negative and significantly different from 0. Line represents approximate average
peat temperature at a depth of 60 cm, as measured in 1990.
The mean relationship between buoyant force and water-table height derived from laboratory studies (Fechner
and Hemond 1992) was 740 (dyn cm-l) cm-2. Therefore,
for a slope of -0.03 cm mb-‘, the rate at which the
volume of CH4 bubbles (per cm2 of bog surface) must
change with changing pressure is
pg.%
cm mb-l
= -0.03
x 740 dyn cm-3
(34
or
dVg_ -0.03
-dP
cm mb-’ x 740 dyn crnm3
980 dyn cm-3
= -0.023
(cm3 cmd2) mb.
(W
Here, 980 dyn cm-3 is equal to pg, the weight density of
water. If we assume that the peat is 2 m thick and has a
total porosity of 0.8, the volume of pore water per cm2
of peatland surface is 160 cm3. Therefore, the rate of
change of the ratio of gas bubble volume to water volume
(V,l VW)with pressure is
d(VglKJ
dP
d Vg
1
’ 160 cm3 cm-2
= dP
= (-1.4
Vg can then be determined
--Vg--p
Kv
dVgW+J
=0.15
mb
- ;.
by combining
(4)
Eq. 2b and 4:
1
dP
= -1,110
x 10h4) mb-I.
-E
x
[(-1.4
x
10-4) mb-11 - i
(5)
The value of l/H ranges from 0.04 to 0.05 for the range
of temperatures encountered over the year in the bog; it
is 0.04 at 15”C, which corresponds to summer months
when gas volumes in the peat arc greatest. Thus, for a
summertime relative mat level vs. atmospheric pressure
slope of -0.03 cm mb-l, the calculated fraction of the
peat porosity occupied by gas bubbles is estimated to be
11% (15% if equilibrium
with dissolved CH4 is not invoked). The gas bubble content for the slope of -0.016
cm mb-r is 3-4’~ (8% if equilibrium is not invoked). The
calculated gas bubble volumes become somewhat larger
if bubbles contain N2, whose lower solubility moves the
calculated gas bubble volume in the direction of the nonequilibrium
result.
These estima.;ed gas bubble volumes correspond to a
sizable fraction of the total CH, content of the bog. For
example, 3-4% CH,-filled pore space corresponds to a
gaseous CH4 reservoir -90% the size of the dissolved
CH4 reservoir, assuming a dimensionless Henry’s law
constant for CH4 of 24 (at 15°C). An 11% CH,-filled pore
space would correspond to -3 times the dissolved CH4
reservoir.
Discussion
The results of this analysis support the hypothesis that
significant amounts of CH4 can be stored as gas bubbles
at the study site. Calculations indicate that the gas bubble
reservoir is at times large enough to serve as an important
storage buffer between microbial production of CH4 and
the release of Cl-I4 bubbles from the system. Release of
CH4 by ebullition presumably occurs when the stored gas
bubble volume increases beyond some critical value. Although we have :30 data that address details of this release
process, such as’ information
on bubble tube geometry
and location, knowledge of the size of the stored gas bubble volume permits assessment of the effects of changes
in atmospheric pressure, temperature, and water-table
elevation on ebullition.
Modulation
qf CH4 ebullition by atmospheric pres-
sure-Calculated
gas bubble volumes are large enough
that changes in atmospheric pressure are expected to affect the rate of ebullition from this site. For illustrative
purposes, assun:e that the CH4 production rate in this
peatland during warm weather is 4 x 1OA2 mol m-2 d-l,
a value consistent both with rates of primary production
in the bog (Herr ond 1980) and with CH4 flux estimates
based on CH4 gradients in the acrotelm (Fechner and
Hemond 1992). For the gas bubble volume, we use a
representative value of 4% of total pore space, based on
the results of the preceding analysis. We further assume
that the gas bubble volume contains 100% CH4. If all
produced CH, immediately enters the gas phase, bubble
volume growth resulting from methanogenesis is - 1 liter
rnw2 d-l. For comparison, when atmospheric pressure is
increasing at a rate of 10 mb d-l, the calculated effect of
decreasing bubble volume due to dissolution and compression (Eq. 2a:I is - - 1 liter m-2 d-l. Thus, we suggest
that periods of rapidly rising atmospheric pressure are
capable of arresting bubble volume growth, thereby halting CH4 ebullition. Conversely, periods of decreasing atmospheric pressure should lead to significantly enhanced
CH4 volume and ebullition
1381
Table 1. Statistics for the 33 blocks of data.
No. Slope*
data (cm
points cm- ‘)
Block
No.
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
24 Jun-8 Jul 88
29 Jul-6 Aug 88
7-23 Aug 88
26-29 Aug 88
30 Aug-4 Sep 88
5-9 Sep 88
23-27 Nov 88
29 Nov-3 Dee 88
3-9 Dee 88
9-12 Apr 89
30 May-2 Jun 89
2-6 Jun 89
18 Jun-5 Jul 89
l-7 Aug 89
14-19 Aug 89
8-14 Sep 89
8-l 1 Ott 89
lo-15 Nov 89
17-20 Nov 89
l-26 Jun 90
3-12 Jul90
17-24 Jul 90
28 Jul-6 Aug 90
12-18 Aug 90
30 Aug-15 Sep 90
23-27 Sep 90
11 Nov-3 Dee 90
16-20 Apr 9 1
22-26 Apr 91
7-12 Jun 91
21-28 Jun 91
1-12 Jul91
15-24 Jul 9 1
46
25
50
11
16
12
13
14
17
10
10
12
54
19
17
17
10
15
10
76
28
21
29
20
47
13
69
14
15
15
23
35
29
1.0
0.59
0.95
0.50
0.57
0.79
0.43
0.40
0.72
0.83
0.79
0.79
0.66
0.89
0.49
1.0
1.6
0.67
0.78
0.81
0.69
0.72
0.72
0.67
0.92
1.2
1.0
0.91
0.73
0.88
0.76
0.96
0.83
r2
Slope?
(cm mb-l)
Difference$
(cm)
0.98
0.98
0.99
0.95
0.96
0.87
0.90
0.90
0.84
0.96
0.87
0.79
0.98
0.98
0.94
0.99
0.83
0.93
0.99
0.99
0.98
0.99
0.98
0.98
0.99
0.98
0.98
0.97
0.97
0.99
0.99
1.0
0.99
-0.017
0.0045
-0.010
-0.0068
0.013
-0.003 1
-0.0026
-0.0020
-0.014
-0.0024
-0.00025
-0.0089
-0.029
-0.0020
-0.0073
-0.0028
-0.011
-0.0075
-0.0024
-0.016
-0.014
-0.0030
-0.030
-0.03 1
-0.016
0.0016
-0.0049
-0.0058
-0.00010
-0.0030
-0.0077
0.0082
-0.00056
0.92§ -0.0099
0.36
1.0
0.16
0.00097
0.35§
0.53
0.14
0.17
0.51§ -0.0048
0.19
0.094
0.24
-0.019
0.76§
0.18
0.25§ -0.00035
0.25
0.41
0.28§ -0.0034
0.086
-0.011
0.658
0.37§ -0.0091
0.21
-0.022
0.49$
0.40$ -0.0078
0.848 -0.0088
0.20
0.71# -0.0016
0.13§ -0.0022
0.36
0.18
0.36
0.00066
0.48§
0.43
95% C.L. for slope
(cm mb-l)
to -0.025
to 0.025
to -0.022
to -0.039
to -0.014
to -0.012
to -0.022
to -0.018
to -0.039
to -0.053
to -0.023
to -0.0083
to -0.0094
to 0.0 16
Mean of 14 slopes significantly different from 0: - 0.0 12 + 0.0 13
Mean of 12, negative slopes significantly different from 0: -0.0 16 +0.0094
Mean of 29 negative slopes: -0.0090&0.0088
* Slope of absolute mat level vs. absolute water level.
t Slope of relative mat level vs. atmospheric pressure.
$ Difference between maximum and minimum relative mat levels.
0 Slopes significantly different from zero at an a level of 0.05 (two-tailed
bubble volume
ullition.
growth and thus trigger episodes of eb-
Modulation of CH, ebullition by temperature-Changes
in peat temperature are also expected to have a strong
effect on ebullition rates. Consider again the above methanogenesis rate and its driving effect on stored bubble
volume. To compare the bubble volume growth rate due
to methanogenesis with the temperature dependency of
the gas bubble volume, we can differentiate the expression
for total mass of dissolved and gaseous CH, in the pore
water:
PV,
RT+
V*P
-
Hd
= total mass of CH4.
@a)
Differentiation
t-test).
with respect to temperature
yields
dV, _ Vg + -xV,,,RT dHd
-_0)
dT
T
Hd2
dT ’
Hd is the Henry’s law constant expressed as pressure divided by aqueous concentration (note that Hd = HRT).
A suppression of bubble size during cooling results from
two reinforcing effects - the contraction of the bubbles at
constant pressure according to the ideal gas law (first term
of Eq. 6b), and the repartitioning
of CH4 from gas to
aqueous phase as a result of the Henry’s law constant
decreasing as temperature decreases (the second term).
The Henry’s law constant decreases by - 11 (liter atm)
mol-l per “C at typical bog temperatures of 6-l 6°C.
Change in the Henry’s law constant results in the larger
1382
Fechner-levy and Hemond
of these two effects; for example, at 12”C, the second term
of Eq. 6b is - 6 times larger than the first term. The change
in gas bubble volume per degree of temperature change
can be estimated from Eq. 6b to be - 1.6 liters mm2.
Therefore, in this example, the result is a decrease in
bubble volume at a rate that could offset the calculated
bubble growth rate due to methanogenesis if the rate of
cooling were 0.6”C d- l. Although this is larger than the
average rate of cooling of the bog during autumn, such a
cooling rate is not implausible on an episodic basis.
A third, additive effect of decreasing temperature is a
decrease in the rate of methanogenesis. This effect, even
for a Qlo value as high as 10 or 15 (Dunfield et al. 1993),
has little influence on the analysis of a short episode of
rapid cooling but has a large cumulative impact on ebullition over the cooling season. We suggest that CH4
emissions from many peatlands may decline precipitously with the onset of cool weather, with the decline
being much more rapid than would be caused by the
temperature dependence of methanogenesis alone. Conversely, the system should respond in the opposite manner to warming. In particular, a rapid spring “turn-on”
of ebullition-more
rapid than would be predicted from
the temperature dependence of methanogenesis aloneis expected.
Efect of variation in water-table elevation-The gas
bubble volumes in many peatlands may be similar to
those calculated for our study site, even if the peatlands
are not floating. Although changes in buoyancy are much
more difficult to measure in a nonfloating peatland, bubble volume should respond in the same way to atmospheric pressure and tempcraturc changes. Additionally,
because the position of the water table relative to the
sediment in a nonfloating peatland is more variable than
it is in a floating peatland, changes in water-table elevation represent yet another mechanism that should be able
to affect ebullition by decreasing or increasing hydrostatic
’ pressure within the pore water. This water-table effect is
entirely independent of the known tendency of a falling
water table to expand the unsaturated thickness of the
peat and thus give rise to increased CH4 oxidation. The
magnitude of this water-table effect may be comparable
to that caused by atmospheric pressure change; for example, 1 mb corresponds to -3 cm of water, and thus
temporary suppression of ebullition by an increase in
atmospheric pressure of 10 mb would be mimicked by a
30-cm rise in water table. This suppression effect would
only require the addition of 3 cm of precipitation if the
storage-coefficient (specific yield plus elastic storage) were
0.1. Conversely, a lowering of the water table could enhance CH, ebullition (while possibly leading to enhanced
CH4 oxidation). Episodic CH4 releases due to decreases
in hydrostatic head on the order of 5-l 0 cm in peatlands
have been proposed (Moore et al. 1990).
Implications for jlux measurement - Measuring
modeling CH, fluxes are rendered mom complicated
the existence of ebullition as a major export process.
suggest that CH4 ebullition is significantly modulated
and
by
We
by
atmospheric pressure, temperature, and water-table elevation (as well as being triggered by physical disturbance
of wetland sediments). The volumes of stored gas may
be large enough that periods of rising atmospheric pressure, decreasing temperature, or rising water table can
suppress the atmospheric release of CH, for hours or days,
which is longer than the period of typical flux chamber
experiments. Conversely, decreasing atmospheric pressure, rising temperature, a falling water table, and mechanical disturbilnce may trigger release of CH4 by ebullition. These fictors need to be examined in the design
and analysis of CH4 flux experiments. Further measurements of CH4 g2.s volumes stored in peatlands, particularly as they change in response to seasonal meteorological variables, are warranted to confirm the extent and
consequences of gaseous CH, storage in peatlands.
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Submitted: 18 July 1995
Accepted: 11 January 1996
Amended: 8 May 1996