4.1. Components of Ecosystem Respiration
 Rates of ER and heterotrophic respiration in the saturated zone were in agreement with earlier eddy covariance tower measurements, suggesting that the chamber and peat bag incubation measurements were sufficiently accurate, and that the selected plots was adequately representative for the Mer Bleue bog. Yearly ER has been estimated at 105 mmol m−2 d−1 of C with heterotrophic respiration accounting for 44–66 mmol m−2 d−1 [Frolking et al., 2002; Lafleur et al., 2001, 2003; Moore et al., 2002]. Similar rates were also found in other peatlands [Lafleur et al., 2001]. We determined an average heterotrophic respiration of 54 mmol m−2 d−1 in the unsaturated zone and an ER of 40–350 mmol m−2 d−1 from the flux measurements in summer 2003 (Table 1). Previously determined rates of heterotrophic respiration in intact soil segments [Scanlon and Moore, 2000] also lay within the range of 0.3 to 13.8 μmol C g−1 d−1 obtained in this study in situ. Such rates are at the lower end of rates (2–110 μmol g−1 d−1) reported from aerobic incubation studies with intact cores and slurries of similar peats [e.g., Bergman et al., 1999; Bridgham et al., 1998; Moore and Dalva, 1997].
 This study demonstrates that only a very small fraction of ER stemmed from the saturated zone of this relatively dry peatland (Table 1). During the summer, or warm temperatures, O2 was consumed within 1 to 3 cm below the water table and rapidly depleted following water saturation [Blodau et al., 2004; Blodau and Moore, 2003]. Anaerobic total respiration, as determined from the pore water profiles, thus contributed on average less than 1 % to ER (Table 1). Even in the wet summer of 2004, accounting for changes in dissolved CO2 storage, and with temperatures at the high water table being about 15°C, total respiration in the saturated zone remained at or below 7 mmol C m−2 d−1. It thus contributed less than 5% to ER, which was recorded at 154 to 310 mmol m−2 d−1 in the summer months of 2003 (Table 1). Frolking et al.  used the PCARS model to estimate that depths below 50 cm contributed 10 to 20 mmol C m−2 d−1 and depths between 10 and 50 cm contributed 10 to 60 mmol m−2 d−1 to ER at the Mer Bleue bog. Our data suggest that these model estimates are probably high for depth increments that are water saturated. Our observed CO2 concentration profiles are fairly typical for peatlands, albeit somewhat at the lower end [Hornibrook et al., 1997; Nilsson and Bohlin, 1993]. Total respiration in the saturated zone of peat bogs should thus be negligible compared to that of the unsaturated zone.
 Autotrophic respiration in the saturated zone decreased by defoliation by an estimated 28 % (Figure 6) suggesting that root activity contributed to respiration in the saturated zone. This respiration related to root activity (0.37 mmol m−2 d−1) was only very small compared to the total root related respiration. The latter has been quantified at 162–176 mmol m−2 d−1 by comparing C emission from control to defoliated plots and recording a difference in emission of 50 to 60% [Stewart, 2006]. Mapping of root biomass in hollows showed that below a depth of 40 cm shrub roots were absent. The depth increments of 10 to 20 cm and 20 to 30 cm contained about 400 g m−2 each, and the increment of 30 to 40 cm about 150 g m−2 of root biomass, of which 15–25% consisted of fine roots [Moore et al., 2002]. A considerable fraction of the shrub root biomass was thus located below the water table (20–25 cm) in the wet July of 2004. Since its contribution to total root related respiration was negligible, the saturated conditions apparently inhibited respiratory activity of the submersed roots.
 The reduction of heterotrophic and autotrophic respiration by anoxia, and the low kd values of 0.002 yr−1 to 0.0012 yr−1 calculated for the saturated zone, emphasize the potential impact of the water table on these processes. However, ER and water table position did not correlate, and did not either in experiments with intact ombrotrophic bog mesocosms [Updegraff et al., 2001], and in a multiyear study of ER at the Mer Bleue bog [Lafleur et al., 2005]. We believe that the lack of correlation is caused by the small contribution of the deeper peat to ER, which is diminished by increasing recalcitrance of the peat, and decreasing soil temperature and availability of oxygen with depth [Lafleur et al., 2005; M. Heitmann and C. Blodau, unpublished data, 2004]. The bog vegetation may also have some capability to relocate activity to the unsaturated zone when only part of the rooted zone is inundated and maintain respiration. Stronger effects of water table level fluctuations on ER should thus be restricted to wetter peatlands. Instructive in this respect are results from earlier experiments with intact Mer Bleue peat soil columns at constant temperature (10°C), and with the water table being either near the moss surface or 35 cm below [Blodau et al., 2004]. Lowering the water table raised “ER” in these phytotron experiments from 47 to 77 mmol m−2 d−1.
 In this dry ombrotrophic bog, soil temperature was thus the more important control on heterotrophic respiration and also ER, as reported by Lafleur et al. . Under peat temperature ranges, heterotrophic respiration generally increases by a factor of 2–3 for every 10°C temperature increase (Q10) [McKenzie et al., 1998; Moore and Dalva, 1993; Yavitt et al., 1997]. This was also reported for the Mer Bleue bog [Lafleur et al., 2001; Scanlon and Moore, 2000]. Higher Q10 values were mainly derived from field studies that related CO2 fluxes and soil temperatures in shallow peats [Bridgham et al., 1995; Bubier et al., 1998; Silvola et al., 1996], and may reflect the influence of roots and root exudates. In forest soils, Q10 values of 4.6, compared to 3.5 for bulk soil, and 2.5 in root-free soil, have been determined [Boone et al., 1998]. Part of the observed Q10 value of 5–6 for heterotrophic respiration in the unsaturated zone was apparently caused by a factor that covaried with the temperature. When peat was additionally incubated at constant temperature, higher rates were measured in midsummer than at other times (Figure 2). DOC reaching the peat bags with seepage water may have contributed to this effect.
 DOC was the most important fraction of dissolved C and was net released and consumed at much larger rates than either CO2 or CH4 in the peat (Table 1). On the basis of measurements in summer or the ice-free season, peatlands have been estimated to export DOC in the range of 1 to 10 mmol m−2 d−1 to discharging streams [Koprivnjak and Moore, 1992; Moore, 1987, 1988; Urban et al., 1989; McKnight et al., 1985]. For the Mer Bleue peatland, a value of 2.6 mmol m−2 d−1 was previously determined from discharge data [Fraser et al., 2001] and has recently been updated to a 6-year average of 3.7 mmol m−2 d−1 (N. T. Roulet, unpublished data, 2005). Such rates are much smaller than the minimum release and immobilization rates obtained in this study. In earlier mesocosm experiments with intact peat soils from the Mer Bleue bog, net DOC production rates were 14–15 mmol m−2 d−1, independent of water table levels [Blodau et al., 2004]. In incubations with peats from different depths and temperatures of 22°C, Moore and Dalva  reported average release rates of 2.6–3.8 μmol g−1 d−1 over a 60-day period with a threefold flushing with distilled water. Assuming a bulk density of 0.05 to 0.1 g cm−3, these rates were about 1–4 times the maximum release determined in this study. From this collation it may be inferred that internal seasonal production and consumption strongly exceeds net export from peatlands, and that maximum rates of release during short-term incubation of peat are probably seldom reached in situ.
 Soil temperature and runoff may control DOC dynamics [Moore and Dalva, 2001]. During summer, increases in DOC concentrations were observed at the Mer Bleue bog in the hydrologically average year of 2003, as previously at other peatlands [Dalva and Moore, 1991; Waddington and Roulet, 1997]. In the wet summer of 2004 we could not observe this pattern and concentrations decreased over time, possibly because of dilution from precipitation.
 Methane production and diffusive fluxes remained small at the Mer Bleue bog and the gross CH4 production amounted to only 17 % of below water table CO2 production, when averaged over the sampling period, and to less than 10% in summer. Accordingly, heterotrophic respiration mostly proceeded by utilization of electron acceptors such as sulfate, which can support respiration rates of 1 to 40 mmol m−2 d−1 in bogs [Nedwell and Watson, 1995; Vile et al., 2003; Wieder et al., 1990]. Decreasing sulfate concentrations coincided with increasing CH4 concentrations over the sampling period (auxiliary material, Figure S4) but these changes did not affect CH4 production rates, which remained low. Thus we could not identify an effect of sulfate availability on methanogenic activity.
 Particularly in winter, when oxygen penetrated deeper into the peat (C. Blodau, unpublished data, 2001), CH4 was oxidized below the water table (Figure 4), in agreement with the methanotrophic activity reported from the saturated zone of peatlands [Kettunen et al., 1999]. Production and emission of CH4 were very low in 2003 and only began increasing in 2004, reaching 0.4–2.7 mmol m−2 d−1. The chamber fluxes then exceeded diffusive fluxes across the water table and also the gross production of CH4 in the near-surface peat (Table 1). Emission was at this point probably driven by ebullition, which may occur at partial pressures above 0.21 atm, or 358–389 μmol L−1 at 8° to 12°C, depending on the partial pressure of N2, which is stripped from the peat pore waters with continuing CH4 production [Fechner-Levy and Hemond, 1996].
 The difference in CH4 emission rates between 2003 and 2004 may be attributed to the threshold of CH4 concentrations allowing for ebullition, which drew nearer to the peatland surface in 2004 (Figure 4). The high water table may have also shortened the residence time of CH4 in the unsaturated zone. In September 2004, emission rates peaked at 2.7 mmol m−2 d−1 after the water table rose by about 15 cm five days before the flux measurement. Since CH4 was not produced in the added water column within the first week (Figure 4), the increase in CH4 flux can only be explained if a larger fraction of the CH4 escaped oxidation. A similar effect was previously reported by Kettunen et al. , who observed emissions to increase for short periods after rainfall and associated rises in water tables in a boreal pine sedge fen. Such an effect would be opposed to decreases in ebullition with rising water table and hydrostatic pressure in floating mats [Fechner-Levy and Hemond, 1996]. As 3 cm of water column equals a change in atmospheric pressure of about 1 hPa, low atmospheric pressure owing to the storm event in September 2004 might have offset the effect of the rising water table and triggered additional ebullition.