Journal of Conference Abstracts

The Effect of Flooding on Gas Fluxes from Wetlands

C. A. Kelly¹ (ckelly@bldgwall.lan1.umanitoba.ca), N. Roulet² (roulet@felix.geog.mcgill.ca), J. W. M. Rudd¹ (rudd@umanitoba.ca), V. St. Louis¹ (vstlouis@starfish.acs.brockport.edu), T. Moore¹ (Moore@felix.geog.mcgill.ca), K. Scott¹ (kscott@cc.umanitoba.ca), S. Schiff³ (sschif@sciborg.uwaterloo.ca), G. Edwards (gedwards@net2.eos.uoguelph.ca), R. Aravena³ (roaravena@sciborg.uwaterloo.ca) & B. Warner³ (bwarner@watdcs.uwaterloo.ca)

¹ Department of Microbiology, University of Manitoba, Canada.

² Department of Geography, McGill University, Canada.

³ Department of Earth Sciences, University of Waterloo, Waterloo, ON N2L 3G1 Canada.

The effects of reservoir construction on gas fluxes from wetlands were studied by carrying out a flooding experiment at the Experimental Lakes Area, northwestern Ontario. The experiment lasted 5 years, with two years of preflood and three years of postflood measurements. The wetland consisted of a 14.4 ha peatland surrounding a 2.39 ha central pond (Lake 979). In its natural state, the pond was a source of both CO₂ (740-1,600 mg CO₂ m^-2 d^-1) and CH₄ (17 mg CH₄ m^-2 d^-1 to the atmosphere, which is characteristic of many natural wetland ponds. Gas fluxes from the peatland areas were quite different. There was a net uptake of CO₂ (-67 mg CO₂ m^-2 d^-1, or 4 g C m^-2 d^-1, for the 1992 growing season, due to net photosynthetic activity of the vegetation. This year was probably somewhat atypical, due to cloudy cool conditions, because on the long term the net C accumulation in the peatland was estimated by ¹⁴C dating to be 20 to 30 g C m^-2 d^-1, compared to CH₄ fluxes from the peatland areas were in the direction of the atmosphere, but of much smaller magnitude (2 mg CH₄ m^-2 d^-1) than the pond fluxes, due to a high efficiency of oxidation as the CH₄ passed through the upper layers of peat above the water table. The wetland as a whole was a small carbon sink, on the long term, of 7.6 g C m^-2 d^-1.

After flooding, the wetland became a carbon source emitting 130 g C m^-2 d^-1 , as fluxes of CO₂ and CH₄, from all areas of the wetland to the atmosphere. As a whole, there was greater CH₄ production and slightly less CH₄ oxidation, and the average postflood flux was 48 mg CH₄ m^-2 d^-1 from the peatland area and 88 mg CH₄ m^-2 d^-1 from the pond area. The postflood CO₂ fluxes were 1,100-2,400 mg CO₂ m^-2 d^-1 from the peatland area and 3,600-3,700 mg CO₂ m^-2 d^-1 from the pond area. Litterbag measurements indicated that most of the increased gas flux was from decomposing vegetation. A small part was from peat that continued to decompose after flooding, probably at a slightly higher rate due to general warming caused by the loss of an insulating layer between the water table and the peat surface. A greater proportion of the decomposition products was CH₄, due to greater anoxic habitat, and this contributed to increased CH₄ production. The fluxes measured in the 3 year-old experimental reservoir were not different from fluxes measured in two of the largest northern Quebec reservoirs (1,700-2,200 mg CO₂ m^-2 d^-1; Duchemin et al. 1995), and were similar to fluxes predicted from flooded biomass (Rudd et al., 1993).

Pre-flood results demonstrate the importance of the ratio of vegetated peatland area to water cover, or pond area in wetlands on gas fluxes, adding to previous observations on the Hudson Bay Lowland (Roulet et al., 1994). The postflood results show that changes in greenhouse gas fluxes due to flooding should be considered in evaluating the environmental consequences of reservoir construction. Reservoirs currently cover approximately 500,000 km², and this area is increasing.

References

Duchemin, E., M. Lucotte, R. Canuel, & A. Chamberland, Global biogeochem. Cycles 9, 529-540 (1995).

Roulet, N. T. and 6 others, J. Geophys. Res. 99, 1439-1454 (1994).

Rudd, J.W.M., R. Harris, C.A. Kelly, & R.E. Hecky. Ambio 22, 246-248 (1993).