Focus: Physical Geography of Wetlands 85

and numerous dissecting channels ensure a more wide- spread, but irregular, distribution orwater and salt. There may be lateral salinity gradients between the channels and the inland marsh. The vertical salinity profiles tend to change seasonally from more saline at the surface in summer, to less saline at the surface in the wet seasons (spring and fall). Below the range of water table fluctua- tion in the marsh sediments, the salinity profile tends to be relatively constant, reflecting the long-term average surface salinity (Figure 3b).

A knowledge of the relationship between morphology, hydrology, and salinity i s basic to understanding vegeta- tion patterns on coastal marshes and will contribute to sound management strategies.

THE BIOCEOCHEMISTRY OF SMALL HEADWATER WETLANDS Alan Hill Department of Geography, York University, North York, Ontario, Canada M3J 1P3

Freshwater wetlands are increasingly viewed as impor- tant components of the landscape because of their role in the regulation of water chemistry (Howard-Williams 1985). Nevertheless, little information is available on the biogeochemistry of small headwater wetlands. Small palustrine (upland) wetlands with outlet streams are typical of many drainage basins in glaciated areas of eastern Canada and adjacent regionsof the United States. The location of these wetlands within headwater drain- age basins makes them potentially important in modify- ing the chemistry of water fluxes between upland areas and streams.

Consideration of hydrology provides an essential framework for understanding wetland biogeochemistry (Hemond 1980). This section will examine the influence of wetland/groundwater linkages and hydrological path- ways within wetlands on water chemistry. These aspects of hydrology may help to explain major differences in the role of headwater wetlands as sources, sinks, and trans- formers of mineral elements.

The relationship between headwater wetlands and groundwater constitutes an important control on wetland hydrology and biogeochemistry. Headwater wetlands that are sustained by a perched water table (ephemerally connected wetlands) often exhibit variable discharge, whereas discharge from wetlands connected to regional groundwater systems is more evenly distributed through- out the year (Roulet, this issue).

The timing of water movement through headwater wetlands i s critical to their role in the regulation of

nutrient fluxes to downstream ecosystems. Wetlands with intermittent streams have little influence on down- stream chemistry, except during periods of runoff. In late winter and spring, snowmelt on the wetland area com- bined with runoff from surrounding slopes frequently produces large fluxes of water, which often represent 50 to 75 per cent of annual runoff (Whitely and Irwin 1986). The magnitude of this flux, combined with short water- residence times and low temperatures, may limit the capacity of these wetlands to retain or transform mineral elements. input-output budgets for several small wet- lands with ephemeral streams on the Precambrian Shield in central Ontario revealed low annual retention of total phosphorus and nitrogen, mainly as a result of large exports of N and P during the late winter and spring (Devito et al. 1989). Groundwater-connected wetlands can influence downstream chemistry throughout the year. A large proportion of annual groundwater inputs flow through these wetlands during the summer and fall months. Warm-season plant uptake and rapid microbial assimilation may increase the capacity of these wetlands to retain or transform nutrients.

Pathways of water movement within wetlands are also important in influencing nutrient flux and transformafon. Water can be transported through headwater wetlands by a variety of routes. In groundwater-connected wet- lands, water may flow by subsurface paths or emerge as springs producing zones of overland flow. In wetlands sustained by a perched water table, most runoff occurs as saturated overland flow during rainfall or snowmelt events.

Hydrochemistry i s affected by differences in the resi- dence times and in the environments encountered by water moving along different flow paths. Linkages be- tween groundwater flow paths and nutrient transforma- tions have been examined recently in a groundwater- connected hemlock-cedar swamp on the Oak Ridges Moraine near Toronto (Hill and Warwick 1987; Warwick and Hill 1988). Three major groundwater flow paths were identified. Shallow groundwater emerges as springs at the upland perimeter of the wetland, producing numerous surface flow lines, which cross the swamp. A second pathway involves deeper subsurface ground- water, which flows upward through the organic soils and enters the surface flow lines within the wetland. These two pathways contribute about 60 per cent of the groundwater input to the outlet stream (Roulet 1988). The third pathway, representing about 40 per cent of the groundwater input, reaches the stream directly as bed and bank seepage.

Groundwater entering the stream as bed and bank

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86 Focus: Physical Geography of Wetlands

seepage had an ammonium concentration of 30 to 50 pg N-L-’ and represented a major ammonium input to the stream ecosystem. In contrast, ammonium concentrations in the surface flow lines were very low (< 5 pg N.L-’), although ammonium levels in spring water and subsur- face groundwater entering flow lines were 30 to 50 k g N.L-’ (Hill and Warwick 1987). Field enrichments of flow lines with ammonium and laboratory experiments using flow line substrates showed rapid depletion of ammonium due mainly to microbial immobilization. Whereas a high demand for ammonium occurs in the aerobic flow line environment, the higher ammonium concentration in bank and bed seepage probably re- sults from the low nitrogen requirement of anaerobic metabolism.

Rates of wetland discharge is surface runoff pathways may also influence nutrient retention. Field enrichment with nitrate of spring-fed surface flow lines revealed an absence of nitrate depletion (Warwick and Hill 1988). Laboratory incubations of flow line sediments showed low rates of denitrification. The efficiency of nitrate removal jn these flow lines i s probably limited in part by the short residence time of <1 hour for water transported through the wetland. High dissolved oxygen levels associated with increased flow velocities in surface flow lines may also inhibit denitrification.

Ephemerally connected wetlands often act as variable source areas producing saturated overland flow during spring snowmelt and intense rainfall events (Taylor and Pierson 1985). The extent of interaction between satu- rated overland flow and wetland soils and vegetation influences nutrient regulation. Wels and Devito (1 988), using lithium bromide, traced water flow paths through a small conifer swamp during spring runoff. The spatial distribution of the tracer suggested that a component of the water rapidly crossed the swamp via open channels. Water, however, moved more slowly as shallow subsur- face flow through living Sphagnum and poorly decom- posed peat. Water flow over the surface and through the upper layers of organic soil may be sufficient to flush mineral elements out of wetlands, particularly during spring runoff. Pierson and Taylor (1985) found that potassium concentrations downstream from an ephemer- ally connected wetland were highest during early spring runoff. In areas of standing water, leaching of leaf litter probably accumulated potassium during winter, which was flushed by saturated overland flow in early spring.

These studies indicate that small headwater wetlands exhibit variable behaviour with respect to element flux and transformation. Additional studies which integrate

hydrology and water chemistry will be required to develop reliable generalizations about the role of small wetlands in the regulation of water chemistry in head- water drainage basins.

GAS EXCHANGE BETWEEN PEATLANDS AND THE ATMOSPHERE Tim Moore Department of Geography, McCill University, 805 Sherbrooke Street West, Montreal, Quebec, Canada H3A 2K6

Exchange between wetlands, such as peatlands, and the atmosphere occurs in a number of gases, such as carbon dioxide, methane, hydrogen sulfide, and nitrogen oxides. Probably the most important exchanges are the fixation of carbon dioxide in plant material by photosynthesis and its storage in peatlands, and the emission of methane. Both carbon dioxide and methane concentrations in the atmosphere are increasing (at 0.5 and 1 . 1 per cent yr-’, respectively), with consequences for the so-called ’green- house effect’ because of the radiatively active nature of these gases (Ramanathan 1988).

Peatlands act as storage for a large amount of organic carbon trapped in slowly decomposing plant tissues. Rates of peatland growth in Canada generally average between 0.2 and 1.0 mm yr-’ (National Wetlands Working Group 1988), although little is known about the regional variations in growth rates, compared to the vast amount of information available on the evolution of peatlands. Paucity of data on bulk density and organic carbon content of the peat prevents accurate calculation of the storage rate of carbon in these peatlands, but values of 10 to 50 g C m-2 yr-’ are common. Storage of organic carbon in peatlands can thus amount to up to 5 x 1 O5 g rn-’ and, given the large area of Canada covered by peatlands, this amounts to a substantial total of about 1 x 10” g C stored in Canada‘s peatland.

A shift in the pattern of carbon storage in peatlands can be produced by both natural and anthropogenic changes. The projected rise in atmospheric temperatures, asso- ciated with the ‘greenhouse effect,’ and reductions in precipitation and soil wetness (a lowering of the water table), could change peatlands from carbon sinks to sources, as has been suggested for the Alaskan tundra (Billings et al. 1982). Several hundred square kilometres of wetlands in Canada have been drained for urban expansion, agriculture, forestry, and peat harvesting.

The Canadian Geographer I Le Ghgraphe canadien 34, no 1 (1 990)