Seasonal boundary dynamics of a groundwater/surface-water ecotone

Ecology, Sept, 1998 by Brian G. Fraser, D. Dudley Williams

MATERIALS AND METHODS

The study area

The study was conducted near the source of the Speed River, southern Ontario, Canada [ILLUSTRATION FOR FIGURE 1 OMITTED]. The Speed flows through gently undulating hills, drumlin fields, glacial spillways, and swampy depressions with an average gradient of 2 m/km (Ontario Department of Planning and Development 1953, Chapman and Putnam 1984). Bedrock in this region belongs to the Guelph dolomite horizon and soils are grey-brown podsols and humic glysols intermixed with outwash sand, gravels, and coarser glacial tills (Hoffman et al. 1964). Approximately 80% of the watershed is used for mixed farming, although floodplains of the upper course are largely wooded, reforested, or maintained as rough pastures (Bishop and Hynes 1969).

The main sampling area, the Rowan Farm study site (43 [degrees] 43 [minutes] 54 [seconds] N, 80 [degrees] 16 [minutes] 24 [seconds] w), consisted of a 40-m riffle and the adjacent riparian corridor. Along this reach, river width varied from 4 to 6 m and water depth ranged from 7 to 12 cm during baseflow conditions to 70 cm at the height of spring run-off. Typically, the seasonal discharge pattern along this portion of the Speed River is characterized by extended periods of high flow during the spring and fall and low flow during the summer and winter (unpublished data; Water Survey of Canada, Environment Canada, Guelph, Ontario, Canada).

To a depth of 30 cm, the substrate is composed primarily of gravels ([less than]10 cm diameter) intermixed with silts and sands together with a few larger dolomite slabs. Below 30 cm, substrate heterogeneity is low and substrate composition is dominated by medium and fine sands (sensu Cummins 1962). The subsurface sediment profile has been described in detail by Stocker and Williams (1972). Riparian vegetation is dominated by eastern white cedar (Thuja occidentalis L.) and paper birch (Betula papyrifera) mixed with willow (Salix sp.) along the north bank of the river and by goldenrod (Solidago sp.) and various grasses along the south bank.

Data collection

Sampling program. - As sampling effort was directed towards monitoring seasonal trends, biological, chemical, and hydrogeological data were collected during a single week-long sampling period per season from winter 1992-1993 through fall 1993. The exact time of sampling was based on seasonal river discharge extremes [ILLUSTRATION FOR FIGURE 2 OMITTED]. Spring and fall sampling commenced immediately following the peaks of seasonal discharge (29 April through 4 May and 27 October through 1 November, respectively). Summer and winter sampling occurred during the period when baseflow accounted for the greatest proportion of stream discharge (18 August through 23 August and 27 February through 4 March, respectively).

All data were collected from nine permanently installed sampling stations positioned at the head of the riffle in a transect across the river and into the adjacent banks [ILLUSTRATION FOR FIGURE 1B OMITTED]. The sampling device, the colonization corer, combines aspects of an artificial substrate sampler (after Panek 1991) and a bundle piezometer (Cherry 1983), which made it possible to collect faunal samples, interstitial water samples, and potentiometric head measurements at 0.2-m intervals between 0.1 and 1.1 m below the river bed surface. A detailed description of the colonization corer can be found in Fraser et al. (1996) and its performance, in comparison with other hyporheic samplers, is reported in Fraser and Williams (1997).