Seasonal boundary dynamics of a groundwater/surface-water ecotone

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

INTRODUCTION

Through initiatives cosponsored by the International Unions' Scientific Committee on Problems of the Environment (SCOPE) and UNESCO's Man and the Biosphere Programme (MAB), the characterization of landscape boundaries, or ecotones (sensu Holland 1988), has become an important research objective. It is now widely recognized that ecotones function as regulatory sites in the movement of nutrients (e.g., Lowrance et al. 1984), organic materials (e.g., McArthur and Marzolf 1986), and biotas (e.g., Ward and Palmer 1994) across landscapes. Concomitantly, there has developed a body of literature concerning the importance of ecotone management (see review in Di Castri et al. 1988, Naiman and Decamps 1990, Holland et al. 1991). While ecosystem boundaries often are defined by conspicuous changes in ecosystem characteristics (e.g., transition zone from forest to grassland), the ecotone between the surface and subsurface environments in lotic ecosystems, the hyporheic zone, is not precisely delineated as it cannot be reduced to a simple physical boundary (Gibert 1991).

Early attempts to describe the spatial extent of hyporheic zones were based largely on the vertical and lateral distributions of epigean (derived from the surficial river bed environment) or hypogean (derived from the true groundwater environment) fauna. Where hypogean taxa were absent or poorly developed, studies focused on the depth of penetration of epigean fauna. Both Coleman and Hynes (1970) and Williams and Hynes (1974) suggested that the hyporheic zone extended deep into the substratum (30-70 cm) beneath a riffle on the Speed River, Ontario, Canada, as they collected high numbers of typical epigean taxa there. Where hypogean taxa were more populous, others have attempted to demarcate hyporheic limits based on epigean-hypogean associations (e.g., Danielopol 1989, Ward 1989, Bretschko 1991). In these studies, the distinction between the hyporheic zone and the true groundwater zone was less clear, however, as hypogean-epigean associations were often more spatially complex.

More recently, a few studies have heeded Hynes' (1983) plea to more actively integrate the principles of groundwater science into studies of river ecology and have attempted to delineate hyporheic zones using hydrogeological and chemical measures. based on work with conservative tracer injections, Triska et al. (1989) suggested a functional means by which the spatial extent of the hyporheic zone could be identified. Their two-component hyporheic zone included the surface region (the area beneath the bed surface that was chemically indistinguishable from the channel water, containing [greater than]98% advected channel water) and the interactive zone (characterized by [less than]98% but [greater than]10% advected channel water). Williams (1989) identified several chemical discontinuities in two Canadian rivers, with break lines occurring from the river margin obliquely downwards under both the river bed and bank. He proposed that these discontinuities were indicators of the position of the hyporheic/groundwater interface. Using depth-to-groundwater temperature (i.e., the depth at which water temperature is isothermal with true groundwater) as an indicator of the location of the hyporheic/groundwater interface, White (1993) generated a three-dimensional model of the hyporheic zone beneath a 9 m long pool-riffle-pool sequence. These more recent studies are unique, as they represent the first explicit attempts to identify parameters that define the location of the hyporheic/groundwater interface. Yet, they provide no data to describe how the interface may shift in response to changes in relative inputs of the various sources contributing to flow in rivers. Temporal fluctuation in the volume of hyporheic interstices is likely to affect the retention and processing of material exchanged between the surface and subsurface environments. Accordingly, there is a need to identify patterns of hyporheic volume fluctuation in order that a more holistic interpretation of: (1) the nature and causes of the changes in water chemistry; and (2) the quantification of the import, export, and transformation of dissolved and particulate organic matter, which occur during exchange between contiguous groundwaters and streams, can be acquired (Williams 1993).

The goal of this study was to examine seasonal boundary fluctuation of the hyporheic/groundwater interface by quantitatively assessing the biotic and abiotic parameters beneath a stream riffle. Specifically, we were interested in the following aspects of hyporheic/groundwater dynamics: (1) do the hyporheic and groundwater zones have distinct and distinguishable chemical signatures and therefore could chemical discontinuities identified in the subsurface delineate the location of the hyporheic/groundwater interface?; (2) is the subsurface community divisible into a number of subunits whose relative positions identify the location of the hyporheic/groundwater interface?; (3) does faunal subunit distribution track seasonal shifts in the position of the hyporheic/groundwater interface?; and (4) is hyporheic/groundwater interface fluctuation related to the relative strengths of the upward force of baseflow and downward force of advected channel water?