sources in an adjacent watershed. Watershed-scale issues therefore must be examined above and below ground. To date, management has generally under-emphasized ground water problems and responses as a component of water resource conservation and rehabilitation (GAO, 1991). Indeed, watershed management too often focuses only on surface waters, or if ground water resources are recognized only volume and accessibility receive notice whereas quality and biologic characteristics are also important.
Research has established at least five important attributes of groundwater ecology that are essential to a holistic view of watersheds (Gibert et al., 1994b). First, there is a continuum of geohydrologic units involving surface and groundwater flow pathways. For example, in one area surface runoff may flow into caves and fissures and that ground water may then feed directly into a river via large springs. In another, water from a river may penetrate alluvial soil at the upstream end of a floodplain, flow through unconfined aquifers within the floodplain and upwell back into the channel via a network of springbrooks at the downstream end (Jaffe and Dinovo, 1987). In either case, surface water becomes ground water that then re-enters the surface systems. This simply illustrates that geohydrologic units should be viewed as a diverse, interconnected mosaic.
A second essential ecological feature of ground water is that as water and materials move from one underground unit to the next, significant biogeochemical transformations usually occur. Types and concentrations of solutes change as the geomorphology and hence flow rate changes along the geohydrological gradient. Likewise, biotic species composition and abundance may change significantly along the geohydrological gradient. Thus we recognize biophysical (e.g., flow, temperature, redox, ion concentration, biodiversity, bioproduction) gradients as water moves through the geohydrological continuum. Gradients may be very steep (i.e., conditions may change very quickly) at boundaries or ecotones between different units (Vervier, 1992).
Third, virtually all ground water has some sort of biotrophic (food) web composed of microbes (bacteria and protozoa) as well as larger, more complex organisms, except in situations where oxygen is insufficient. As in surface water, the food web relies on microbial activity that uses dissolved or particulate organic matter as the primary energy source (Stanford and Ward, 1993). Since photosynthesis cannot occur in ground water, a supply of organic matter from soils or surface waters upstream is critical. Limited supplies of this organic matter generally makes ground water much less productive than surface waters. Nonetheless, ground water can support food webs (including in some cases vertebrates) that play vital ecological roles in transforming solute concentrations (including pollutants) in waters moving through the ground.
Fourth, water's movement through ground water systems and its associated biogeochemical transformations adds complexity to our view of landscapes. For example, we now examine river ecosystems in four dimensions: upstream-down-stream (longitudinal), channel-riparian (lateral), channel-groundwater (vertical),