Framework

Riverine Ecosystems

Groundwater discharge to streams can play a critical role in the viability of riverine ecosystems by:

1. directly maintaining in-stream habitats, with groundwater baseflow contributing to the dry-season low-flow regime;
2. discrete springs and seepage zones creating specific habitat;
3. the hyporheic zone of saturated sediments below and along side the stream channel where surface water and groundwater exchange occurs also having ecological significance (Boulton and Hancock, 2006); and
4. associated shallow watertables maintaining vegetation along the riparian corridor.

Instream habitats

Groundwater seepage dominates the low-flow regime for many Australian rivers, and so can be critical in maintaining habitats over extended dry periods. This is the case for many coastal rivers, particularly in northern Australia (SKM, 2001). The platypus is considered to be one example of fauna that relies on groundwater to sustain perennial stream flow or provide refugia in pools during droughts (Hatton and Evans, 1998). Groundwater can not only contribute volumetrically to stream flow but due to greater residence time can also be an important source of minerals and organic nutrients for aquatic biota. Groundwater discharge can also have an ecological impact by moderating stream temperature.

Discrete springs

In certain geological settings such as karst terrain, groundwater discharge can occur as discrete springs, rather than diffuse seepage. These can support unique and specialised aquatic biota, by providing a relatively stable environment in terms of water flow, quality and temperature. For example, springs in the upper reaches of streams in the Flinders Ranges, South Australia provide initial flows enriched in dissolved nitrogen. Such groundwater discharge provides a nutrient source required for aquatic productivity and control downstream gradients in algal-community composition (Boulton and Hancock, 2006). Springs, which may not necessarily be linked with any surface drainage, are an important water source for animals in the northern tropics and the arid inland.

Riparian Vegetation

The shallow watertables typically associated with gaining stream systems, can also be accessed by vegetation within the riparian corridor. This includes wetland ecosystems that can be waterlogged or inundated during the wet season. These wetlands show a wide diversity across Australia and include forests or woodlands of mesophyll palm vine, swamp paperbark (melaleuca spp) or swamp sclerophylls (such as river red gum), lignum-dominated shrublands, swamp grasslands (such as phragmites and typha) and sedgelands (SKM, 2001). As groundwater can maintain vegetation health compared with the rest of the landscape, fauna retreat to these wetlands during dry periods.

Hyporheic zone

The hyporheic zone is the transition between aquifer and stream consisting of permeable sediments saturated entirely with surface water or it may be mixed with groundwater. This zone can have significant biological and chemical activity due to this mixing of waters, resulting in a higher diversity of microfauna than the in-stream environment.

The hyporheic zone can play a significant role in nutrient cycling and organic matter decomposition in the overall stream ecosystem (Boulton and Hancock, 2006). Upwelling from the hyporheic zone, typically controlled by fine-scale features such as changes in sediment texture or streambed topography, can generate a thermal refuge for biota (such as fish), or encourage greater algal and macrophyte growth due to supply of nutrients. Such upwelling can be affected by stream bed clogging due to algal growth and saltation. Also structures such as reservoirs or diversions can change stream sediment loads and water velocities, leading to changed bed depth.

Relevant Links

The North American Benthological Society
UK Environment Agency High resolution in-situ monitoring of hyporheic zone biogeochemistry
UK Environment Agency Groundwater-surface water interactions in the hyporheic zone

References

Boulton AJ, Hancock PJ, 2006. Rivers as groundwater-dependent ecosystems: a review of degrees of dependency, riverine processes and management implications. Australian Journal of Botany 54, 133-144.

Hatton T, Evans R, 1998. Dependence of ecosystems on groundwater and its significance to Australia. LWRRDC Occasional Paper No 12/98.

SKM, 2001. Environmental Water Requirements of Groundwater Dependent Ecosystems. Technical Report Number 2, Sinclair Knight Mertz for Environment Australia.