River ecosystem health down under: Assessing ecological condition in riverine groundwater zones in Australia

Below many rivers and extending laterally beneath their banks lies a zone of saturated sediments-the hyporheic zone. This region is the site of dynamic exchanges of water and materials between the groundwater below, lateral alluvial aquifers, and the river flowing above. In this centrally located ecosystem component, the actions of the microbial biofilms coating the sediments are considered to act like a biological filter, enhancing water quality during the exchange process. However, the hyporheic zones in many rivers are threatened by siltation, toxicants and increasing acidity, physical extraction for gravel, or altered groundwater inputs through pumping. Current protocols to assess river health do not explicitly consider that of the hyporheic zone, despite its central role in many rivers. This oversight is due largely to the relative infancy of the discipline of hyporheic research, limited communication to river managers by ecologists, and technical difficulties in sampling the hyporheic zone. Ecosystem health assessments in rivers with potentially important hyporheic zones could include measurements of the extent of hydrological exchange between the river and the hyporheic zone at a range of scales, the rates of some key interstitial chemical processes, and perhaps the biodiversity of hyporheic invertebrates. In Australia, the potential of some of these measurements is being investigated, especially in relation to assessing the health of "groundwater-dependent ecosystems."