Imperilled subsurface waters in Australia: Biodiversity, threatening processes and conservation

Abstract

Subsurface waters in Australia span massive aquifers to small cave streams and fluctuating hyporheic zones where stream water exchanges with groundwater. Groundwater resources have been exploited heavily, especially in the arid zone, and usage is predicted to increase. Ironically, preliminary surveys of some groundwater habitats in arid northwestern Australia indicate an extraordinarily diverse subsurface fauna with apparently highly localised distributions. Elsewhere in Australia, changes to river flows, gravel extraction, and poorly managed catchment land uses have altered the extent and ecological integrity of the hyporheic zone in most gravel and sand bed rivers. In many cave streams and karst aquifers, sedimentation, pollution and changes to the water table have caused extinction or reduction of the dependent biodiversity. Most of these subsurface habitats harbour ancient groups absent from surface waters (relictual stygofauna) and are ‘hotspots’ of unexpected aquatic biodiversity. Unfortunately, our knowledge of the regional extent of this biodiversity and its functional significance is fragmentary. Current threats vary according to the subsurface habitat. For example, lowering of the water table in calcrete aquifers by water abstraction may jeopardise isolated, endemic relictual faunas in Australia's arid zone whereas in many gravel bed rivers, siltation threatens the biodiversity and filtration capacity of the hyporheic zone. Groundwaters in karst, especially cave streams and their dependent fauna, are vulnerable to impacts in their surface catchments but these linkages are seldom obvious. Recognition of the intimate linkages between groundwater and many surface ecosystems has led recently to policies aimed at protecting ‘Groundwater Dependent Ecosystems’ in Australia. However, such protection is hampered by our scant taxonomic and ecological knowledge of these ecosystems. Successful conservation and management of groundwaters and their dependent ecosystems rely on better public understanding of their unique fauna and ‘ecosystem services,’ further research on subsurface processes and taxonomy, and legislative protection of rare and threatened subterranean communities and species.