Twixt two worlds: taxonomic and functional biodiversity at the surface water/groundwater interface

WA Museum Records and Supplements | Updated 8 years ago

ABSTRACT – Between the familiar surface habitats of rivers and streams and the less well known groundwater environment lies an ecotone whose ill-defined boundaries oscillate in response to changes in surface discharge and groundwater pressure. This ecotone, termed the hyporheic zone, plays a crucial role as a dynamic chemical and physical filter between the adjacent ecosystems. It also potentially harbours a diverse fauna whose taxonomic composition in Australian waters is only now being explored. Given the dominance of this fauna by crustaceans in most parts of the world, coupled with Australia's rich hoard of syncarids and other Gondwanan relicts, it is likely that the biodiversity of the hyporheic zone of many of Australia's streams and rivers is considerable and exhibits high endemicity.

Functionally, invertebrate biodiversity in the hyporheic zone probably underpins the success and efficiency of these subsurface biological filters in gravel- and sand-bed rivers. Microbial biofilms coating the sediment particle surfaces are grazed by detritivorous hyporheic invertebrates. Hyporheic invertebrates, traditionally considered as just scientific oddities, may regulate the activity of these biofilms as they actively transform interstitial nutrients and alter flow paths and porosity. The impacts of river regulation and other human activities on the biodiversity of the hyporheic zone is unknown and little studied.

This paper reviews the published literature on the fauna of the hyporheic zone of Australia's rivers, seeking patterns in taxonomic representation in the scant data set. Drawing on overseas research, the functional significance of the biodiversity of the interstitial fauna is explored and related to a recent classification of groundwater and hyporheic invertebrates. Knowledge gaps in taxonomy of many groundwater groups and ignorance of the life histories of most hyporheic invertebrates confound present studies. There is a need for a national, standardised inventory of groundwater and hyporheic biodiversity to help focus future research efforts and use funding efficiently to expand our understanding of this little-known underworld.