Groundwater Ostracods from the arid Pilbara region of northwestern Australia: distribution and water chemistry

Abstract

An attempt has been made at a comprehensive study of the diversity and distribution of subterranean ostracods in the Pilbara region, northwestern Australia. The area is a “hot spot” for subterranean biodiversity, some of which is currently under threat from extensive mining operations. Both bore and well sites were targeted, totalling 445 sites, to obtain a thorough coverage of the 200,000 km2. In addition, physical and hydrochemical measurements were obtained for all of the samples (temperature, conductivity, dissolved oxygen, pH, Eh, turbidity, nutrients, major ions). Ostracods were retrieved from approximately 47% of the samples and 56% of the sites. Twenty-one genera and around 110 species of ostracods have been identified. Of these, 72 are new species and a further 10 are currently in open nomenclature, due to the lack of suitable material for formal taxonomic description. The Candoninae are particularly well represented with 12 genera; some, such as Areacandona and Deminutiocandona, with 25 and 10 species respectively. Most sites (80%) were dominated by only one or two species, with up to six species at some sites. Population density varied from 1–370 individuals/sample. The most abundant and diverse sites occur in fresh, bicarbonate-rich aquifers utilised for water extraction, such as Pannawonica (Robe River), Cane River and Millstream. There is a clear distinction between taxa at the genus level from coastal and low-lying alluvial sites, and upland sites (>300 m altitude). Beyond this, the majority of species are confined within a surface water catchment, or in many cases, a specific aquifer. There are, however, some morphological similarities of the carapaces between different species within similar hydrogeologic settings. Ornate and ridged-valved species are common in the Mg-HCO3 waters of the Newman and Marillana Creek areas, whereas smooth-shelled, tapered forms are prevalent in alluvial aquifers. The more saline, Na-Cl rich aquifers at the edge of Great Sandy Desert have a particularly distinctive fauna, including one almost triangular species. The distribution of the stygobitic ostracod species in relation to the hydrogeology and water chemistry is discussed.