Groundwater calcrete aquifers in the Australian arid zone: the context to an unfolding plethora of stygal biodiversity

Abstract

-The Western Shield of Australia has been above sea level since the Palaeozoic. It is incised by ancient palaeovalieys now containing groundwater calcretes that are found throughout much of arid Australia. The calcretes are deposited, owing to surface evaporation, from a groundwater flow path that increases in salinity from fresh to hypersaline groundwater each terminating at a salt lake (playa). The calcrete aquifers contain a diverse obligate groundwater fauna (stygofauna: predominantly Crustacea) in one of the oldest non-marine landscapes on Earth. Each of the 11 calcrete areas examined so far contains a unique fauna; there are over 210 large discrete calcrete bodies in the Western Australian arid zone. This paper examines a working hypothesis that the stygofauna became isolated in the upper tributaries of the palaeodrainage systems by the progressive upstream movement of salinity from the Eocene onwards. Although each calcrete body does have a unique fauna, as predicted by the hypothesis, the hypothesis has proved too simple because the hydrochemical cycles are repeated along the length of the palaeochannels, each terminating in a saltlake immediately downflow of the calcrete. A new hypothesis has to incorporate these intervals of hypersaline groundwater / calcrete discontinuities acting as barriers to dispersal of the stygofauna. The paper focuses on the geological and landscape setting, and the hydrochemical and groundwater context in which this stygal diversity is found. It is intended to serve as a background resource for further work on this diverse stygofauna. A synopsis is provided of the knowledge of this stygofauna. The presence of such highly diverse and locally endemic stygofauna, often ancient relictual lineages, in a major economic resource within arid Australia poses delicate and chalienging conservation issues. At the same time the scientific challenge is considerable to understand the development and functioning of these systems in order that they can be managed sustainably.

VIEWPOINT. Is the Australian subterranean fauna uniquely diverse?

“…2), largely due to the sustained research efforts of several groups over the last decade, in addition to the numerous EIAs fuelled by Australia's mineral exploration boom ). Geologically, the Pilbara and Yilgarn cratons of WA comprise the Western Shield, an area that has been continually emergent since the Proterozoic (Humphreys 1999(Humphreys , 2001 (Fig. 1).…”

Biodiversity, functional roles and ecosystem services of groundwater invertebrates

“…These groundwater metazoans (stygofauna) include many unique representatives of lineages from various geological periods and can exhibit extraordinary endemicity (Poore and Humphreys 1992;Harvey 2002;Humphreys 2006Humphreys , 2008. Globally, stygofauna comprises a significant component of total biodiversity (Rouch and Danielopol 1997;Sket 1999) and there are several 'hot-spots' of high subterranean biodiversity (Culver and Sket 2000;Danielopol and Pospisil 2001;Castellarini et al 2007), including parts of Australia (Bradbury and Williams 1997;Humphreys 2001;Karanovic and Marmonier 2003;Karanovic 2007). The arid zone in Western Australia is yielding especially interesting and diverse groundwater faunas (Humphreys 2001;Cooper et al 2002;Leys et al 2003) yet these regions are also where human pressures on groundwater are heaviest (Humphreys 2000;Hancock et al 2005).…”

Evidence for population fragmentation within a subterranean aquatic habitat in the Western Australian desert

“…This process of speciation by transition from surface to subterranean environments is suggested as the main mechanism for the evolution of subterranean species, although there is some debate about whether it requires extinction of the surface ancestral species (climate relict hypothesis; for example Peck and Finston (1993)) or can proceed in the presence of gene flow between the surface and subterranean populations (adaptive shift hypothesis; for example Desutter-Grandcolas and Grandcolas (1996)). However, given the extraordinary number of species observed in cave systems (Poulson and Culver, 1969;Humphreys, 2000Humphreys, , 2008Buhay et al, 2007), this transition, from surface to subterranean environments, is unlikely to be the sole source of species diversification in cave animals and it is possible that some species evolved from troglomorphic ancestors fully adapted to life underground (Juan and Emerson, 2010;Ribera et al, 2010). The difficult nature of gaining access to cave habitats makes this latter hypothesis difficult to investigate; however, a number of key studies have examined these questions (Caccone, 1985;Buhay and Crandall, 2005;Guzik et al, 2009).…”