Different depths, different fauna: habitat influences on the distribution of groundwater invertebrates

At regional and catchment scales, geology and hydrogeology strongly influence the distribution of groundwater invertebrates (stygofauna), but the fine scale distribution of stygofauna in sedimentary aquifers remains poorly studied. In this study, we examine the small-scale distribution of stygofauna in sediments of a perched aquifer in an upland swamp in south eastern Australia. We installed a series of piezometers which accessed either the full sediment profile or one of four discrete sedimentary layers in the swamp. Piezometers were sampled for stygofauna and 2H and 18O isotopes in the groundwater. The swamp contained a taxonomically diverse and abundant stygofauna, which was distributed throughout the swamp and similar in composition to that of other aquifers in the region. There were strong temporal changes in the faunal assemblages but the stimuli for these changes remain unknown. Isotope analysis indicated that the swamp water was well mixed despite localised inputs of groundwater from springs. Accordingly, we could not explore the relative influence of groundwater inputs on fauna; however, we have shown clearly that stygofauna were strongly influenced by sediment properties, with the abundance of stygofauna in the dense, fine sandy sediments being significantly lower than in the coarser sedimentary layers above and below.