Aspects of the hyporheic zone below the terminus of a South Australian arid-zone stream

Abstract

Diel variation in physicochemistry and assemblage composition of the hyporheos, below and beyond the terminus of an arid-zone intermittent stream, was studied in Brachina Creek, Flinders Ranges, South Australia. Surface water downwelled at the terminus, and physicochemical conditions in the hyporheic zone at 30 cm resembled those in the stream above. With increasing distance beyond the terminus and presumably greater hyporheic residence time, diel fluctuations in water temperature and dissolved oxygen became dampened, and were absent 80 m beyond the terminus. At least 31 taxa were collected, 19 of which were surface species temporarily occupying the hyporheic zone. These epigean taxa commonly occurred as tiny instars in the hyporheos below the stream in regions of strong downwelling. Further downstream, the hyporheos became dominated by phreatic species (e.g. blind amphipods, syncarids) that are obligate subsurface residents. At the most downstream site, where dissolved oxygen concentrations were lowest and water temperatures highest, species richness was least, although biomass (mainly amphipods) was greatest. There was no diel variation in assemblage composition, species richness, total abundance and biomass, or the abundance of common species at any of the sites below the stream. The longitudinal pattern in the spatial distribution of the hyporheos appeared to be related to stream hydrology (downwelling), bed permeability, and the hyporheic residence time of the water.