Sedimentation in dryland river waterholes: a threat to aquatic refugia?

In dryland river systems subject to prolonged low and no flow periods, waterholes, or sections of river channel that are deep relative to the rest of the channel and that retain water for longer periods of no flow, provide refugia for aquatic biota and hence are critical to the resilience of aquatic ecosystems. This study examined physical, chemical and bio-stratigraphy in refugial waterholes situated along four distributaries of the Lower Balonne River system in semi-arid Australia. In doing so we reconstructed environmental histories for the waterholes, calculated how sedimentation rates have changed in response to land use change over the past two centuries, and assessed whether they are threatened by increased sedimentation through potential effects on waterhole depth and hence persistence times and habitat quality. Our study found that sedimentation rates have increased substantially since European settlement, most likely in response to removal of groundcover by grazers. The increased sediment accumulation rates are estimated to have reduced persistence times during low and no flow periods of the waterholes by 2-4 months. Despite evidence from other similar systems in Australia that increased influx of sediment coincided with loss of submerged macrophytes, stratigraphic records of preserved pollen and diatoms did not provide consistent evidence of biotic or habitat quality changes within the waterholes associated with European settlement.