Water-quality variability in dryland riverine waterholes: a challenge for ecosystem assessment

Abstract

Waterholes in intermittent dryland rivers are important biotic refugia, but detailed water-quality descriptions of these habitats are rare. Their possible variability in water quality, due to variable flow regimes, may impede development of appropriate water-quality guidelines. We investigated this issue by describing spatial and temporal variation in water quality in waterholes in a tropical Australian dryland system. Water quality varied considerably among three rivers, between sites within rivers and within sites, reflecting lithology, hydrology, bathymetry, local catchment influences, stratification and within-site patchiness. Seasonal variation was due to differences in insolation, temperature, flow and depth. Diel variation was high for pH and dissolved oxygen (DO) at some sites. Variation among locations within waterholes was low for DO, temperature and pH, but higher for chlorophyll and nutrients, due to shade-related patchy photosynthesis. Neither the rivers nor the waterholes within them could indicate water-quality reference conditions because of their different characteristics. Our results highlight the need for strategic timing and location of sampling to capture spatial and temporal variation in key water-quality variables and to identify critical ecological conditions in dryland riverine waterholes.