Do Organic and Anthropogenic Acidity Have Similar Effects on Aquatic Fauna?

Abstract

In Westland, New Zealand, there are many brownwater steams with naturally low pH (often around 4) brought about by high concentrations of organic acids. Up to 90% of dissolved aluminium in these humic streams is bound to organic matter and is therefore non-toxic. This condition enabled us to investigate the effects of long-term, natural acidity on aquatic biota without the compounding effects of high concentrations of toxic aluminium that are found in many anthropogenically-acidified, clearwater streams of the Northern Hemisphere. Most fish are absent from Northern Hemisphere clearwater habitats with pH < 5. However, in Westland 9 out of 14 native fish species were found in brown waters with pH below 5, and 7 species were taken from waters with pH < 4.5. Furthermore, 34 of the 37 most widespread aquatic insect taxa were recorded in Westland streams with pH < 5, and 24 were taken from sites with pH < 4.5. Clearly, many members of the freshwater fauna in Westland are well-adapted for life in waters of low pH. Physiological adaptations enabling this tolerance may be the same as those that evolved in response to the physicochemical variability associated with the unpredictable flow regimes of Westland streams. Tolerances of low pH have been documented in aquatic fauna from some naturally acidic, humic waters of the Northern Hemisphere, although degree of tolerance can vary within as well as between species. Colonisation of recently-acidified waters by acid-tolerant strains may cause more subtle changes in community structure than can be detected by conventional ecological techniques.