The influence of humic substances on the toxicity of low pH and acid mine drainage to freshwater organisms

"Contrasting effects with respect to toxicity, have been reported for freshwaters acidified via anthropogenic or natural means. Decreased diversity, species richness and ecosystem functioning is often reported from waterways acidified via anthropogenic means, whilst on the other hand, naturally acidic waters are reported to contain diverse and functioning ecosystems. Previously it has been suggested that dissolved organic carbon (DOC) containing high amounts of humic substances (HS) may play a role in buffering aquatic organisms in naturally acidic waters against the detrimental effects of low pH. This research used field studies to investigate the effect of decreased pH in naturally acidic waters. These were complemented by laboratory studies to investigate the ability of HS to influence the toxicity of low pH in waters of varying chemistry and whether this ability can extend to acidified streams affected by multiple stressors (low pH and heavy metal mixtures) such as those contaminated via acid mine drainage (AMD).