Effects of Acidification on Biological Communities in Aquatic Ecosystems

Abstract

Extensive information is available on the effects of acidification on biological communities in aquatic ecosystems. Whole-system experiments, meso-cosm experiments, and field surveys have demonstrated major shifts in species composition and decreases in species richness with increasing acidity. Sensitive species may be lost even at moderate levels of acidity. For example, effects on important zooplankton predators, such as Mysis relicta and Epischura lacustris, occur at pH 5.6 to 5.9; acid sensitive mayfly and stonefly species (e.g., Baetis lapponicus) are affected at levels near 6.0; and sensitive fish species, such as the fathead minnow (Pime-phales promelas), experience recruitment failure and extinction at pH 5.6 to 5.9. At the same time, however, acid tolerant species may appear or increase in abundance, resulting in little or no overall decrease in standing crop. Attached filamentous algae, especially Mougeotia spp., often become visibly more abundant in acidified lakes and streams. Many ecosystem level processes (primary productivity, rates of decomposition, and nutrient concentrations) also appear relatively robust and unaffected at least at moderate levels of acidification (pH 5.0 to 5.6). Specific mechanisms for effects of acidification on aquatic biota have been studied particularly for fish communities. The toxicity of acidic waters to fish is determined primarily by pH, calcium, and inorganic monomeric aluminum. Fish population declines associated with acidification may result most commonly from recruitment failure. Relatively few quantitative models are available for predicting biological responses to acidification; most deal with effects on fish populations. All these models require further development and testing prior to their use for regional assessments of effects from acidic deposition.¹