Decreases in pH and ANC are often paral- leled by changes in element concentrations including increases in Al +3 concentrations and decreases in Ca +2 . High dissolved Al +3 concen- trations can have toxic effects on many types of aquatic biota, and at extreme levels few aquatic species can survive (Table 1). Organic forms of Al +3 are much less toxic than inorganic forms. Emerging research suggests that Ca +2 concentra- tions in streamwater are also an important bio- logical indicator. Acidifying deposition has accelerated the leaching of Ca +2 from soils to surface waters gradually decreasing the avail- able pool of Ca +2 in soils and lowering Ca +2 concentrations in runoff. This soil depletion together with decreases in leaching associated with declines in acidifying deposition is con- tributing to.
