Relative Importance of Seasonal, Short-Term pH Disturbances During Discharge Variation on a Stream Ecosystem

Abstract

Effects of short-term, experimental pH reduction on water chemistry and structure of benthic invertebrate communities were investigated in a south-central Ontario stream during spring and fall. With increased acidity, inorganic monomeric Al and Ca concentrations were significantly increased in stream water during spring and fall, whereas Fe, Mn, and Zn concentrations did not change. Total Al and DOC concentrations in stream water showed no constant pattern. Benthic density and generic richness during spring decreased in mineral, but not organic sediments. Total drift density did not change, but mayflies increased in the drift. In contrast, in the fall, benthic density and generic richness fluctuations were not correlated with pH disturbance, whereas drift density increased significantly during elevated acidity. Diversity and evenness of benthic invertebrates did not change above or below pH disturbance in either season. Non-metric multidimensional scaling analysis showed that discharge was the primary correlate of spring drift, whereas pH played a secondary role. In autumn, analyses demonstrated that Ca and DOC were predominant factors associated with increased drift but were related to hydrology. This study highlights the interrelationship between life history strategies of benthic invertebrates and the frequency and intensity of physical (discharge fluctuations) and chemical (pH depressions) disturbances.