Influence of acidification and aluminum on the density and biomass of lotic benthic invertebrates

Experiments were carried out in plasticized wooden channels fed by a small creek in the Reserve des Laurentides, 80 km north of Quebec city. Channels were naturally colonized by invertebrates for 65 d before treatment. Treated channels were acidified in August with dilute H₂SO₄ only, or with acid plus a solution of Al sulfate (final concentration of 0.19 mg L^?1). The control channel received creek water only (pH 6.3 to 6.9). The addition of Al had no effect on invertebrate density and biomass. After 73 d of acidification, invertebrate densities were only one third the number found in the control channel. Invertebrate hiomass was not different within channels, although biomass was generally higher in the two acidified channels. Difference in densities between acidified and non-acidified channels was attributed to lack of colonization and not to an increase in drift. Microtendipes, a large and resistant larva of Chironomidae constituted a large fraction of the hiomass, largely outweighing numerous very small larvae. Effects on the density were attributed to the direct effect of low pH and not to indirect action through food limitations.