Acidification effects on macroinvertebrates and fathead minnows ( Pimephales promelas) in outdoor experimental channels

Abstract

Three outdoor experimental channels at the U.S. EPA Monticello (Minnesota) Ecological Research Station were employed to assess in the field acidification effects on macroinvertebrate communities and populations of fathead minnows ( Pimephales promelas). One channel served as ambient (pH 8), the second was acidified with sulfuric acid to pH 6 and the third to pH 5 for 17 weeks. Acidification did not markedly increase toxic metal concentrations in the channels; however, there was some evidence of possible mercury mobilization. Benthic macroinvertebrate densities were lower in the acidified channels during most of the study and final diversity indices were 2.1, 1.7 and 1.2 in the ambient, pH 6 and pH 5 channels, respectively. Drift of amphipods and leeches was stimulated in the pH 5 channel early in the study. The number of insect taxa and percentages emerging from the ambient, pH 6 and pH 5 channels, respectively, were 30 and 56%, 28 and 31%, and 18 and 13%. Macroinvertebrate tolerance to the acidification was classified as follows: damselflies, isopods and leeches most tolerant; chironomids, some amphipods ( Crangonyx) and flatworms of intermediate tolerance; and other amphipods ( Hyalella azteca) and snails ( Physa gyrina) most sensitive. Fathead minnow spawning and embryo production were similar in the ambient and pH 6 channels with little spawning and no eyed-embryos found in the pH 5 channel. Newly-hatched larval fish did not survive to the juvenile stage in the pH 6 channel. Continued acidification would likely have further decreased the diversity of the macroinvertebrate community, increased relative abundance of more tolerant species, and caused extinction of the minnow population.