Effects of an Algivorous Minnow on Temperate Stream Ecosystem Properties

Abstract

We used a field experiment to test for effects of the algae—grazing minnow Campostoma anomalum on stream ecosystem dynamics. Eight whole pools in Brier Creek, in south—central Oklahoma (USA), served as replicates. Following a stream—resetting flood, four pools with natural densities of Campostoma and four pools with no Campostoma were maintained by fences across riffles for 61 d. Subsequently, a changeover was conducted. Fish were removed from two of the previously grazed pools and added to two of the previously nongrazed pools; the other four pools remained as grazed and nongrazed controls. The changeover was maintained for 33 d. A repeated—measures analysis of variance showed significant grazing or grazing ° time effects for variables related to organic matter processing, algal primary productivity and community composition, and invertebrate density. Principal components analysis showed complete separation of grazing treatments on the combined first two axes (accounting for 34.6% of the variance). Correlations of variables with these axes corresponded to results of the univariate ANOVAs; pools with Campostoma had lower algal standing crops, higher biomass—specific net primary productivity, greater percent blue—green algae, lower areal coverage by filamentous green algae, lower bacterial density, smaller benthic particulate organic matter fractions, and higher invertebrate density than pools without Campostoma. The changeover indicated that effects of grazing by these fish can be rapid when they enter nongrazed pools, but that effects can linger in formerly grazed pools despite markedly reduced densities of these algivorous fish.