Effects of algivorous minnows (Campostoma) on spatial and temporal heterogeneity of stream periphyton.

The effect of grazing minnows (Campostoma) on spatial and temporal heterogeneity of the vertical height of attached filamentous algae (Spirogyra and Rhizoclonium) was measured in natural and artificial streams. Measurements were made at 1-m intervals across natural-stream pools during April-May, and at 0.3-m intervals longitudinally in smaller artificial streams during February-May. Spatial heterogeneity was calculated weekly, as the mean standardized difference in algal height (spatial MDH) between adjacent fixed points. Temporal MDH was calculated as the mean standardized difference in algal height at fixed points between adjacent weeks. Reduction in spatial and temporal MDH, detected only in artificial streams, suggested that grazing by Campostoma promoted and maintained more uniform algal height in contrast to ungrazed algae. Heterogeneity of algal export was greater for ungrazed algae, and decreased over time after sloughing, but that for grazed algae was lower and increased after sloughing. The contrasting experimental systems suggested that Campostoma can reduce spatial and temporal heterogeneity of algae, but that additional variation in depth, substratum characteristics, or presence of other biota may modify effects in natural streams.