Effects of Periodically Disturbing a Small Area of Substratum in a Brown-Water Stream of Alberta, Canada

A small substratum area of a brown-water stream was thoroughly disrupted at about 15-day intervals for 2.5 months. The disruptions resulted in a more homogeneous substratum, with relatively more gravel and less fine sand in the experimental area. Twenty of the 24 taxa had a smaller mean density in the experimental area by the end of the study. Of these taxa, 11 exhibited significantly (P < 0.05) smaller numbers in the experimental area. Nevertheless, considering how frequent and thorough the disturbances were, the overall effect could not be described as severe. Due to life cycle features of various species, many taxa increased in absolute abundances throughout the study in both the experimental and control areas. Unavoidable disturbances of substrata must be common in most streams where the fauna is studied intensively over long periods. Results of this study would indicate that routine sampling of the benthos in the Bigoray River would not have a pronounced effect, probably not even a discernible effect, on the structure of the stream community. Data of the Bigoray River study, and most other studies pertaining to recolonization, deal only with structure of the stream's community. The disturbances undoubtedly had a much greater impact on the underlying functional organization of the community.