Colonization of artificial substrata by macro‐invertebrates in a stream and variations according to stone size

Abstract

SUMMARY. Plastic cages containing artificial substrata were placed on the stony bottom of a stream in a section where the environmental conditions appeared to be homogeneous. The section was divided into eight equal sub‐sections (A‐H) and four cages were placed in each sub‐section. The stone‐size of the artificial substrata was similar within each cage but differed between the four cages with average diameters of 14, 24, 48 and 96 mm, respectively. The baskets were removed one month later and the catches of macro‐invertebrates were recorded.Analysis of the catches (density and number of taxa in each cage) revealed no significant differences in connection with the position of the cages in the section of stream. Cages with 48‐mm stones contained the least abundant fauna. The taxa which colonized cages with 14‐ or 24‐mrn stones were more numerous than those collected from cages with 48‐ or 96‐mm stones.Statistical analysis by laxon confirmed the preceding results. Variability in connection with the position of the cages along the section of stream was never significant. A little more than half the taxa were distributed independently of stone size but always showed a contagious type distribution. The density of the other taxa was higher, either in cages with 14‐ or 24‐mm stones (particularly Gammarus) or only in cages with 96‐mm stones (especially Hydropsyche). Only one taxon was more abundant in cages with 48‐mm stones, the type of substratum in which the overall density was lowest.Catches in the cages were not the same as those taken with a Surber sampler because the two samplers did not take samples from the same habitats and also because the baskets offered a more specialized habitat than the surrounding bottom. The basket with 24‐mm stones gave the least distorted picture of the natural population.