Sample number and colonization patterns of benthic macroinvertebrates and organic material on artificial and natural substrata

We determined differences in colonization by benthic macroinvertebrates among two types of artificial substratum (single rocks and bricks) and natural substratum after it was physically disturbed. We also determined the number of samples required for the same level of precision using the artificial substrata and direct sampling of the natural substratum. The study was conducted in a slow moving riffle-run in the Battle River in Alberta, Canada. All measures of the macroinvertebrate fauna (densities of each taxon, total number of organisms, and total number of taxa) and quantities of organic material were greater on the natural substratum than on artificial substrata. Physical disturbance of the natural substratum substantially reduced the macroinvertebrate community and quantity of organic material, and changed the composition and size of the natural substratum. But this disturbance to the substratum did not affect the colonization patterns of the macroinvertebrate fauna and organic material on the substratum types. Artificial substrata, similar those used in our study, are unlikely to give representative samples of macroinvertebrate fauna, especially for densities of zoobenthos taxa, or quantities of organic material on the natural substratum. The artificial substrata, in contrast to the natural substratum, required a larger number of samples for the same level of precision. Using the artificial substrata instead of direct sampling of the natural substratum therefore, does not appear to increase the precision of population estimates of benthic macroinvertebrates or quantities of organic material. Our results suggest that benthic macroinvertebrate communities and organic material in lotic systems might be sampled more efficiently using direct sampling of the natural substratum than the use of artificial substrata.