Studies on the spatial heterogeneity of shallow water benthos with particular reference to the Chironomidae

Abstract

The examination of numbers of 6-in. Ekman grab samples from areas of 2 m2 or less in a variety of shallow lentic habitats revealed heterogeneity in the spatial distribution of most species of macrobenthos in each habitat. Chironomus abortivus Malloch, Glyptotendipes barbipes (Staeger), Procladius freemani Sublette, and the Oligochaeta were invariably aggregated when their population densities were adequate to determine a distributional pattern. The Chironomidae, considered as a group, also displayed an aggregated distribution in all habitats. Other benthic species had distributional patterns that varied from random to highly aggregated in the different habitats. No evidence was found that any species are normally randomly distributed. Analyses of mean crowding of the common species in each habitat suggest that the species often respond independently to microenvironmental variations. The factors which produce well-defined aggregations of one species may be neutral, or they may have the opposite effect on the distribution of other species. Experimental examination of the possible occurrence of behavioral mechanisms of aggregation in the two chironomids, Chironomus attenuatus Walker and Glyptotendipes barbipes, revealed that they lend to aggregate at low population densities (≈ 1/10 cm2) but are randomly distributed in a uniform environment when present in higher densities. Because of the aggregated nature of most benthic species four 6-in. Ekman grab samples were usually required to obtain a population estimate of the total fauna, total chironomid fauna, and the dominant species within ± 30% of the mean obtained with a larger number of samples.