Rapid Biological Assessments: How Many Animals Are Enough?

Abstract

Rapid bioassessment data using counts of benthic macroinvertebrates from the littoral zones of 5 lakes in south-central Ontario were examined to determine if subsamples of 100 animals provided sufficient statistical power to distinguish these lakes. One-way analysis of variance and power analysis were completed using 17 biological indices based on counts of 100, 200, and 300 animals. Despite the common perception that more is better, the doubled or tripled effort required to sort, identify, and enumerate more animals resulted in very little improvement in our ability to distinguish lakes. Eight of the 17 indices were of limited value for separating the 5 lakes. High correlations, indicating redundancies among the best indices, suggested that 1 or 2 metrics would be sufficient to characterize the lakes. Three indices including % amphipods, % insects, and a multivariate metric representing the 1st axis from a correspondence analysis ordination were the best metrics for separating the 5 lakes. A variant of the commonly used EPT index, composed of the total number of individuals of Ephemeroptera, Plecoptera, and Trichoptera, was also useful for characterizing the lakes. From this comparative analysis, subsamples based on counts of 100 animals are sufficient to distinguish the littoral benthic communities of small inland lakes in south-central Ontario.