Km. Somers et al., RAPID BIOLOGICAL ASSESSMENTS - HOW MANY ANIMALS ARE ENOUGH, Journal of the North American Benthological Society, 17(3), 1998, pp. 348-358
Rapid bioassessment data using counts of benthic macroinvertebrates fr
om the littoral zones of 5 lakes in south-central Ontario were examine
d to determine if subsamples of 100 animals provided sufficient statis
tical power to distinguish these lakes. One-way analysis of variance a
nd power analysis were completed using 17 biological indices based on
counts of 100, 200, and 300 animals. Despite the common perception tha
t more is better, the doubled or tripled effort required to sort, iden
tify, and enumerate more animals resulted in very little improvement i
n our ability to distinguish lakes. Eight of the 17 indices were of li
mited value for separating the 5 lakes. High correlations, indicating
redundancies among the best indices, suggested that 1 or 2 metrics wou
ld be sufficient to characterize the lakes. Three indices including %
amphipods, % insects, and a multivariate metric representing the Ist a
xis from a correspondence analysis ordination were the best metrics fo
r separating the 5 lakes. A variant of the commonly used EPT index, co
mposed 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 a
re sufficient to distinguish the littoral benthic communities of small
inland lakes in south-central Ontario.