Evaluation of invertebrate biometrics and the influence of subsample size using data from some Westland, New Zealand, lowland streams

Abstract

The accuracy and variability of six invertebrate biometrics (taxa richness, number of Ephemeroptera, Plecoptera, and Trichoptera (EPT) taxa, % EPT abundance, % dominant taxon, the Macroinvertebrate Community Index (MCI), and Quantitative MCI (QMCI)) were examined using data from four Westland, New Zealand, streams. All metrics except QMCI differed significantly among sites. Taxa richness and number of EPT taxa distinguished the most impacted site from the remaining sites. Significant temporal differences among streams occurred for % EPT abundance and MCI, and there were interaction effects for these plus % dominant taxon and QMCI, indicating that any differences among sites varied through time. Within individual streams, MCI and QMCI had low (<5%) variation about the mean from sampling of multiple transects, and the variation for all metrics generally stabilised when three transects were included. Increasing subsample size (100, 200, and 300 individuals) resulted in significant increases in estimates of richness measures and a decrease in MCI values. Metric values obtained from random 100‐count subsamples with replacement had low variability (<20%). Overall, our results suggest that 100‐count subsamples collected from multiple transects within a site provide accurate assessments of most indices. However, richness measures, and biotic indices that are based on presence/absence (e.g., MCI), were more sensitive to low subsample size.