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Fixed- count and proportional benthic invertebrate subsampling methods: A comparison of efficacy and cost

Project Abstract
By Tom F. Cuffney, Stephen R. Moulton, II, James L. Carter, and Terry M. Short

Abstract

The efficacy and cost of fixed-count (100-600 organisms) and proportional (1-25% of volume) subsampling methods were compared using computer simulations. Data from over 600 invertebrate samples collected by the USGS's National Water-Quality Assessment Program were used to define communities from which fixed-count and proportional subsamples were drawn. Subsamples were then compared to the original sample data to assess the efficacy of the method. Costs were assumed to be directly proportional to the number of organisms processed. The accuracy of fixed-count subsampling tended to decrease as the number of organisms and taxonomic diversity of the sample increased. Proportional subsampling provided a more accurate and more consistent representation of the underlying community over a wide range of sample richnesses and abundances. However, this increase in accuracy was accompanied by widely variable (800 fold) processing costs. Average processing costs were reduced only when the proportion of the sample processed was low. Under these conditions, the proportional method provided results that were not significantly different from the fixed-count method. Therefore, the fixed-count subsampling method is probably the more cost effective processing strategy for most monitoring programs.