Evaluating subsampling approaches and macroinvertebrate taxonomic resolution for wetland bioassessment

Abstract. Methods for wetland bioassessment using macroinvertebrates are not well developed. Two of the most controversial issues in stream bioassessment, subsampling and taxonomic resolution, have yet to be quantitatively addressed for wetlands. Using a multivariate approach, we evaluated the efficacy of family-, genus-, and species-level assemblage data in reflecting the environment and distinguishing impaired sites from the reference condition. We used 5 basic levels of subsampling (100-, 200-, and 300-organism fixed counts; 10 % and 25 % fixed areas), an integrated subsample requiring a minimum fixed count and fixed area (100&10%), and 100-count and 10%-area subsamples coupled with a supplementary large-rare (LR) search. Data were obtained from 1.5-m2 composite samples collected from 126 plots along a 10-km-long eutrophication gradient in the Florida Ever-glades. Our results suggest that effectiveness of subsampling depended more upon the minimum number of individuals retained than minimum area or proportion of the sample picked. Fixed-area subsamples were generally less efficient than fixed counts, with 200- and 300-individual fixed counts resulting in significantly greater assemblage–environment relationships and much higher accuracy in detecting impairment than 10 % fixed area, despite averaging similar numbers of individuals. The greatest improvement with increasing subsample size was observed between fixed counts of 100 an