Quantitative sampling of benthic macroinvertebrate communities from 60 sites in New York City's drinking-water-supply watersheds was undertaken in 2000, 2001, and 2002 as part of a large-scale enhanced water-quality monitoring project (the Project). Sampling yielded 543 macroinvertebrate taxonomic units, most of which (including Chironomidae) were identified to the genus/species level. Our goals were to investigate the effect of level of taxonomic resolution on statistical macroinvertebrate–environment relationships, the effect of including rare taxa on among-site similarity and macroinvertebrate–environment relationships, and the correlations between the common and rare components of the total community at each site. Mean site richness ranged from 90.8 to 101.2 taxa for sites west of Hudson River (WOH) and 62.2 to 78.5 taxa for sites east of Hudson River (EOH). Species-level identifications provided the greatest separation of sites in multivariate space, but genus- and family-level identifications discriminated between most- and least-impacted sites, particularly in the EOH region where anthropogenic impact was greatest. Of the 543 taxa, 175 (32%) were found at ≤3 sites, and nearly ½ of the taxa within a given site were occasional (found once in 3 y). Numerically rare taxa (defined as either <1% or <0.3% relative abundance within each site) accounted for 42 to 75% (WOH) and 37 to 73% (EOH) of mean site richness. Ordinations of data sets including or excluding rare taxa revealed similar impact gradients, and the % of spatial variance explained by environmental factors was similar with and without rare taxa included. Common taxa contributed noise to site-similarity patterns in the WOH region, and rare taxa provided information that was redundant with information provided by common taxa in the EOH region.
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