Publication Summary

Biological assessment of benthic macroinvertebrate communities was completed at forested stream reference sites in three ecoregions of Washington State: Puget Lowlands, Columbia basin, and Cascades. Characteristic chemical and biological patterns were explored through reference sites within each ecoregion. Physical characteristics of the reference sites within an ecoregion were reflective of mid-order stream types and conformed, as closely as possible, to the predefined site selection criteria. (See link to appendices below.)

For a printed copy of this publication, call the DNR Library at 360-902-1419. Ask for TFW Publication No. TFW-WQ11-92-001.

Biological assessment of benthic macroinvertebrate communities was completed at forested stream reference sites in three ecoregions of Washington State: Puget Lowlands, Columbia basin, and Cascades. Characteristic chemical and biological patterns were explored through reference sites within each ecoregion. Physical characteristics of the reference sites within an ecoregion were reflective of mid-order stream types and conformed, as closely as possible, to the predefined site selection criteria.

Habitat and biological conditions in each ecoregion were determined by using a modified version of the Environmental Protection Agency's Rapid Bioassessment Protocols (RBP). Habitat condition determined through the qualitative RBP scoring system indicated specific seasons that habitat availability to benthic macroinvertebrate communities was reduced due to changing wetted stream bottom surface areas. Each region had characteristic natural disturbances that determined timing of habitat instability.

Benthic macroinvertebrate communities and surface water conditions were examined for uniqueness by ecoregions and change by calendar seasons. The benthic macroinvertebrate information was initially examined by detrended correspondence analysis (DCA), and best distinctions among ecoregions occurred during the fall, spring, and summer seasons. Two-way indicator species analysis (TWINSPAN) produced lists of genera that were considered unique to each ecoregion. The functional attributes of these "unique assemblages" were used to relate water quality and physical habitat influences that were thought to shape community patterns. Seasonal taxonomic lists were also constructed for each ecoregion that included macroinvertebrates assumed to appear in streams similar to those used in this project.

Seven RBP biometrics were used to define ecoregion macroinvertebrate conditions. Each of the biometrics was examined individually during each calendar season. Three of the metrics commonly used by benthologists were problematic. The "shredders/total abundance of sample organisms" ratio had consistently low values in each ecoregion during the fall and winter. The "EPT/Chironomidae abundance" ratio was not useful for Cascades ecoregion reference streams because of highly variable results. The "scrapers/collector-filterer abundance" ratio was least useful during winter 1991 in this ecoregion, also.

Surface water information was examined through use of principal components analysis to define parameter relationships among the three ecoregions. Many of the parameters measured in this project revealed close associations between the Columbia basin and Puget Lowland reference sites. The Cascade streams maintained distinct surface water conditions from the other two regions, probably due to increased streamflows and higher gradients. Biological, chemical, and physical instream information surveyed in this project contrasted the mountain ecoregion streams with the valley/plains ecoregion streams.