Genus-level, trait-based multimetric diatom indices for assessing the ecological condition of river and stream across the conterminous United States.
Taxonomic inconsistency in species-level identifications has constrained use of diatoms as biological indicators in aquatic assessments. This study addressed this problem by developing diatom multimetric indices (MMIs) of ecological condition using genus-level taxonomy and trait-based autecological information. The MMIs were designed to assess river and stream chemical, physical and biological condition in three large ecoregions across the conterminous United States. Trait-based approaches have the advantage of using genus-level data, which require less effort and expense to acquire than traditional species-based approaches and eliminating the persistent taxonomic bias introduced over vast geographic scales. For large-scale assessment programs that require multiple taxonomic laboratories to process samples, such as the United States Environmental Protection Agency’s (U.S. EPA’s) National Rivers and Streams Assessment (NRSA), the trait approach can eliminate discrepancies in species-level identification or nomenclature that can render diatom data difficult to interpret. We applied our trait-based MMI to NRSA data collected within three large ecoregions within the U.S. - the East, Plains, and West. The MMI for the East had the greatest ability to discriminate least-disturbed from most-disturbed sites, followed by MMIs for the Plains and West, respectively. The performance of the MMIs was comparable to that observed in existing NRSA fish and macroinvertebrate MMIs. Our research shows that trait-based diatom indices constructed on genus-level taxonomy can be effective for large-scale assessments, and may also allow programs such as NRSA to assess trends in freshwater condition retrospectively, by revisiting older diatom datasets. Moreover, our genus-based approach may allow the inclusion of diatoms in state assessment programs with limited resources if data with genus-level taxonomic resolution can meet state decision-making needs.