BenthicdiatomsinUSArivers-ANSPPhycologySection.PDFVIP

Journal of Biogeography, 29, 167–187 Benthic diatoms in USA rivers: distributions along spatial and environmental gradients Marina G Potapova* and Donald F Charles Patrick Center for Environmental Research, The Academy of Natural Sciences, Benjamin Franklin Parkway, 1900 Philadelphia, PA, 19103 USA Abstract Aim To study large-scale patterns of benthic diatom assemblages in rivers, to assess the relative importance of environmental and geographical factors affecting their compo- sition, and to evaluate the implications of these patterns for the use of diatoms as indicators of water quality, particularly nutrient enrichment. Location The United States Geological Survey (USGS) National Water Quality Assessment (NAWQA) Programme data set covers the conterminous United States. Methods We employed gradient analysis to understand the floristic structure of the data set and to discover major ecological gradients underlying variation in species composition at different spatial scales (entire US, Omernik Level 1 and Level 2 ecoregions). We used variance partitioning to separate the effect of environmental and spatial characteristics. Results At the national scale, three major ecological gradients were evident. The first was a complex ‘downstream