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Ecoregions and benthic diatom assemblages in Mid-Atlantic Highlands streams, USA

Ecoregional differences in geology and hydrology may affect physical and chemical conditions in streams and, consequently, the species composition of algal assemblages. Stresses resulting from human disturbance, however, may constrain species membership in algal assemblages and reduce regional diversity. We expected that ecoregional differences in diatom assemblages, if they were present, would be more evident in relatively undisturbed sites than in randomly selected sites. Benthic diatom and water chemistry samples were collected from streams in 7 ecoregions of the Mid-Atlantic Highlands to evaluate correspondence between ecoregional classification and diatom assemblages. Ecoregional differences were assessed using 196 randomly selected stream sites (probability sites) and 60 sites with less disturbance by humans (reference sites). Multivariate analyses showed that significant ecoregional differences in diatom assemblages were observed only in probability sites and not in reference sites. Water chemistry was significantly different among ecoregions, both for probability sites and for reference sites. Significant differences in diatom assemblages and water chemistry were, however, evident only among ecoregions grouped by topography (i.e., montane, high plateau, and low plateau/valley). Ecoregional differences between montane regions or low plateau/valley regions were subtle. Stream sites grouped by catchments were also significantly different in water chemistry but not in diatom assemblages, both for probability sites and for reference sites. Our data suggest that diatom assemblages respond to land use, especially agricultural activities, and thus may correspond to the ecoregional classification when land use differs significantly among these ecoregions (e.g., montane vs valley ecoregions). Diatom assemblages that lack a region-specific feature may be ideal as unbiased indicators of stream water quality.