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

Ecoregional differences in geology and hydrology may affect physical and ch emical 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 diver sity. We expected that ecoregional differences in diatom assemblages, if th ey were present, would be more evident in relatively undisturbed sites than in randomly selected sites. Benthic diatom and water chemistry samples wer e collected from streams in 7 ecoregions of the Mid-Atlantic Highlands to e valuate correspondence between ecoregional classification and diatom assemb lages. Ecoregional differences were assessed using 196 randomly selected st ream sites (probability sites) and 60 sites with less disturbance by humans (reference sites). Multivariate analyses showed that significant ecoregion al differences in diatom assemblages were observed only in probability site s and not in reference sites. Water chemistry was significantly different a mong ecoregions, both for probability sites and for reference sites. Signif icant differences in diatom assemblages and water chemistry were, however, evident only among ecoregions grouped by topography (i.e., montane, high pl ateau, and low plateau/valley). Ecoregional differences between montane reg ions or low plateau/valley regions were subtle. Stream sites grouped by cat chments were also significantly different in water chemistry but not in dia tom assemblages, both for probability sites and for reference sites. Our da ta suggest that diatom assemblages respond to land use, especially agricult ural 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.