Spatial patterns and ecological determinants of benthic algal assemblages in Mid-Atlantic streams, USA

We attempted to identify spatial patterns and determinants for benthic alga l assemblages in Mid-Atlantic streams, Periphyton, water chemistry, stream physical habitat, riparian conditions, and land cover/use in watersheds wer e characterized at 89 randomly selected stream sites in the Mid-Atlantic re gion. Cluster analysis (TWINSPAN) partitioned all sites into six groups on the basis of diatom species composition, Stepwise discriminant function ana lysis indicated that these diatom groups can be best separated by watershed land cover/use (percentage forest cover), water temperature, and riparian conditions (riparian agricultural activities). However, the diatom-based st ream classification did not correspond to Omernik's ecoregional classificat ion, Algal biomass measured as chi a can be related to nutrients in habitat s where other factors do not constrain accumulation. A regression tree mode l indicated that chi a concentrations in the Mid-Atlantic streams can be be st predicted by conductivity, stream slope, total phosphorus, total nitroge n, and riparian canopy coverage. Our data suggest that broad spatial patter ns of benthic diatom assemblages can be predicted both by coarse-scale fact ors, such as land cover/use in watersheds, and by site-specific factors, su ch as riparian conditions, However, algal biomass measured as chi a was les s predictable using a simple regression approach, The regression tree model was effective for showing that ecological determinants of chi a were hiera rchical in the Mid-Atlantic streams.