Abstract
We examined the structure of periphyton and macroinvertebrate assemblages in 18 Oregon Coast Range perennial headwater streams bordered by mature (6), clearcut (5), and thinned (7) forest treatments. Summer base flow discharge ranged from 0.02 to1.98 l/s (mean − 0.5 l/s). Periphyton was almost entirely diatoms (72 species) and assemblage characteristics were most strongly influenced by substrate characteristics. Mature treatment sites had fewer species of diatoms and less biomass than other treatments. Diatom richness was highest at sites with higher unit area discharge. Diatom assemblages were dominated by a single species (Achnanthes lanceolata [Brébisson] Grunow), which composed more than 50% relative abundance of the assemblage at nine sites. Macroinvertebrate assemblages were rich, with 194 taxa collected across all sites and 42 taxa found at a single site. Macroinvertebrate assemblages differed across treatments with higher abundance, more Chironomidae taxa, and more biomass at the clearcut treatment sites. We observed no difference in functional feeding group percentage composition across treatments, with collector-gatherers and shredders composing at least 50% abundance at all treatments. We found little difference in either periphyton or macroinvertebrate assemblages between thinned and mature treatments.