COMMUNITY DYNAMICS AND METABOLISM OF BENTHIC ALGAE COLONIZING WOOD AND ROCK SUBSTRATA IN A FOREST STREAM

Colonization dynamics and metabolism of algal communities on wood (sma ll pieces of Douglas fir) and artificial rock (tiles) substrata were i nvestigated in an open reach in Mack Creek, Cascade Mountain Range, Or egon for 42 days. Chlorophyll a concentration was not significantly di fferent between the two algal communities (ANOVA, P = 0.119). Even tho ugh differences in cell densities were not significantly different bet ween the two algal communities (ANOVA, P = 0.063), biovolume and diver sity were greater in epixylic than in epilithic communities (ANOVA, P = 0.011 and 0.002, respectively). The same alg al taxa occurred on bot h substrata, but some of them (e.g. Ceratoneis arcus Grun., Cymbella m inuta Hilse ex Rabh., Zygnema sp.) were more numerous on wood, whereas a few others preferred tiles (e.g Achnanthes lanceolata (Breb.) Grun. ). Scanning electron microscopy revealed a much higher surface roughne ss on wood, which likely bd to a patchy distribution of the algae and the development of stalked diatom species. However, adnate species ada pted much better to the mainly flat surfaces of tiles. Net community p rimary productivity (NCPP) measured in 42-day-old substrata was modera te on tiles but negative on wood. Community respiration (CR) was signi ficantly higher on wood than on tiles. Epixylic algal communities exhi bited greater nitrogen demand than epilithic communities after ammoniu m addition. NCPP shifted to positive values on wood but did not increa se on tiles. Despite metabolic differences, the variations between the two communities are more closely related to the physical characterist ics of the substrata and the attachment abilities of the algal taxa.