BIOFILM DEVELOPMENT AND INVERTEBRATE COLONIZATION OF WOOD IN 4 NEW-ZEALAND STREAMS OF CONTRASTING PH

1. Biofilm development and activity on wood substrata (Nothofagus menz iesii) were examined at four forested sites in a South Island, New Zea land, river catchment over a period of 6 months. Two of the sites had brown waters and mean pH of 3.7 and 4.5, whereas the other two had cle ar waters and mean pH of 6.3 and 6.8. 2. Fungi and other filamentous h eterotrophs were the dominant colonizers of wood at all sites; few alg al cells were present. Incorporation of C-14-glucose by biofilms was g reatest in all four streams after 3 months, whereas endocellulase acti vity fluctuated over time and temporal patterns differed among streams . 3. No clear relationship was found between the incorporation of C-14 -glucose or endocellulase activity of biofilms and pH, although at one near-neutral pH site C-14-glucose uptake increased in response to nut rient (N + P) additions. 4. After 6 months, incorporation of C-14-gluc ose and endocellulase activity of biofilms on Pinus radiata dowels bur ied vertically in the stream beds did not differ at depths of 3-9 cm a nd 19-25 cm in each stream. 5. Radiotracer experiments with a grazing amphipod (Paraleptamphopus sp.) demonstrated that biofilms on wood fro m all four sites could be ingested and at least partially assimilated. Chironomid larvae and harpacticoid copepods were the most abundant in vertebrates colonizing wood substrata at all sites. Different chironom id species dominated at acidic and near-neutral pH sites. 6. Overall, our findings provide little support for the hypothesis that microbial activity on organic substrata is necessarily lower in streams of low p H.