Biofilm development and invertebrate colonization of wood in four New Zealand streams of contrasting pH

Abstract

SUMMARY1. Biofilm development and activity on wood substrata (Nothofagus menziesii) were examined at four forested sites in a South Island, New Zealand, 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 clear waters and mean pH of 6.3 and 6.8.2. Fungi and other filamentous heterotrophs were the dominant colonizers of wood at all sites; few algal cells were present. Incorporation of 14C‐glucose by biofilms was greatest in all four streams after 3 months, whereas endocellulase activity fluctuated over time and temporal patterns differed among streams.3. No clear relationship was found between the incorporation of 14C‐glucose or endocellulase activity of biofilms and pH, although at one near‐neutral pH site 14C‐glucose uptake increased in response to nutrient (N + P) additions.4. After 6 months, incorporation of 14C‐glucose and endocellulase activity of biofilms on Pinus radiata dowels buried vertically in the stream beds did not differ at depths of 3–9 cm and 19–25 cm in each stream.5. Radiotracer experiments with a grazing amphipod (Paraleptamphopus sp.) demonstrated that biofilms on wood from all four sites could be ingested and at least partially assimilated. Chironomid larvae and harpacricoid copepods were the most abundant invertebrates colonizing wood substrata at all sites. Different chironomid 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 pH.