Bryophytes as invertebrate habitat in two New Zealand alpine streams

Abstract

SUMMARY. 1. Quantitative samples of lotic benthic invertebrates and associated organic material were taken monthly for 18 months from bryophyte and gravel habitats in two New Zealand alpine streams in Arthur's Pass National Park, South Island; an unshaded site above the tree‐line and a shaded stream flowing through mountain beech forest. This was to assess the ecological role of bryophytes in these habitats and to determine the relative effects on the fauna of algal and detrital resources.2. Bryophytes were confined to stable bedrock substrata in shallow (mean depth of 4.5 and 10.5 cm), fast water (mean velocity of 0.21 ms−1 and 0.30ms−1) at the unshaded and shaded sites. Mean ash‐free dry weight (AFDW) of organic material from bryophyte areas was similar at the unshaded and shaded sites (335.6 and 272.4 gm−2). Low stream‐bed stability of the gravel areas resulted in poor retention of organic matter, with average AFDW of 59.1 and 77.9 gm−2 at the unshaded and shaded sites, respectively. Periphyton biomass, however, was much higher at the unshaded site.3. Within bryophytes, water velocity is reduced and large quantities of periphyton and detritus accumulate. This explains the greatly enhanced invertebrate densities within bryophytes (218400 and 53500 individuals m−2), compared with gravel areas (20900 and 7400 individuals m−2) in the unshaded and shaded sites, respectively. The higher invertebrate densities at the unshaded site reflect greater periphyton biomass there.4. Invertebrate densities followed clear seasonal patterns, attributable to changes in chironomid densities. These reflected larval chironomid hatching following adult oviposition. This illustrates the importance of these plants as oviposition sites, and as protective nurseries for small larvae where they can obtain shelter during floods and consume the abundant food sources available to them.5. DECORANA ordination revealed taxonomically distinct invertebrate communities in each stream, and in bryophytes and gravels. Several taxa appeared restricted to either the unshaded site (e.g. Acroperla spiniger and Hydrobiosis silvicola) or the shaded site (e.g. Zelolessica