Stable surface stones as refugia for invertebrates during disturbance in aNew Zealand stream

We tested whether stable surface stones serve as invertebrate refugia in a New Zealand gravel-bed stream. Two-hundred stones were marked in situ in a systematic grid across 20 transects 40-60 cm between stones, 1 m between tr ansects). Six days prior to a spate, ive sampled invertebrates on 24 unmark ed stones. Twelve were randomly chosen among well-embedded stones, the othe rs among stones of similar size lying loosely on top of the bed. (Previous work had shown the former to be more stable than the latter during high-flo w events.) As soon as possible after the spate, we sampled another set of s tones. Ten were marked stones that had remained stable during the spate, wh ereas 12 were chosen at random among loose stones. Nineteen days after the spate, we sampled a final set of 8 stable and 12 unstable stones in the sam e way. Taxon richness, area-standardized total densities of invertebrates, and densities of Chironomidae, Deleatidium spp., Austrosimulium spp., Zelan doperla spp., and Oligochaeta were similar on both stone categories before the spate. Shortly after the spate, their densities land taxon richness) we re all significantly higher on stable than on unstable stones, and values o n the former exceeded pre-disturbance levels in several cases. By day 19, m ost of these differences had disappeared, although densities of 3 of the 5 taxa on unstable stones had not vet recovered to pre-disturbance levels. We concluded that stable surface stones were important invertebrate refugia d uring the spate. The higher densities of several taxa relative to pre-distu rbance levels implied that invertebrates may actively seek stable refugium stones, whereas many leave or are dislodged from unstable stones. It remain s to be determined how invertebrates can identify stones that remain stable during high-flow events.