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

Abstract

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 transects). Six days prior to a spate, we sampled invertebrates on 24 unmarked stones. Twelve were randomly chosen among well-embedded stones, the others 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-flow events.) As soon as possible after the spate, we sampled another set of stones. Ten were marked stones that had remained stable during the spate, whereas 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 same way. Taxon richness, area-standardized total densities of invertebrates, and densities of Chironomidae, Deleatidium spp., Austrosimulium spp., Zelandoperla spp., and Oligochaeta were similar on both stone categories before the spate. Shortly after the spate, their densities (and taxon richness) were all significantly higher on stable than on unstable stones, and values on the former exceeded pre-disturbance levels in several cases. By day 19, most of these differences had disappeared, although densities of 3 of the 5 taxa on unstable stones had not yet recovered to pre-disturbance levels. We concluded that stable surface stones were important invertebrate refugia during the spate. The higher densities of several taxa relative to pre-disturbance levels implied that invertebrates may actively seek stable refugium stones, whereas many leave or are dislodged from unstable stones. It remains to be determined how invertebrates can identify stones that remain stable during high-flow events.