Influence of reach geomorphology on hyporheic communities in a gravel-bed stream

The surface sediment, subsurface sediment, and hyporheos were sampled by fr eeze-coring at three sites of contrasting geomorphology in a New Zealand gr avel-bed stream. No differences in size class composition were detected amo ng the subsurface sediments of the three sites but the forced pool-riffle ( a stretch of stream where the sequence of pool and riffles is controlled by in-channel bedrock outcrops) had significantly finer surface sediments tha n the other sites. The hyporheos of all sites was dominated numerically by insects, including nymphs of Deleatidium and Oniscigaster, and chironomid l arvae. Of the non-insect taxa, Isopoda, Oligochaeta, and Acari were most co mmon. No significant differences existed between total invertebrate density or taxon richness at the sites. Aoteapsyche was found only in the plane be d site, whereas Nesameletus was found only in the forced pool-riffle. Psilo chorema and Nematoda occurred at significantly lower densities in the force d pool-riffle than in the plane bed or floodplain sites. For most taxa, loc al variation within a site was greater than that between sites. Total inver tebrate density was highest near the sediment surface but no significant di fferences were found between the three sites in this or any other depth lay er. Significant site-depth interactions were obtained for three taxa (Zelan dobius, Ostracoda, and Oligochaeta) indicating that local geomorphology may result in differential depth distributions of hyporheic species.