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

The surface sediment, subsurface sediment, and hyporheos were sampled by freeze-coring at three sites of contrasting geomorphology in a New Zealand gravel-bed stream. No differences in size class composition were detected among 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 than the other sites. The hyporheos of all sites was dominated numerically by insects, including nymphs of Deleatidium and Oniscigaster, and chironomid larvae. Of the non-insect taxa, Isopoda, Oligochaeta, and Acari were most common. No significant differences existed between total invertebrate density or taxon richness at the sites. Aoteapsyche was found only in the plane bed site, whereas Nesameletus was found only in the forced pool-riffle. Psilochorema and Nematoda occurred at significantly lower densities in the forced pool-riffle than in the plane bed or floodplain sites. For most taxa, local variation within a site was greater than that between sites. Total invertebrate density was highest near the sediment surface but no significant differences were found between the three sites in this or any other depth layer. Significant site-depth interactions were obtained for three taxa (Zelandobius, Ostracoda, and Oligochaeta) indicating that local geomorphology may result in differential depth distributions of hyporheic species.