Vertical distribution and abundance of invertebrates in two New Zealand stream beds: a freeze coring study

Distribution of the invertebrate fauna within the stony beds of two upland streams was examined by freeze coring to a depth of 30 cm. Faunas of both streams were dominated by aquatic insects, and the only abundant group that might typically be described as hyporheic was the Ostracoda. Of 34 taxa recorded in each stream 13 and 19, respectively, were found at depths of 20-30 cm. About 50% of individuals occurred in the upper 10 cm of both stream beds, but the depth distributions of four common taxa differed from each other and within each stream bed. The greater relative abundance of invertebrates at 20-30 cm in one stream may indicate that more individuals move down into the bed to escape predation by fish (absent from the second stream), or that a higher quality hyporheic food resource occurred there. Comparisons of invertebrate densities in freeze cores and Surber samples showed that the latter substantially underestimated the abundances of some taxa even in the top 10 cm of stream bed. This has serious implications for the estimation of production using conventional surface sampling methods.