TEMPORAL AND SPATIAL-DISTRIBUTION OF BENTHIC MICROFAUNA IN SEDIMENTS OF A GRAVEL STREAMBED

Densities of microfauna (large heterotrophic flagellates and three ciliate size classes) inhabiting surface sediments and the hyporheic zone of a gravel-bed brook were investigated at 14-d sampling intervals between October 1991 and October 1992. Abundances of both flagellates and ciliates were higher in the hyporheic zone than in surface sediments. Flagellates were distributed at all depths over the sampling period, but densities were highest at 30-40-cm depth before a spate in April 1992. Ciliate depth distribution also showed high densities from 10 to 40 cm in April 1992, and patches of high abundance occurred at 30-40 cm in summer 1992. Preliminary estimates of resilience suggested that flagellates were more resilient than ciliates and that large flagellate individuals and ciliates <50 mu m in the hyporheic zone had higher resilience values than those in the streambed surface sediments. The combined effect of hydrophysical variables on flagellate and ciliate densities differed between microfauna inhabiting the surface of the streambed and the hyporheic zone.