A study of drift in the Kakanui River, New Zealand

Abstract

The bottom fauna and drift in the Kakanui River, New Zealand, were studied for a period of one year. The fauna was dominated by Ephemeroptera and Chironomidae. A diurnal drift rate was found with maximum rates just after sunset. A mechanism is postulated to account for the observed correlation of decrease in light intensity at sunset and increase in drift rate. There was a quantative change in the drift at night with the occurrence of comparatively large numbers of mayfly nymphs and Rhyacophilidae larvae. Lowest drift rates were recorded during the winter. There is a close interrelationship between drift and bottom fauna: proportional occurrence in the bottom fauna is similar to that in drift but is modified by differences in behaviour of the animals, and the occurrence of pieces of algae, containing animals. During the study a flood occurred which halved the density of the bottom fauna but within three weeks the denslty returned to near its pre-flood value. Some species had higher densities after the flood than before it. Recolonization of the substrate by the animals could be followed by interdependent changes in bottom fauna and drift samples. A large increase in drift rate was recorded at sunset immediately after the flood had occurred.