The drift of zooplankton and microzoobenthos in the river Strandaelva, western Norway

Abstract

The drift of zooplankton (rotifers, cladocerans, cyclopoid copepods) and microscopical zoobenthos (mainly bdelloid rotifers and small chironomid larvae) was investigated by filtering samples of river water. The number of drifting benthic rotifers varied between 1 000 and 6 000 ind. m−3 in the lake inlet, and between 30 and 500 ind. m−3 in the lake outlet, without any seasonal trend. The number of drifting insect larvae was approx. equal in the lake inlet and outlet, with a maximum in summer (250–300 ind. m−3) and minimum in winter (ca. 10 ind. m−3). Increasing water flow resulted in an increasing number of drifting zoobenthos. Downstream from the lake, the number of drifting benthic rotifers was increasing from approx. 300 ind. m−3 in the outlet to 6 500 ind. m−3 3.4 km downstream, while the number of insect larvae was ca. 100 ind. m−3 in the outlet and leveled off at approx. 300 ind. m−3 after 200 m. The number of drifting zooplankton in the lake outlet varied between 20 and 2 000 ind. m−3 for crustaceans, and between 300 and 20 000 ind. m−3 for rotifers, both with a maximum in late summer/autumn and a minimum in winter. The number of drifting zooplankton decreased by some 45% in the first 200 m from the lake outlet, but some zooplankton was still found in the drift 3.4 km downstream. The largest species was removed first from the drift. The diurnal variation in the number of drifting zooplankton in lake outlets appear to be related to the vertical migration in the lake, i.e. the largest number drifting when most animals are in the upper water layers.