MICROCRUSTACEA IN FLOWING WATER - EXPERIMENTAL-ANALYSIS OF WASHOUT TIMES AND A FIELD-TEST

1. Flow-chamber experiments were conducted to evaluate the ability of microcrustacea to maintain position in moving water. These results were compared to distributions of zooplankton and water velocity in a stream pool to determine the relationship of animal density to water movement and swimming ability. 2. Cladocerans exhibited negative rheotaxis (directed behaviour against a current) but poor ability to maintain position at velocities >2.5 cm s-1. Daphnia and Scapholeberis were better at avoiding washout than Moina and Diaphanosoma. At velocities <2.5 cm s-1 Eucyclops (Cyclopoida) tended to exhibit no rheotaxis. 3. Washout of Daphnia was complete at velocities >2.5 cm s-1, Scapholeberis >3.2 cm s-1 and Eucyclops >7.75 cm s-1. Washout time of Daphnia and Scapholeberis was positively related to body size and negatively to water velocity and possession of eggs. Washout was inversely related to water velocity for Eucyclops. 4. Highest densities of microcrustacea in a stream pool were found in non-flowing or downstream zones of the pool. Benthic (Hydracarina, harpacticoid copepods, ostracods) and fast-swimming (cyclopoids) forms were most common in flowing zones. Facultatively benthic Cladocera were abundant in regions of no flow. Rotifers and immature copepods were most abundant at the downstream end of the pool. 5. Behavioural mechanisms for remaining in stream pools at times of high flow appear to include: (i) flow avoidance (Simocephalus, Chydorus, Scapholeberis and cyclopoids), (ii) use of benthic habitat (ostracods, harpacticoids, Hydracarina), (iii) strong swimming ability (cyclopoids).