Richardson, W. B., 1992, Microcrustacea in flowing water - experimental-analysis of
washout times and a field-test: Freshwater Biology, v. 28, no. 2, p. 217-230.

Abstract

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).

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