The effect of acid stress on survivorship and reproduction of Daphnia pulex (Crustacea: Cladocera)

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Abstract
Laboratory simulation of acid stress to a common cladoceran, Daphnia pulex, was conducted in pond water of varying acidity due to addition of H2SO4. An acute test using exposure times of 1 to 96 h and pH levels of 3.7 and 6.5 revealed virtually no effect at 4.3 and higher, while 4.2 and lower severely reduced survivorship. Very short (3-h) exposures caused nearly complete mortality at pH 3.7, while > 12 h exposure caused high mortality at pH of 4.0–4.2. A chronic 21-d life table test indicated a gradually increasing impairment of population growth rate potential (r) at pH 5.0 and below. This was due primarily to reduced survivorship and delayed onset of reproductive maturity. Those individuals which survived at lower pH levels produced broods equivalent to unstressed daphnids. While the tolerance of D. pulex to low pH does not appear to have been reported from field surveys, our results are in close agreement with the observed effects of increasing acidity on the distribution of other zooplankton species.