Interacting Effects of pH Acclimation, pH, and Heavy Metals on Acute and Chronic Toxicity to Ceriodaphnia dubia (Cladocera)

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Abstract
Understanding those factors that modify the sensitivity of the zooplankter Ceriodaphnia dubia to toxicants is of vital importance to the interpretation of chronic toxicity data generated for the purpose of granting industrial permits. Early reports of high sensitivity of Ceriodaphnia to brief pH excursions led toxicologists to question the use of C. dubia as a routine toxicity test organism. Acute and chronic toxicity of pH and heavy metals, pH acclimation to acidic and alkaline conditions, and the role of pH in modifying heavy metal (copper and zinc) toxicities were investigated. Ceriodaphnia dubia acclimated near neutral pH had acute (48-h) LC50s of 4.6 (acidic) and 10.3 (alkaline) SU (Standard Units). Using a Mount-Norberg design, reproduction and mortality were not impaired between pH 6.14-8.99 regardless of pH acclimation history, but beyond these extremes, reproduction was significantly impaired (.alpha. = 0.05). Acute (48-h) exposures to both heavy metals at pH 6, 8, and 9 and in water hardnesses of 180, 110, and 100 mg/l showed C. dubia was consistently most sensitive in low pH, low hardness waters and was least sensitive in high pH, high hardness waters. However, reproduction and mortality of C. dubia were not so affected by pH in chronic exposures. Similar concentrations of metals at all pH levels resulted in equivalent reductions in offspring per female. The results strongly suggest that effluent guidelines for pH at 6-9 are sound, and that, for this important test species, toxicant activity in chronic time frames (i.e., exposures spanning several or all life stages) is directed primarily by concentration and water hardness, not by pH.

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