Inter- and Intra-specific Variability in the Response of Zooplankton to Acid Stress

We measured the toxicity of sulfuric acid to natural populations of Daphnia pulex, D. galeata mendotae, Simocephalus serrulatus, Mesocyclops edax, Chaoborus americanus, and C. punctipennis. Organisms were collected in the spring and fall from acidic (pH 5.4) and circumneutral (pH 6.3–7.5) ponds, and their response to low pH was compared in 48- or 96-h acute bioassays. Based on 48- and 96-h LC50, cladocerans were most susceptible to acid stress, followed by Mesocyclops and Chaoborus larvae. Simocephalus was the most tolerant cladoceran, followed by D. pulex then D. galeata mendotae. Daphnia populations tested in the spring or early summer were more tolerant of low pH than those tested in the fall. Mesocyclops edax from an acid pond were more tolerant than those from a neutral pond. The response of the two Chaoborus species to low pH were quite similar; their 96-h LC50 (2.00, 2.09) was two pH units lower than those of cladocerans and one pH unit lower than that of M. edax. Our data demonstrate interspecific variability in acid sensitivity and suggest intraspecific variability due to habitat and season.

Acute and Chronic Toxicity of Cadmium to the Fathead Minnow (Pimephales promelas)

Two continuous-flow bioassays on the chronic toxicity of cadmium to the fathead minnow (Pimephales promelas Rafinesque) indicated that the maximum acceptable toxicant concentration (MATC) for this species is between 57 and 37 μg Cd/liter. The experimental concentration of 57 μg Cd/liter decreased survival of the developing embryos, the embryos being the most sensitive life stage. At lower concentrations of 37 to 4.5 μg Cd/liter, no adverse effect on survival, growth, or reproduction was found. Application factors based on the acute bioassays are discussed in relation to the great variation in toxicity values.