In tests with newborn and one-week-old Daphnia magna, 48-h EC50 values of 21–24 μg/L and 16 μg/L pirimicarb, respectively, were found. Older animals thus were as sensitive to pirimicarb as newborn animals. In an experiment with sediment included in the test system, all mother animals survived for 72 h at 20 μg/L, and the number of offspring was not reduced relatively to the control. Addition of sediment thus reduced the toxicity of pirimicarb toward Daphnia magna.
Pirimicarb was accumulated 12–16 times (5–7% of total) in the sediment, but the water concentrations of pirimicarb were not reduced significantly during the experiment, due to the small amount of sediment used. Accumulation in the sediment was found independent of the water concentration used. This was also the case with bioaccumulation in Daphnia magna, where a bioaccumulation factor of 31–37 was found on a dry weight basis. In water without sediment a BCF of 50 was found. Addition of sediment also reduced the accumulation of pirimicarb in the daphnids. The reduced bioavailability of pirimicarb may derive from humic acid and related compounds released from the sediment.