Effect of sublethal concentrations of pesticides on the feeding behavior of Daphnia magna.

Daphnia magna was exposed to sublethal levels (1/4, 1/2, and 2/3 LC50 and LC50) of endosulfan and diazinon to determine the effect of these pesticides on filtration and ingestion rates. The experiments were performed with the unicellular algae Nannochloris oculata in a density of 5 x 10(5) cells/ml. Prior to these experiments, the acute toxicity of both pesticides was evaluated to calculate the LC50s. The 24-hr LC50 values were 0.62 mg/liter and 0.9 microgram/liter for endosulfan and diazinon, respectively. Rates of filtration and ingestion declined with increasing toxicant concentrations after a short exposure of 5 hr. The effective concentration at which feeding rates were reduced to 50% of those in controls (EC50) was calculated for both pesticides. These values were 0.44 and 0.61 mg/liter for filtration and ingestion rates, respectively, in the case of endosulfan and 0.47 and 0.60 microgram/liter, respectively, in the case of diazinon.