Effect of Methyl Parathion on the Population Growth of the Rotifer Brachionus patulus (O. F. Müller) under Different Algal Food (Chlorella vulgaris) Densities

The population growth of the rotifer Brachionus patulus was studied under a combination of different concentrations of Chlorella vulgaris and methyl parathion. To obtain sublethal concentrations of the pesticide a 24-h LC50 bioassay was performed under two food densities (1.5 and 3.0×106 cells ml−1) using neonate rotifers. For the population growth experiments, seven toxicant concentrations were used ranging from 0.16 to 10.0 mg L−1 and four food densities (0.75, 1.5, 3.0, and 6.0×106 cells ml−1), each with four replicates and thus in all, 128 test jars. A medium withChlorella alone and without toxicant acted as control. The initial density of the rotifers in each replicate were 5 individual ml−1. Experiments were conducted for 20 days. The LC50 values of B. patulus were 8.8 and 10.7 mg L−1 at low and high algal food density, respectively. Data on the population growth studies revealed a significant effect of both food density and toxicant concentration on the maximum abundance, day at which maximum abundance was reached, and rate of population increase per day. At any toxicant concentration, an increase in food density reduced the toxicity of methyl parathion to rotifers. In controls the maximum rotifer density under the highest food level (6×106 cells ml−1) was 795±46 individuals ml−1. The rate of population increase per day (r) in controls varied from 0.22±0.01 to 0.34±0.01 depending on food density. Algal food density had a significant influence in reducing the toxicity of methyl parathion to B. patulus under both acute and chronic toxicity test conditions.