Effect of methyl parathion on the population growth of the rotifer Brachionus patulus (O. F. Muller) 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 p arathion. To obtain sublethal concentrations of the pesticide a 24-h LC50 b ioassay was performed under two food densities (1.5 and 3.0x10(6) cells ml( -1)) using neonate rotifers. For the population growth experiments, seven t oxicant concentrations were used ranging from 0.16 to 10.0 mg L-1 and four food densities (0.75, 1.5, 3.0, and 6.0x10(6) cells ml(-1)), each with four replicates and thus in all, 128 test jars. A medium with Chlorella alone a nd without toxicant acted as control. The initial density of the rotifers i n each replicate were 5 individual ml(-1). Experiments were conducted for 2 0 days, The LC50 values of B. patulus were 8.8 and 10.7 mg L-1 at low and h igh algal food density, respectively. Data on the population growth studies revealed a significant effect of both food density and toxicant concentrat ion on the maximum abundance, day at which maximum abundance was reached, a nd rate of population increase per day. At any toxicant concentration, an i ncrease in food density reduced the toxicity of methyl parathion to rotifer s, In controls the maximum rotifer density under the highest food level (6x 10(6) 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 de pending on food density. Algal food density had a significant influence in reducing the toxicity of methyl parathion to B. patulus under both acute an d chronic toxicity test conditions. (C) 2001 Academic Press.