Acute and chronic toxicity of the pesticide methyl parathion to the rotifer Brachionus angularis (Rotifera) at different algal (Chlorella vulgaris) food densities José Luis Gama-Flores 1, * , S.S.S. Sarma 2 and S. Nandini 3 1 Doctoral Programme, Autonomous Metropolitan University, Campus Xochimilco, Calzada del Hueso No. 1100, Villa Quietud, C.P. 04960, Mexico City, Mexico; 2 Laboratory of Aquatic Zoology, Edificio UMF, Division of Research and Postgraduate Studies, National Autonomous University of Mexico, Campus Iztacala, Av. de los Barrios s/n, Los Reyes, AP 314, CP 54090, Tlalnepantla, State of Mexico, Mexico; 3 UIICSE, Division of Research and Postgraduate Studies, National Autonomous University of Mexico, Campus Iztacala, Av. de los Barrios s/n Los Reyes, AP 314, CP 54090, Tlalnepantla, State of Mexico, Mexico; * Author for correspondence (e-mail: joluga@servidor.unam.mx) Received 21 October 2002; accepted in revised form 13 October 2003 Key words: Food effect, Population dynamics, Rotifers, Toxicity, Zooplankton Abstract Methyl parathion is a commonly used insecticide in Mexico to eradicate insect pests. We evaluated the effects of this insecticide on rotifer B. angularis using both acute and chronic toxicity tests. Median lethal concentration LC 50  of methyl parathion for B. angularis for a 24-h bioassay in the presence and absence of an algal diet was derived. Elevated LC 50 due to the survival of a greater number of test individuals in the presence of food was observed. Regardless of the toxicant concentration, population growth curves of the animals maintained at the low food level 0.75 10 6 cells ml –1  had a longer lag phase than those at the high food level 1.5 10 6 cells ml –1 . Regardless of food level, an increase in the toxicant concentration in the medium resulted in decreased population growth. The lowest peak population density 50 ind. ml –1  was observed at the highest toxicant con- centration and the lower food level. The highest population density 200 ind. ml –1  was observed in the controls at high food level. The rates of population increase per day r in the controls were higher from 0.14 to 0.37 depending on the food level. Irrespective of food level, there was a decrease in the r values with increasing pesticide concentration in the medium. In order to detect the effect of population density on the growth rates in relation to the toxicant stress, we plotted the daily growth rate against initial density for the entire duration of the experiment. We observed the existence of a significantly inverse relation at all treatments except at the low food level and high toxicant concentrations 0.625 and 1.25 mg l –1 . We discuss the role of algae in the toxicity of methyl parathion to zooplankton. Introduction Ecotoxicological studies with zooplankton have been traditionally used to characterise toxic stress by con- sidering different endpoints such as median lethal concentration LC 50 , swimming speed, bioaccumula- tion, morphological, morphometric and demographic responses or biochemical responses Snell and Jans- sen 1995. Population growth studies yield variables which are generally more sensitive and permit us to evaluate the sublethal toxic effect on individuals of different age groups and generations, which occur si- multaneously Sarma et al. 2001a. 27 © 2004 Kluwer Academic Publishers. Printed in the Netherlands. Aquatic Ecology 38: 27–36, 2004.