Journal of Environmental Science and Health Part A (2007) 42, 393–398 Copyright C  Taylor & Francis Group, LLC ISSN: 1093-4529 (Print); 1532-4117 (Online) DOI: 10.1080/10934520601144717 Combined effects of temperature and lead concentration on the competition between the rotifers Brachionus havanaensis and Brachionus rubens (Rotifera: Brachionidae) ARAMEN IV ´ AN MONT ´ UFAR-MEL ´ ENDEZ 1 , JONATHAN RA ´ UL S ´ ANCHEZ-ORT ´ IZ 1 , S. S. S. SARMA 1 and S. NANDINI 2 1 Laboratory of Aquatic Zoology, Division of Research and Postgraduate Studies, National Autonomous University of Mexico, Campus Iztacala, Los Reyes, Tlalnepantla, State of Mexico, Mexico 2 UIICSE, Research and Postgraduate Studies, National Autonomous University of Mexico, Campus Iztacala, Los Reyes, Tlalnepantla, State of Mexico, Mexico The combined effects of temperature (22 and 32 ◦ C) and the concentration of Pb (0, 0.05, 0.1 and 0.2 mg L −1 ) on the competition between B. havanaensis and B. rubens using population growth was evaluated. At 22 ◦ C, increase in Pb concentration had little effect on the population growth of B. havanaensis, while it had adverse effect on B. rubens. At this temperature, the presence of B. rubens had adversely affected the growth of B. havananensis. However, under similar conditions, B. rubens was not influenced by B. havanaensis. Increase in Pb concentration decreased the population growth of B. havanaensis at 32 ◦ C regardless whether the population was alone or together with B. rubens. When grown alone, B. havanaensis was numerically more abundant than B. rubens. B. havanaensis grown at 32 ◦ C had population growth rates of 0.32 to 0.56 day −1 , depending on Pb level and the presence of competitor. At this temperature, B. rubens had mostly negative growth rates. Further, our study showed that Pb at a concentration as low as 0.05 mg L −1 had adverse effect on B. rubens at both the temperatures. Though B. rubens is known to occur naturally at wide temperature range, in our study this species appeared to have adapted to low temperature. Keywords: Temperature, Pb toxicity, competition, rotifera, population growth rate. Introduction Rotifer species of the genus Brachionus are widely used as bioassay organisms for evaluating effects of pesti- cides, heavy metals and other toxic substances for aquatic ecosystems. [1] There are about 40 species of this genus around the world and in Mexico about half of them have been reported. [2] Most members of Brachionus have similar food requirements [3] and therefore competition is expected to be intense. [4] Though some Brachionus species do coex- ist, their relative abundances vary seasonally suggesting the role of biotic (e.g., food density) and abiotic (such as tem- perature) factors as the governing forces. [5] The interaction of food level and temperature on the population growth of several rotifer species has been documented. [6] Generally, Address correspondence to S.S.S. Sarma, Laboratory of Aquatic Zoology, National Autonomous University of Mexico, Campus Iztacala, AP 314, CP 54000, Los Reyes, Tlalnepantla, State of Mexico; Mexico; E-mail: sarma@servidor.unam.mx Received June 15, 2006. increase in temperature and food level have stimulatory ef- fect on the population growth rates of rotifers. However, higher temperature and lower food level or lower temper- ature and higher food level may have adverse effect on the population growth of rotifers. [7] Competition is yet another natural stress on one or more of the competing rotifer species. [8] Within a favorable range of temperature, food level and the relative inoculation den- sities of the competing species decide the competitive out- come in a predictable way. For example, if one of the com- peting species was inoculated at higher initial densities than the others, it is more likely that it outcompetes the rest. [9] However, toxicants interfere with the natural process of competition among zooplankton species causing changes in the population growth rates in many different ways. For example, when Moina macrocopa and B. calyciflorus were grown together, depending on the concentration of cad- mium and the duration of exposure, the growth of Moina was favoured or adversely affected. [10] Lead, like cadmium, is one of the heavy metals with no known positive role for aquatic organisms. On the other