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Heavy metals may interact with ecological factors such as temperature, food level and salinity, causing both mortality and reduced reproduction in organisms. Among different heavy metals, copper compounds are commonly used for eliminating algal blooms in aquaculture tanks. At certain concentrations, copper is toxic to rotifers. In the present work, we evaluated the combined effects of salt concentrations (2.5 and 5.0 g l )1 NaCl), copper levels (0, 0.03125, 0.0625, 0.125 and 0.25 mg l )1 as CuCl 2 ) and two temperatures (20 and 25°C) on the population growth of B. rotundiformis using Chlorella as the algal food (at 0.5 · 10 6 cells ml )1 for every 24 h). Regardless of salinity and temperature, copper at concentrations as low as 0.03 mg l )1 had an adverse effect on the population growth of rotifers and above 0.125 mg l )1 , the populations did not grow. The effect of the toxicant on B. rotundiformis was more severe at 25°t han at 20°C at lower salinity. In general, we observed peak densities of rotifers around day 12 at 20°C but 6-8 days earlier at 25°C. Peak population densities of B. rotundiformis in the controls at the salinity of 2.5 g l )1 ranged from 90 to 180 ind. ml )1 , depending on temperature; at a salinity of 5.0 g l )1 , these were lower. The population growth rates, r, in our study varied from +0.31 to -0.12 depending on the test conditions. There was a significant impact of temperature, salinity and toxicity level on the population growth rate of B. rotundiformis. Our results suggested that even narrow changes in salinity could negatively influence the toxicity of heavy metal on the population growth rates of B. rotundiformis.

Competition among zooplankton is a natural phenomenon and often cladocerans are competitively superior to rotifers. However, anthropogenic factors including the release of industrial effluents, may influence this interaction. In this study, we evaluated the effect of cadmium (0.05 and 0.1 mg L(-1) as CdCl(2)) on competition between the cladoceran Moina macrocopa and the rotifer Brachionus calyciflorus. Since the release of industrial effluents is generally pulsed, we also exposed the test zooplankton species at different exposure periods (3, 6, 12 and 24 h) to the heavy metal. Regardless of exposure time and the presence of competing species, an increase in concentration of Cd resulted in decreased population growth of M. macrocopa and B. calyciflorus. Regardless of presence of the competing species and Cd concentration, an increase in exposure period resulted in decreased population growth rates of both the zooplankton species. In mixed cultures, in general, M. macrocopa outcompeted B. calyciflorus and completely eliminated it under conditions of high toxicant concentrations and longer exposure time.

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