Combined Effects of Copper and Microalgal (Tetraselmis suecica) Concentrations on the Population Growth of Brachionus plicatilis Müller (Rotifera)

Copper compounds are commonly used to kill algal blooms in fish and shrimp ponds. However, indiscriminate use of copper in the aquaculture ponds may have a negative impact on the production of rotifers, which, in turn, can influence the final yield. In this work we studied the combined effects of chronic toxicity of copper (0, 0.125, 0.25, 0.5 and 1.0 mg L^-1 of copper sulphate) and algal (Tetraselmis suecica) levels (0.05 × 10⁶ and 0.10 × 10⁶ cells ml^-1) on the population growth of the saline water rotifer B. plicatilis at 25 °C, pH 8.5 and salinity 36‰. The population of B. plicatilis increased with increasing levels of T. suecica. Regardless of the food level, rotifers exposed to 1 mg L^-1 of copper did not survive beyond 3 days. Copper of 0.125 mg L^-1 caused a reduction in the population of B. plicatilis but only at low algal food level; at the high (0.2 × 10⁶) food concentration, the growth was comparable to that observed in the control suggesting that higher algal levels had an ameliorating effect on the toxicity of copper to B. plicatilis. The relation between the rotifer density and the egg-ratio was inverse and statistically significant in controls. However, this relation was non-significant in rotifers subjected to the copper concentration. The peak population density of B. plicatilis in controls varied from 36 to 251 ind. ml^-1 depending on the food level. The rate of population increase per day (r) in the controls varied from 0.24–0.63 depending on the food level. In general, an increase in food level resulted in higher peak population abundance and growth rates. The presence of copper resulted in the reduction of both r values and peak population densities of B. plicatilis. The results have been discussed in light of protective role of algae in reducing the toxicity of heavy metals to rotifers.