Effect of cadmium and chromium toxicity on the demography and population growth of Brachionus calyciflorus and Brachionus patulus (Rotifera)

This paper reports on the assessment of the effect of Cd⁺² and Cr⁺⁶ to the rotifers Brachionus calyciflorus and B. patulus using life table demography and population growth. Based on acute toxicity tests, cadmium was nearly 100 times more toxic than chromium for both the rotifer species. Age-specific fecundity curves of both the brachionids showed increased offspring beyond the age of 4 days and after that the production nearly stabilized for about 10 days and later declined. At any given treatment, B. calyciflorus had the higher rate of offspring production than B. patulus. For any given rotifer, increased metal concentration (Cd or Cr) resulted in decreased offspring production. While chromium at a concentration of 8.0 mg L^-1, permitted reproduction in B. calyciflorus, the egg production of B. patulus was inhibited at half that level (4.0 mg L^-1). Cd at concentrations as low as 0.056 mg L^-1 caused 10-30% reduction in survivorship and reproductive parameters of B. calyciflorus. The lowest chromium concentration chosen for B. calyciflorus (0.5 mg L^-1) had, however, a lower impact (2-22%) on the life history variables. Corresponding values of life history variables for B. patulus at the lowest Cd (0.0028 mg L ^-1) and Cr (0.25 mg L^-1) concentrations varied greatly (2-40% and <1-24%, respectively). Compared to controls, both the rotifer species in Cd or Cr treatments had lower densities. In controls, B. calyciflorus reached a peak abundance of about 100 ind. mL^-1, while the corresponding value for B. patulus was slightly higher (120 ind. mL ^-1). The rate of population increase (r) derived from the population growth data varied from +0.42 to -0.40 depending on the concentration of the heavy metal in the medium.