Effects of Mercury on the Life Table Demography of the Rotifer Brachionus calyciflorus Pallas (Rotifera)

Abstract

Mercury is highly toxic to a variety of aquatic organisms including zooplankton. The functioning of freshwater ecosystems can be altered if rotifers, being a natural food link between phytoplankton and fish larvae, are contaminated by mercuric compounds. In order to detect age-specific responses of the rotifer Brachionus calyciflorus to mercury toxicity (5 nominal concentrations as chloride viz. 0, 0.000625, 0.00125, 0.0025 and 0.005 mg l⁻¹), we used the standard life table method at two different food (Chlorella vulgaris) levels (0.5 × 10⁶ and 1.5 × 10⁶ cells ml⁻¹). Data indicated that increase in mercury concentration had an increasingly intense negative effect on many of the life history variables, while at higher food levels, its impact was less. A nearly rectangular survivorship pattern was obtained in controls, especially at higher food levels. This trend gradually changed to a steep fall as the concentration of the heavy metal in the medium increased from 0 to 0.005 mg l⁻¹. At any given food density, increase in the mercury concentration resulted in decreased age-specific reproduction. A maximum of 3.5 offspring female⁻¹ was observed in controls at higher food density. The average lifespan varied from 6 to 8 days at low food level, depending on the heavy metal concentration in the medium. The corresponding values at high food level varied from 8 to 12 days. Regardless of mercury concentration in the medium, gross and net reproductive values varied from 10 to 33 and 4 to 19 offspring female⁻¹. The longest generation time (about 9 days) of B. calyciflorus was obtained at 1.5 × 10⁶ cells ml⁻¹ food density in control, while the shortest was 5 days at low food level and high (0.005 mg l⁻¹) mercury concentration in the medium. Depending on the food level and heavy metal concentration in the medium, the rate of population growth (r) varied from 0.32 to 0.62 d⁻¹. In general, higher food level resulted in higher r. Except generation time, all other derived variables were significantly influenced by food level and the heavy metal concentration in the medium.