Comparative population growth of Ceriodaphnia dubia and Daphnia pulex (Cladocera) exposed to zinc toxicity

Abstract

Population growth of two cladocerans (Ceriodaphnia dubia and Daphnia pulex) exposed to 4 different concentrations of ZnCl₂ (0.125, 0.25, 0.5 and 1.0 mg L⁻¹, plus controls) at one algal food (Scenedesmus acutus) density (0.5X10⁶ cells mL⁻¹) was quantified for 30 days. Population densities of C. dubia and D. pulex decreased with increasing concentration of Zn in the medium. At a concentration of 1 mg L⁻¹ of ZnCl₂, both C. dubia and D. pulex did not reproduce and died within a week. The peak population densities of C. dubia ranged from 0.2 to 6.0 ind. mL⁻¹, depending on the Zn level in the medium, whereas this range was lower for D. pulex (0.2 to 4.1 ind. mL⁻¹). The peak population density was inversely related to the Zn concentration. The rate of population increase (r) varied from −0.12 to +0.14 and −0.02 to +0.23 per day for C. dubia and D. pulex, respectively, depending on the Zn level in the medium. Statistically, both the peak population density and the r were significantly affected by the heavy metal concentration in the medium. Multiple comparison tests showed that the rate of population increase (r) of D. pulex in the lowest ZnCl₂ level (0.125 mg L⁻¹) was significantly higher than controls. However, under similar conditions, the r of C. dubia was significantly lower than controls. With a further increase in Zn level, the growth rates of both the cladoceran species were significantly reduced as compared to controls. The results are discussed in relation to published data on the toxicity of zinc to freshwater zooplankton.

Keywords:

• Heavy metal
• zooplankton
• population dynamics
• chronic toxicity

Acknowledgments

The first author is grateful to the Mexican National Council for Science and Technology (CONACyT) for a scholarship and to PCMyL (UNAM) for encouragement. This study was supported by a project PAPIIT-201907 (UNAM).

Notes

*Analysis of variance (ANOVA); DF: degrees of freedom, SS: sum of squares; MS: mean square; F: F-ratio

*** = P < 0.001.