Comparative population growth of Ceriodaphnia dubia and Daphnia pulex (Cladocera) exposed to zinc toxicity
´ ´ ´ JONATHAN RAUL SANCHEZ-ORTIZ1 , S.S.S. SARMA1 and S. NANDINI2
Laboratory of Aquatic Zoology,National Autonomous University of Mexico, Campus Iztacala, Mexico UIICSE, National Autonomous University of Mexico, Campus Iztacala, Mexico
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 quantiﬁedfor 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 to4.1 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. Theresults are discussed in relation to published data on the toxicity of zinc to freshwater zooplankton. Keywords: Heavy metal, zooplankton, population dynamics, chronic toxicity.
Zinc is among the few heavy metals that are abundant in nature and essential for all organisms. However, in recent times the increased production, consumption and release of zinc into aquatic ecosystems havegreatly inﬂuenced zooplankton production and dynamics.[1,2] In freshwater bodies, zooplankton is mainly composed of rotifers, cladocerans and copepods. In terms of biomass, cladocerans often are dominant and because they lack naupliar stages (unlike copepods), they mature rapidly and occupy all the available microhabitats (e.g., limnetic, benthic and periphytic regions) in ponds and lakes.Compared to the vast number of littoral and benthic cladocerans, planktonic species are few. However, in contaminated waterbodies, dissolved heavy metals including Zn more likely affect the planktonic genera such as Moina, Ceriodaphnia, Daphnia and Diaphanosoma.
Address correspondence to S.S.S. Sarma, Laboratory of Aquatic Zoology, Building U.M.F., Division of Research and Postgraduate Studies,National Autonomous University of Mexico, Campus Iztacala, A. P. 314, C. P. 54000, Los Reyes, Tlalnepantla, State of Mexico, Mexico. E-mail: firstname.lastname@example.org
Compared to Moina and Diaphanosoma, Ceriodaphnia and Daphnia have been routinely employed for both acute and chronic toxicity tests. Daphnia are predominantly temperate, and many of its species are not found in tropical waters.However, D. magna, a typical temperate species, has been considered as the standard bioassay organism even in some tropical nations (e.g., Mexico). Search for alternative cladoceran species has revealed that Ceriodaphnia is equally sensitive as Daphnia to stress. Published works show that most studies on ecotoxicology have considered either Ceriodaphnia or Daphnia, but rarely both....