Evaluating additive versus interactive effects of copper and cadmium on Daphnia pulex life history

Abstract

A key challenge of standard ecotoxicological risk assessment is to predict the sub-lethal risk of multiple contaminants on aquatic organisms. Our study assessed the sub-lethal mixture toxicity of copper (Cu) and cadmium (Cd) on Daphnia pulex and included manipulations of food level and assessment of three genotypes. We investigated the interaction between essential (Cu) and non-essential (Cd) metals on ingestion rate, reproduction, maturation time, size at maturity and somatic growth rate of three D. pulex genotypes, over 21 days and under standard and high food conditions. We explored the potential interaction of the metals on ingestion and life history by implementing a response surface experimental design combining control and two levels of Cu and Cd and their combinations. Overall, both metals reduced ingestion rates, reduced reproduction, delayed maturation, reduced body size at maturity and lowered somatic growth rate. Our results further indicated pervasive interactions between the metals; numerous instances where the effects of each metal were non-linear; the effect of a metal varied by D. pulex food levels (ingestion rate and size at maturity), and the effect of a metal varied by genotypes (reproduction). Apart from the maturation time and somatic growth rate, our results suggest that life history traits are affected in non-additive ways by three factors that are often discussed and rarely estimated together: mixtures of metals, genotypes and resource levels. Our data that are derived from exposing daphnids to two metals highlight how metals interact with each other and the context of food resource and genetic variation. While interactions make it harder to generate predictions, and ultimately water quality regulations about the effects of metals, those detected in this study appear to be tractable.

Keywords: Cadmium; Copper; Daphnia pulex; Genotypes; Life history; Response surface models.

Fig. 1

Contour plots for the effect of Cu/Cd mixture on the ingestion rate (cell/h) of different genotypes of D. pulex. The plots display the fitted (predicted) values from the response surface model. Each panel is a genotype–food combination and the x- and y-axes are the concentrations of the metals. Yellow versus red colours are higher values of ingestion. Details of significant terms that underpin the shapes that can be seen are described in the text

Fig. 2

Contour plot for the effect of Cu/Cd mixture on reproduction (mean number of neonates per female) across three genotypes of D. pulex. The plots display the fitted (predicted) values from the response surface model. Each panel is a genotype–food combination and the x- and y-axes are the concentrations of the metals. Yellow versus red colours are higher values of reproduction. Details of significant terms that underpin the shapes that can be seen are described in the text

Fig. 3

Contour plot for the effect of Cu/Cd mixture on the maturation age (days) of different genotypes of D. pulex. The plots display the fitted (predicted) values from the response surface model. Each panel is a genotype–food combination and the x- and y-axes are the concentrations of the metals. Yellow versus red colours are later maturation times. Details of significant terms that underpin the shapes that can be seen are described in the text

Fig. 4

Contour plot for the effect of Cu/Cd mixture on body size at maturity (mm) across three genotypes of D. pulex. The plots display the fitted (predicted) values from the response surface model. Each panel is a genotype–food combination and the x- and y-axes are the concentrations of the metals. Yellow versus red colours are larger size at maturity. Details of significant terms that underpin the shapes that can be seen are described in the text

Fig. 5

Contour plot for the effect of Cu/Cd mixture on somatic growth rate g (d^-1) of different genotypes of D. pulex. The plots display the fitted (predicted) values from the response surface model. Each panel is a genotype–food combination and the x- and y-axes are the concentrations of the metals. Yellow versus red colours are higher growth rates. Details of significant terms that underpin the shapes that can be seen are described in the text

Fig. 6

The nominal vs. Actual concentrations for Cu and Cd

Fig. 7

The nominal vs. Actual concentrations for Cu and Cd