Linking sub-individual and population level toxicity effects in Daphnia schoedleri (Cladocera: Anomopoda) exposed to sublethal concentrations of the pesticide α-cypermethrin

Abstract

Synthetic pyrethroids are classified as moderately toxic to mammals and birds; nevertheless, they are highly toxic to non-target aquatic organisms such as fish and zooplankters. Chemical pollutants produce different effects in exposed organisms, ranging from biochemical to population responses. Cladocerans can modify the energy content of their offspring according to the surrounding medium as a way to improve their odds in case they have to cope with stressful conditions at birth. In this study, the effect of a synthetic pesticide on two levels of response in a Daphnia species different from those traditionally used as test organisms was evaluated. With this aim, Daphnia schoedleri neonates (<24 h) were exposed for 21 days to three sublethal concentrations of α-cypermethrin, 0.54, 5.4, and 54 ng L(-1), which correspond to 48-h EC1/100, EC1/10, and EC1, respectively. Effects were measured through a life table analysis for fecundity and survivorship. For effects on progeny, protein, carbohydrates, and lipids were determined and then transformed to caloric content. Biomarkers (BM) were expected to be the most sensitive evaluated response; nevertheless, population parameters such as survivorship and net reproductive rate (R0) were more sensitive since they presented significant differences with respect to controls at the lowest tested concentration. Neonates' caloric content varied during the reproductive period assessed and was negatively correlated to fecundity: as more neonates were born, less energy was provided by the adult females. Macromolecules concentration and caloric content values in cypermethrin-exposed adults were not different from those recorded in the control at the end of exposure time. The results herein presented suggest that stressed daphnids allocate more energy reserves to their offspring, although this strategy can vary depending on the number of reproductive events during the lifecycle, and on the toxicant's concentration. Sub-individual approaches to assess toxicant effects should be accompanied by demographic studies, which support population effect predictions inferred from BMs assessment.