The cladoceran Daphnia magna, clone K6 (Antwerp, Belgium) was used to perform all bioassays in this study. Glass beakers (1 L capacity) were kept with ASTM (American Society for Testing and Materials) hard water ( ASTM, 1998) and daphnids, 10–12 individuals/L, were fed daily with the algae Raphidocelis subcapitata at a rate of 3×105 cells/mL and an organic additive (Marinure seaweed extract, supplied by Glenside Organics Ltd.) ( Baird et al., 1989). The cultures were maintained at a temperature of 20 °C in a 16:8 light: dark cycle. New cultures of daphnids as well as bioassays were initiated using the neonates from the third to the fifth brood.