A new protocol to measure the effects of toxicants on daphnid–algae interactions

Inhibition of zooplankton grazing by toxicants present at sublethal concentrations in freshwater ecosystems can lead to uncontrolled algal growth and consequently exacerbate eutrophication problems. Short-term measurements of cladoceran grazing inhibition have been reported for some toxicants, but these studies do not mimic the actual interactions between microalgae, daphnids and toxicants, since algal growth is not allowed. On the opposite, algal blooms in complex microcosm or mesocosm assays have been interpreted as consequences on zooplankton, but effects on grazing, survival, growth or reproduction could not be easily discriminated. In this study, a simple assay with daphnids and microalgae is proposed to measure effects on grazing in dynamic conditions (algal growth over 6 days), and applied to copper and lindane. In the same time, direct effects on algal growth can be shown and taken into account. Results are compared with daphnid response measured with different endpoints (immobilization test and static grazing assay). For both toxicants, effects at sublethal concentrations were demonstrated. Copper impaired daphnid grazing at 10 μg/l (60% inhibition) in the 48 h-static test and 15 μg/l (40% inhibition) in the 6 day-dynamic test, whereas 48 h-EC50 for daphnid mobility was 47 μg/l. The EC50s for lindane were 50 μg/l for daphnid grazing (48 h-static and 6 day-dynamic tests) and 383 μg/l for the immobilization test (48 h).