Exposure to marine contaminant mixtures with different toxicity drivers reduces microzooplankton diversity

Marine surface waters contain complex mixtures of chemicals that can adversely affect microzooplankton. There is a lack of toxicity data for this organism group, and we used two different methodologies to fill this gap. We tested the toxicity of three chemical mixtures of polar organic chemicals extracted from marine surface water, using a component-based and a whole-mixture approach. The component-based approach estimates cumulative toxic units for each mixture based on concentrations of individual compounds. The observed hazard data for zooplankton was supplemented with ECOSAR-generated QSAR daphnid LC50s when observed data was missing. ECOSAR performance was evaluated for zooplankton, where 65% of the observed hazard data for zooplankton was predicted within a factor of 10. This approach suggested that none of the mixtures should be toxic to zooplankton at their respective measured environmental concentrations. We found contrasting results using the whole-mixture approach with a reduction in ciliates and dinoflagellates, and change in microzooplankton diversity, at the measured environmental concentrations. We suggest an assessment factor of at least 1000 when using additive toxic units in a component-based risk assessment approach to cover for the extrapolation from acute to chronic toxicity data and for the range of sensitivities among microzooplankton species.