Ecotoxicology of neotropical freshwater zooplankton species exposed to toxic mixtures

Abstract

Chemical pollutants in the environment rarely occur as single compounds, but rather as more or less complex mixtures. Aquatic ecosystems near agricultural areas are particularly prone to impact from simultaneous or sequential exposures to combinations of different pesticides, yet environmental regulators normally limit risk assessment to the effects of individual chemicals. Reactive substances such as pesticides or metals have been found to produce interactive effects that cannot be predicted from effects of individual compounds. The precise effects of a mixture will however depend on its composition (in terms of substances present and their relative amounts) as well as the organisms and toxicity parameters investigated. For the assessment of pesticide impacts on a sensitive wetland system in south-east Mexico a standardised bioassay with a standard cladoceran test species, Ceriodaphnia dubia, and two locally isolated tropical species, Ceriodaphnia corn uta and Simocephalus vetulus, was developed. The bioassay determined inhibition of cladoceran feeding rates as a sensitive sublethal endpoint for assessing effects of low, environmentally realistic concentrations of three pesticides and one metal. Zooplankton feeding is an important ecological parameter in aquatic systems, influencing rates of nutrient cycling and affecting all trophic levels. The feeding inhibition bioassay demonstrated feeding rates of the cladocerans to be significantly reduced at sublethal exposure concentrations to aqueous solutions of the pyrethroid pesticide deltamethrin, the herbicide atrazine, and the non-essential metal cadmium. The organophosphate pesticide chlorpyrifos did not inhibit feeding at sublethal concentrations. Median 24 h feeding inhibition values (24hICsos) compared to lethality values (24hLCsos) for C. dubia were 0.19 j.lg Lot deltamethrin (ICso) compared to 0.49 Ilg L-1 (LC50), 5.31 Ilg L-1 compared to 68.8 Ilg L-1 cadmium, and 13.23 mg L-1 compared to 30 mg L-1 atrazine, when food was present at equal concentrations. LC50s were established in the presence and absence of food, for 24 and 48 hours, and differences in LC50S and 1C50s between species were found. The effects of binary and tertiary mixtures of deltamethrin, atrazine and cadmium on feeding rates were assessed by comparing observed to predicted toxicity (% feeding inhibition). All binary and tertiary mixtures were found to produce a greater-thanadditive effect, with the binary and tertiary mixtures of atrazine and deltamethrin producing a pronounced more-than-additive effect, with very high mixture effects at lower individual concentrations, levelling off exponentially towards the total inhibition of feeding. Individual compounds below their NOEC were found to contribute to the overall toxicity of the mixture Findings show that interactive effects of mixtures of reactive chemicals can be assessed from studying sensitive sublethal endpoints, such as feeding inhibition. Effects of binary and tertiary mixtures can differ from those expected from either individual effects or models such as the concentration addition (CA) or independent action (IA) models. The establishment of a database on combination effects of different groups of compounds could allow the use of this bioassay as a predictive tool for assessing effects of mixtures of pesticides and metals on individuals and population dynamics of zooplankton.