Assessing ecological responses of exposure to the pyrethroid insecticide lambda-cyhalothrin in sub-tropical freshwater ecosystems

Pyrethroid insecticides are widely detected in aquatic ecosystems due to their extensive use in agriculture and horticulture, which could pose a potential risk to aquatic non-target organisms. While previous ecotoxicological studies have been conducted mainly with standard tests and local species under temperate conditions, scarce information is available on the effects of pyrethroid insecticides on communities and ecosystems under (sub-)tropical conditions. A single application of lambda-cyhalothrin at concentrations of 0, 9, 30, and 100 ng/L was evaluated in outdoor mesocosms under sub-tropical conditions. Lambda-cyhalothrin was found to dissipate rapidly in the water column, with only 11 % and 7 % of the remaining dose measured at 1 and 3 days after application, respectively. Lambda-cyhalothrin concentrations disappeared considerably faster from the water compartment compared to temperate conditions. Consistent decreases in abundance were observed for Lecane lunaris at the medium and higher treatments (NOEC = 9 ng/L) and at the highest treatment (NOEC = 30 ng/L) for Keratella tropica. On the contrary, two taxa belonging to Cladocera (i.e., Ceriodaphnia sp. and Diaphanosoma sp.) showed the most prominent increase in abundance related to the lambda-cyhalothrin treatments. At the community level, a consistent no observed effect concentrations (NOECs) of 9 ng/L could be calculated for the zooplankton community. A marginal significant overall treatment related effect was observed for the macroinvertebrate community. The results of species sensitivity distribution (SSD) analysis based on results of acute toxicity experiments conducted alongside the mesocosm experiment and obtained from the literature indicated that macroinvertebrates from temperate regions may be generally more sensitive than their counterparts in (sub-)tropical regions. Overall, these findings suggest that environmentally relevant concentrations of the pyrethroid insecticide lambda-cyhalothrin may lead to different ecological outcomes in freshwater ecosystems in the (sub-)tropics relative to temperate regions.