Bouncing Back: The resistance and resilience of aquatic organisms to multiple insecticides

Background/Question/Methods

The global use of insecticides is critical to disease control and crop production, but it can also pose unintended consequences to non-target organisms. Aquatic invertebrates, such as amphipods and zooplankton, are exposed to a variety of insecticides that vary in chemical properties (i.e. breakdown rates and toxicities). As critical drivers of green and brown food webs in aquatic systems, investigating the impact of insecticides on these non-target invertebrates is critical to understanding the link between population-level changes and community level interactions. Surprisingly, while most studies investigate the immediate direct and indirect effects of insecticides, few studies incorporate a time scale appropriate for assessing recovery. Using aquatic mesocosms, we investigated the consequences of three common organophosphate insecticides (chlorpyrifos, diazinon, malathion) applied individually at nominal concentrations of 10 and 40 ppb. Over a 20-month period, we measured the resistance (magnitude of effect) and resilience (time to recovery) of 16 taxa (zooplankton, amphipods, isopods, snails, phytoplankton, periphyton, and larval amphibians) and abiotic conditions (pH, temperature, light attenuation, dissolved oxygen). 

Results/Conclusions

We found that the resistance and resilience of zooplankton assemblages and amphipod populations were insecticide-specific. Invertebrate responses varied along a gradient with chlorpyrifos causing the most effect, diazinon causing moderate effects, and malathion causing the least effect. Compared to the control, we found a 91% decrease in zooplankton abundance when exposed to chlorpyrifos, a 43% decrease when exposed diazinon, and a 31% decrease when exposed to malathion. For amphipods, compared to control, we found an 87% decrease with chlorpyrifos, a 60% decrease with diazinon, and a 35% decrease when with malathion.  Zooplankton abundance recovered to control abundances in 60 weeks when exposed to chlorpyrifos, nine weeks when exposed to diazinon, and four weeks when exposed to malathion.  Amphipod abundances recovered in 72 weeks with chlorpyrifos, in 52 weeks with diazinon, and in 52 weeks with malathion. Thus, various insecticides have unique long-term impacts on invertebrate populations. Investigating these long-term population level consequences is critical to not only understanding the underlying mechanisms dictating community interactions but also future responses to disturbances.