Forested headwaters mitigate pesticide effects on macroinvertebrate communities in streams: Mechanisms and quantification.

Pesticides impact invertebrate communities in freshwater ecosystems, leading to the loss of biodiversity and ecosystem functions. One approach to reduce such effects is to maintain uncontaminated stream reaches that can foster recovery of the impacted populations. We assessed the potential of uncontaminated forested headwaters to mitigate pesticide impact on the downstream macroinvertebrate communities in 37 streams, using the SPEARpesticides index. Pesticide contamination was measured with runoff-triggered techniques and Chemcatcher® passive samplers. The data originated from 3 field studies conducted between 1998 and 2011. The proportion of vulnerable species decreased significantly after pesticide exposure even at low toxicity levels (-4<TUmax≤-3). This corresponds to pesticide concentrations down to 3-4 orders of magnitude below the LC50 value for standard test organisms. The toxicity of pesticides and the length of the forested reaches together explained 78% of variation in the community composition (SPEARpesticides). The proportion of vulnerable species doubled within the measured length of the forested stream section (0.2-18 km), whereas other characteristics of the forest or abiotic water parameters did not have an effect within the measured gradients. The presence of forested headwaters was not associated with reduced pesticide exposure 3 km downstream and did not reduce the loss of vulnerable taxa after exposure. Nevertheless, forested headwaters were associated with the absence of long-term pesticide effects on the macroinvertebrate community composition. We conclude that although pesticides can cause the loss of vulnerable aquatic invertebrates even at low toxicity levels, forested headwaters enhance the recovery of vulnerable species in agricultural landscapes.