DOI: 10.5061/dryad.0f257gq
title: Data from: Stressor dominance and sensitivity-dependent antagonism: disentangling the freshwater effects of an insecticide among co-occuring agricultural stressors

description: 1. Pesticide concentrations are correlated with regional declines in
 stream invertebrate diversity. Experimental studies have identified that
 pesticides can have strong and persistent negative effects on aquatic
 ecosystems. These effects may occur at concentrations orders of magnitude
 lower than laboratory toxicity studies predict. Synergism among stressors
 is one explanation for the observed laboratory-field difference. However,
 the true effect of pesticides on stream invertebrates remains uncertain,
 given interactions between stressors and natural environmental conditions.
 2. We experimentally examined multiple-stressor effects on stream
 invertebrate assemblages and leaf-litter breakdown using 24 independent
 ~900 L re-circulating outdoor mesocosms in a semi-orthogonal design. Two
 pulses of the pesticide malathion (C10H19O6PS2) were delivered at low and
 high concentrations (Pulse 1: low at 0.1 and high at 1 µg L-1¬; Pulse 2:
 at 2.5 and 25 µg L-1). These were crossed with a treatment mimicking the
 combined stressors from agricultural development; nitrogen, phosphorus and
 clay (kaolin) sediment (hereafter agricultural stressors). 3. Malathion
 degradation was rapid (&lt;24 hr half-life) in all treatments, likely
 because of photolysis, hydrolysis, the presence of biofilms and sorptive
 processes. There were significant differences in invertebrate assemblages
 between treatments, where malathion contributed to 48% and 87% of deviance
 during Pulse 1 and 2 respectively. Malathion had strong negative effects
 during Pulse 2, decreasing Ephemeroptera, Plecoptera and Trichoptera
 abundance (P&lt;0.05), invertebrate richness (P&lt;0.01) and total
 abundance (P&lt;0.01). 4. Despite the dominant effects of malathion on
 invertebrates, agricultural stressors inhibited rates of leaf litter
 breakdown (P&lt;0.05), increased invertebrate densities and weakly
 mitigated malathion toxicity in mesocosms (e.g. PERMANOVA, P≈0.1).
 Malathion breakdown analysis indicated sediment addition reduced malathion
 concentrations through alkaline hydrolysis and sorption. Experimental
 assays on common taxa indicated sediment was antagonistic with malathion.
 5. Pesticide effects were not synergistic with the stressors tested here.
 Indeed, sensitivity-dependent stressor antagonisms were observed,
 reinforcing a mechanistic understanding of stressor effects and
 interactions is important. Among aquatic stressors examined, malathion had
 dominant effects on invertebrate assemblages, despite rapid degradation.
 Our results suggest that pesticide effects may be underestimated within
 ecosystems, where pesticides can have severe community wide effects, but
 rapid pesticide breakdown and sorptive processes may conceal causal
 links.01-May-2019
description: Invertebrate and
 physicochemical dataThis file contains
 invertebrate and physicochemical data used in analyses.MesoMalathionFull2.csvChemistry resultsChemistry results for the first and second pulses.MalathionBreakdown2Assay results from the malathion breakdown experiment.Indices and metricsIndices and metrics from the conclusion of first and second treatment pulses.indices3.csvMultipleStressBioassayResults from the laboratory assay conducted examining whether antagonism observed in the mesocosms, and breakdown assay could result in effects within particular specimens in the lab.