Stressor dominance and sensitivity‐dependent antagonism: Disentangling the freshwater effects of an insecticide among co‐occurring agricultural stressors

Sensitivity of Daphnia spinulata Birabén, 1917 to glyphosate at different salinity levels

Daphnia spinulata Birabén, 1917 is an endemic cladoceran species, frequent in the zooplankton communities of the shallow lakes of the Pampean region of Argentina. These lakes have varying salinity levels and, being located in agricultural areas, are frequently subject to pesticide pollution. This study aimed to determine the effects of the herbicide glyphosate (Panzer Gold®) in combination with different salinity levels on the biological parameters of D. spinulata and its recovery ability after a short exposure. Three types of assays were performed: an acute toxicity test, a chronic assessment to determine survival, growth and reproduction, and recovery assays under optimal salinity conditions (1 g L-1). The LC50-48 h of glyphosate was 7.5 mg L-1 (CL 3.15 to 11.72). Longevity and the number of offspring and clutches were significantly reduced due to the combined exposure of glyphosate and increased salinity. The timing of the first offspring did not recover after glyphosate exposure. Our results reveal that D. spinulata is sensitive to the herbicide Panzer Gold® at concentrations well below those indicated in the safety data sheet of this commercial formulation, which causes stronger negative effects in conditions of higher salinity. Further research is needed to shed light on the sensitivity of this cladoceran to glyphosate and its variability under other interactive stress factors.

Do magnesium and chloride ameliorate high sodium bicarbonate concentrations? A comparison between laboratory and mesocosm toxicity experiments

Increasing salinity is a concern for biodiversity in many freshwater ecosystems globally. Single species laboratory toxicity tests show major differences in freshwater organism survival depending on the specific ions that comprise salinity types and/or their ion ratios. Toxicity has been shown to be reduced by altering ionic composition, despite increasing (total) salinity. For insistence, single species tests show the toxicity of sodium bicarbonate (NaHCO3, which commonly is a large proportion of the salts from coalbeds) to freshwater invertebrates is reduced by adding magnesium (Mg2+) or chloride (Cl-). However, it is uncertain whether reductions in mortality observed in single-species laboratory tests predict effects within populations, communities and to ecosystem processes in more complex multi-species systems both natural and semi-natural. Here we report the results of an outdoor multi-species mesocosm experiment to determine if the effects of NaHCO3 are reduced by increasing the concentrations of Mg2+ or Cl- on: a) stream macroinvertebrate populations and communities; b) benthic chlorophyll-a and; c) the ecosystem process of leaf litter decomposition. We found a large effect of a high NaHCO3 concentration (≈4.45 mS/cm) with reduced abundances of multiple taxa, reduced emergence of adult insects and reduced species richness, altered community structure and increased leaf litter breakdown rates but no effect on benthic chlorophyll-a. However, despite predictions based on laboratory findings, we found no evidence that the addition of either Mg2+ or Cl- altered the effect of NaHCO3. In semi-natural environments such as mesocosms, and natural environments, organisms are subject to varying temperature and habitat factors, while also interacting with other species and trophic levels (e.g. predation, competition, facilitation), which are absent in single species laboratory tests. Thus, it should not be assumed single-species tests are good predictors of the effects of changing ionic compositions on stream biota in more natural environments.

Microbial-influenced pesticide removal co-occurs with antibiotic resistance gene variation in soil-earthworm-maize system

Within human-influenced landscapes, pesticides cooccur with a variety of antibiotic stressors. However, the relationship between pesticides removal process and antibiotic resistance gene variation are not well understood. This study explored pesticide (topramezone, TPZ) and antibiotic (polymyxin E, PME) co-contamination using liquid chromatography-tandem mass spectrometry (LC-MS/MS), bacterial-16 S rRNA sequencing and high-throughput quantitative polymerase chain reaction (HT-qPCR) in a soil-earthworm-maize system. After incubating soil for 28 days with TPZ and PME (10 mg kg-1 dry weight), earthworm weight-gain, mortality rates, and maize plant weight-gain only differed slightly, but height-gain significantly decreased. PME significantly increased TPZ-removal in the soil. Accumulation of TPZ in earthworm's tissues may pose potential risks in the food chain. Combined pollution altered the microbial community structure and increased the abundance of functional microorganisms involved in aromatic compound degradation. Furthermore, maize rhizosphere can raise resistance genes, however earthworms can reduce resistance genes. Co-contamination increased absolute abundance of mobile genetic elements (MGEs) in bulk-soil samples, antibiotic resistance genes (ARGs) in skin samples and number of ARGs in bulk-soil samples, while decreased absolute abundance of transposase gene in bulk-soil samples and number of ARGs in rhizosphere-soil samples. Potential hosts harbouring ARGs may be associated with the antagonistic effect during resistance and detoxification of TPZ and PMB co-occurrence. These findings provide insights into the mechanism underlining pesticide removal regarding occurrence of ARGs in maize agroecosystem.

A synthesis of anthropogenic stress effects on emergence-mediated aquatic-terrestrial linkages and riparian food webs

Anthropogenic stress alters the linkage between aquatic and terrestrial ecosystems in various ways. Here, we review the contemporary literature on how alterations in aquatic systems through environmental pollution, invasive species and hydromorphological changes carry-over to terrestrial ecosystems and the food webs therein. We consider both the aquatic insect emergence and flooding as pathways through which stressors can propagate from the aquatic to the terrestrial system. We specifically synthesize and contextualize results on the roles of pollutants in the emergence pathway and their top-down consequences. Our review revealed that the emergence and flooding pathway are only considered in isolation and that the overall effects of invasive species or pollutants on food webs at the water-land interface require further attention. While very few recent studies looked at invasive species, a larger number of studies focused on metal transfer compared to pesticides, pharmaceuticals or PCBs, and multiple stress studies up to now left aquatic-terrestrial linkages unconsidered. Recent research on pollutants and emergence used aquatic-terrestrial mesocosms to elucidate the effects of aquatic stressors such as the mosquito control agent Bti, metals or pesticides to understand the effects on riparian spiders. Quality parameters, such as the structural and functional composition of emergent insect communities, the fatty acid profiles, yet also the composition of pollutants transferred to land prove to be important for the effects on riparian spiders. Process-based models including quality of emergence are useful to predict the resulting top-down directed food web effects in the terrestrial recipient ecosystem. In conclusion, we present and recommend a combination of empirical and modelling approaches in order to understand the complexity of aquatic-terrestrial stressor propagation and its spatial and temporal variation.

Short-term insecticide exposure amid co-occurring stressors reduces diversity and densities in north-east Indian experimental aquatic invertebrate communities

Globally, river pesticide concentrations are associated with regional and local stream invertebrate diversity declines. Pesticides often co-occur with elevated nutrients (e.g. nitrogen and phosphorus) and sediments related to agriculture, making their individual effects difficult to disentangle. These effects are also less well studied in Asia, than in other geographic regions. Within Asia, India is one of the largest producers and users of pesticides and has approximately 60% of total land mass used for agriculture. Here we examine the responses of Indian river invertebrate communities subjected to malathion, nutrients, and sediment additions in a semi-orthogonal design, in three sequential (through time) short-term (120 h) mesocosm experiments. Additionally, a series of single-species toxicity tests were run that used 24 h exposure and 72 h recovery to examine the sensitivity of 13 local invertebrate taxa to malathion, and 9 taxa to cypermethrin, comparing these results to those from other biogeographic regions. Mesocosm results indicate that malathion exposure had a major effect compared to other stressors on communities, with a lesser effect of nutrients and/or sediments. In mesocosms, taxa richness, total abundance and the abundance of sensitive species all declined associated with malathion concentrations. Comparisons of organism sensitivities from other geographic locations and those in the current paper suggest taxa in India are relatively tolerant to malathion and cypermethrin. Our results further reinforce that the high observed aquatic pesticide concentrations known to occur in Asian freshwater ecosystems are likely to be negatively affecting biodiversity, homogenising biota towards those most stress tolerant.

Combined effects of heatwaves and micropollutants on freshwater ecosystems: Towards an integrated assessment of extreme events in multiple stressors research

Freshwater ecosystems are strongly influenced by weather extremes such as heatwaves (HWs), which are predicted to increase in frequency and magnitude in the future. In addition to these climate extremes, the freshwater realm is impacted by the exposure to various classes of chemicals emitted by anthropogenic activities. Currently, there is limited knowledge on how the combined exposure to HWs and chemicals affects the structure and functioning of freshwater ecosystems. Here, we review the available literature describing the single and combined effects of HWs and chemicals on different levels of biological organization, to obtain a holistic view of their potential interactive effects. We only found a few studies (13 out of the 61 studies included in this review) that investigated the biological effects of HWs in combination with chemical pollution. The reported interactive effects of HWs and chemicals varied largely not only within the different trophic levels but also depending on the studied endpoints for populations or individuals. Hence, owing also to the little number of studies available, no consistent interactive effects could be highlighted at any level of biological organization. Moreover, we found an imbalance towards single species and population experiments, with only five studies using a multitrophic approach. This results in a knowledge gap for relevant community and ecosystem level endpoints, which prevents the exploration of important indirect effects that can compromise food web stability. Moreover, this knowledge gap impairs the validity of chemical risk assessments and our ability to protect ecosystems. Finally, we highlight the urgency of integrating extreme events into multiple stressors studies and provide specific recommendations to guide further experimental research in this regard.

Sensitivity and specificity of macroinvertebrate responses to gradients of multiple agricultural stressors

Environmental degradation of rivers in agricultural landscapes is typically caused by multiple co-occurring stressors, but how interactions among stressors affect freshwater ecosystems is poorly understood. Therefore, we investigated the sensitivity and specificity of several measures of benthic macroinvertebrate community response to the individual and combined effects of the pesticide sulfoxaflor (SFX), increased sand sedimentation and elevated nutrients using outdoor recirculating mesocosms. Among the single stressor treatments, nutrients had no observable impact and sand only affected one community response measure compared to controls. High SFX levels had the largest effects on benthic macroinvertebrate communities, negatively affecting six of seven macroinvertebrate response measures. Sulfoxaflor had similar adverse effects on biota when in combination with sand and nutrients in the multi-stressor treatment, suggesting that generally SFX has overwhelming and pervasive effects irrespective of the presence of the other stressors. In contrast to SFX, elevated nutrients had no detectable effect on macroinvertebrate communities, likely as a consequence of nutrients being rapidly taken up by bacteria rather than by benthic algae. Elevated sand sedimentation increased the negative effects of SFX on sediment sensitive taxa, but generally had limited biological effects. This was despite the levels of sedimentation in our treatments being at concentrations that have caused large impacts in other studies. This research points to direct and rapid toxic effects of SFX on stream macroinvertebrates, contrasting with effects of the other stressors. This study emphasises that pesticide effects could be misattributed to other common freshwater stressors, potentially focussing restoration actions on a stressor of lesser importance.

Sub-organism (acetylcholinesterase activity), population (survival) and chemical concentration responses reinforce mechanisms of antagonism associated with malathion toxicity

Within human modified ecosystems the effects of individual stressors are difficult to establish amid co-occurring biological processes, environmental gradients and other stressors. Coupled examination of several endpoints across different levels of organisation may help elucidate the individual and combined effects of stressors and interactions. Malathion is a commonly used organophosphate pesticide that contaminates freshwaters and has strong negative effects on aquatic biota. However, both other stressors (e.g. increased sediment) and common ecosystem components (e.g. macrophytes and variable pH) can reduce the aqueous concentrations of malathion, reducing its toxic effects. We conducted a fully orthogonal bioassay to examine how pH (at 7 and 7.8) and sorptive processes (across two levels of kaoline clay 0 and 24 g L^-1) affected aqueous malathion concentrations and toxicity in an aquatic invertebrate genus. Survival and acetylcholinesterase activity as a sub-organism response were examined in the mayfly Coloburiscoides spp. (Ephemeroptera; Coluburiscidae). Measured aqueous malathion concentrations decreased with increased pH and in the presence of kaolin clay. Survival declined with increasing malathion concentrations and exposure period. Results further identify that antagonism of malathion toxicity was associated with both pH (alkaline hydrolysis) and effects associated with sediment independent of pH (driven by sorptive processes). However, model predictions varied associated with target and measured concentrations and concentrations examined. Antagonistic effects were most apparent using subset target malathion concentrations because of the dominant effect of malathion at high concentrations. Acetylcholinesterase activity, identified repression occurred across all treatments and did not identify antagonistic interactions, but these results were similar to survival responses at the time points examined (i.e. 120 h). Examination of chemistry, acetylcholinesterase, and survival, affords greater understanding of stressor effects and their interactions. Measured malathion concentrations may underestimate effects on aquatic biota; not because of synergism among stressors, but because of strong effects despite antagonism.

Eutrophic status influences the impact of pesticide mixtures and predation on Daphnia pulex populations

Pesticides, nutrients, and ecological stressors such as competition or predation co-occur in freshwater ecosystems impacted by agriculture. The extent to which combinations of these stressors affect aquatic populations and the role of nutrients availability in modulating these responses requires further understanding. In this study, we assessed how pesticides affecting different taxonomic groups and predation influence the response of Daphnia pulex populations under different trophic conditions. An outdoor experiment was designed following a factorial design, with the insecticide chlorpyrifos, the herbicide diuron, and the predation by Notonecta sp. individuals as key stressors. The single impact of each of these stressors, and their binary and tertiary combinations, was evaluated on D. pulex abundance and population structure under mesotrophic and eutrophic conditions for 21 days. Data were analyzed using generalized linear mixed models estimated by means of a novel Bayesian shrinkage technique. Our study shows a significant influence of each of the evaluated stressors on D. pulex abundance; however, the impacts of the herbicide and predation were lower under eutrophic conditions as compared to the mesotrophic ones. We found that binary stressor interactions were generally additive in the mesotrophic scenario, except for the herbicide-predation combination, which resulted in synergistic effects. The impacts of the binary stressor combinations in the eutrophic scenario were classified as antagonistic, except for the insecticide-herbicide combination, which was additive. The tertiary interaction resulted in significant effects on some sampling dates; however, these were rather antagonistic and resembled the most important binary stressor combination in each trophic scenario. Our study shows that the impact of pesticides on freshwater populations depends on the predation pressure, and demonstrates that the combined effect of pesticides and ecological stressors is influenced by the food availability and organism fitness related to the trophic status of freshwater ecosystems.

Can SPEcies At Risk of pesticides (SPEAR) indices detect effects of target stressors among multiple interacting stressors?

Pesticides are increasingly recognised as a threat to freshwater biodiversity, but their specific ecological effects remain difficult to distinguish from those of co-occurring stressors and environmental gradients. Using mesocosms we examined the effects of an organophosphate insecticide (malathion) on stream macroinvertebrate communities concurrently exposed to a suite of stressors typical of streams in agricultural catchments. We assessed the specificity of the SPEcies At Risk index designed to determine pesticide effects in mesocosm trials (SPEAR_mesocosm). This index determines the log abundance proportion of taxa that are considered physiologically sensitive to pesticides. Geographic variation in pesticide sensitivity within taxa, coupled with variation between pesticides and the effects of co-occurring stressors may decrease the accuracy of SPEAR_mesocosm. To examine this, we used local pesticide sensitivity assessments based on rapid toxicity tests to develop two new SPEAR versions to compare to the original SPEAR_mesocosms index using mesocosm results. We further compared these results to multivariate analyses and community indices (e.g. richness, abundance, Simpson's diversity) commonly used to assess stressor effects on biota. To assess the implications of misclassifying species sensitivity on SPEAR indices we used a series of simulations using artificial data. The impacts of malathion were detectable using SPEAR_mesocosm, and one of two new SPEAR indices. All three of the SPEAR indices also increased when exposed to other agricultural non-pesticide stressors, and this change increased with greater pesticide concentrations. Our results support that interactions between other non-pesticide stressors with pesticides can affect SPEAR performance. Multivariate analysis and the other indices used here identified a significant effect of malathion especially at high concentrations, with little or no evidence of effects from the other agricultural stressors.

Repeated insecticide pulses increase harmful effects on stream macroinvertebrate biodiversity and function

We exposed twelve mesocosm stream channels and four instream channels to one, two, and four pulses of the insecticide lambda-cyhalothrin (0.1 μg L-1) applied at two day intervals, each pulse lasting 90 min. Unexposed controls were included. We monitored macroinvertebrate taxonomic composition in the channels and in deployed leaf packs one day before and 29 days after the first exposure. Further, we measured drift in and out of the channels and leaf litter decomposition. Lambda-cyhalothrin exposures induced significantly increased drift in both experiments especially for Gammarus pulex, Amphinemura standfussi, and Leuctra spp. Macroinvertebrate taxonomic composition increasingly changed with increasing number of lambda-cyhalothrin exposures being most pronounced in the mesocosm channels. Further, leaf decomposition significantly decreased with increasing number of exposures in the mesocosm channels. Our study showed that species with predicted highest sensitivity to lambda-cyhalothrin were primary drivers of significant changes in taxonomic composition lasting for at least one month despite continuous recolonization of exposed channels from upstream parts of the natural stream and from the water inlet in the mesocosm channels. The overall results highlight the importance of sequential exposures to insecticides for understanding the full impact of insecticides on macroinvertebrates at the community level in streams.