Chemical Mixtures and Multiple Stressors: Same but Different?

Beyond Dosage: The Need for More Realistic Research Scenarios to Understand Pesticide Impacts on Agricultural Soils

Pesticides play a crucial role in modern agriculture, yet they pose considerable risks to soil health and ecosystem integrity. Current risk assessment research often relies on simplified models, focusing on single substances under standardized conditions and failing to reflect realistic exposure scenarios. We call for a paradigm shift toward incorporating agroecological research that evaluates pesticide effects under more complex and dynamic conditions, including mixtures, application frequency, diverse soil properties, and interactions with other environmental stressors. Additionally, multiseasonal exposure and long-term persistence of pesticides in soils must be considered. By integrating these multidimensional factors, such experimental research can yield valuable insights that improve environmental risk assessment frameworks, ensuring they more accurately represent the complexity of agricultural systems and support more sustainable soil management practices.

Heatwaves, elevated temperature, and insecticide-induced effects at different trophic levels of a freshwater ecosystem

Heatwaves and increasing average temperatures associated with climate change pose severe stress on nature, including freshwater ecosystems. As these thermal stressors do not act in isolation over temporal or spatial scales, interactions with other stressors, like pesticides, may lead to unpredictable combined effects. Empirical studies investigating multiple stressor effects across different trophic levels are scarce and often lack environmental realism. Here, we performed a multiple stressor experiment using outdoor freshwater mesocosms and realistic pond assemblages including microbes, phytoplankton, macrophytes, and invertebrates. The effects of the pesticide imidacloprid at three dosings (0, 1, 10 μg/L) were examined in combination with three temperature scenarios comprising natural ambient, elevated temperatures (+4 °C), and repeated heatwaves (+8 °C). Our results reveal fast imidacloprid dissipation for all temperature treatments with the lowest average dissipation half-lives (DT50: 6 days) in the heatwave treatment. Imidacloprid induced a series of significant effects on the macroinvertebrate community at 10 μg/L across the temperature treatments. Significant declines in abundance appeared throughout the experiment for the most sensitive taxa Cloeon dipterum, Caenis sp., Chironomini, and Dero sp., whereas significant imidacloprid effects on Tanytarsini, Chironomus, and Gammarus pulex occurred only after each dosing. Only Zygoptera showed an imidacloprid-related increase in abundance, whereas significantly adverse time-cumulative effects on abundance occurred for Asellus aquaticus. The zooplankton community showed imidacloprid tolerance (10 μg/L) with increasing abundances of Chydorus sphaericus, Acroperus harpae, and Ascomorpha. Heatwaves induced significantly meliorating effects on Dero sp. and adverse effects on Caenis sp., Tanytarsini, G. pulex, and A. harpae. The macroinvertebrate community demonstrated faster post-exposure recovery dynamics in the highest imidacloprid treatment when previously exposed to heatwaves and elevated temperatures compared to ambient conditions. Overall, heatwaves amplified the effects of imidacloprid on the invertebrate community, manifesting in manifold effects that adversely impacted multiple trophic levels.

Proposal of novel Predicted No Effect Concentrations (PNEC) for metals in freshwater using Species Sensitivity Distribution for different taxonomic groups

Water pollution by metals and metalloids promotes toxic effects to aquatic biota especially in mining regions. Environmental legislation applied to protect aquatic life from the toxicity of metals relies on the definition of protective values (PVs) for each compound. Among methods used to define PVs, Species Sensitivity Distribution (SSD) curves enable the derivation of the Predicted No Effect concentration (PNEC). In this context, this is one of the first studies to propose the construction of acute and chronic split SSD curves built separately for three groups of freshwater organisms (algae, invertebrates and fish) to derive PNEC values for the 14 metals most commonly observed in iron ore mining tailings. Data used to construct split SSD curves were derived from the USEPA ECOTOX knowledgebase and EnviroTox databases and segregated according to the freshwater organism group and as "acute" or "chronic" toxicity. Then, split SSD curves were built using a minimum of nine species for each group to determine the hazardous concentration to 5% of species (HC5) and PNEC values for each group. Once PNEC were derived, a framework was proposed to calculate the Bioavailabity Factor (BioF) used to adjust values for local bioavailability conditions considering water quality characteristics in different regions. The lowest acute PNEC were observed for algae and invertebrates and corresponded to Silver (Ag). Nearly half of calculated PNEC were below current PVs in practice in Brazil, United States (US), United Kingdom (UK), Canada and European Union (EU). Results reinforce the pertinence of: (i) splitting SSD curves to define PVs for metals; and (ii) taking bioavailability into consideration to correct PNEC for local conditions. In addition, outcomes suggest that it is critical to rethink PVs related to metals for aquatic life protection, mainly in Brazil and Minas Gerais state, a region known for extensive mining activity. Finally, PNEC values obtained in this study may be used for ecological risk assessment studies, especially in areas affected by mining and other activities that result in pollution by metals and metalloids, such as Brazil.

Reconciling Variability in Multiple Stressor Effects Using Environmental Performance Curves

Understanding the effects of multiple stressors has become a major focus in ecology and evolution. While many studies have investigated the combined effects of stressors, revealing massive variability, a mechanistic understanding that reconciles the diversity of multiple stressor outcomes is lacking. Here, we show how performance curves can fill this gap by revealing mechanisms that shape multiple stressor outcomes. Our experiments with 12 bacterial taxa, demonstrate that additional stressors alter the shape of temperature, pH and salinity performance curves. This leads to changes in stressor interaction outcomes-for example, shifts between additive, antagonistic or synergistic interactions-along gradients, revealing that small changes in a stressor along nonlinear performance curves can dramatically impact the stressor interaction. These findings help to explain the lack of generality found across multiple stressor studies and highlight how a performance curve approach can provide a more holistic view of multiple stressor interactions.

Environmental realism in molecular ecotoxicology: key considerations to transition experimental data to ecologically relevant scenarios

Molecular ecotoxicology facilitates the mechanistic understanding of chemical-organism interactions and the establishment of frameworks to link molecular events to adverse outcomes. However, the foundation of this sub-discipline must remain focused on the necessity to generate insight at levels of biological organization beyond the individual, namely the population, community, and ecosystem levels, and to strive towards ecological relevance. As planet Earth continues to experience unprecedented levels of chemical pollution, causing significant impact to the integrity and functionality of ecosystems, research efforts in molecular ecotoxicology must prioritize experimentation that quantitatively incorporates the influence of non-chemical stressors to enhance the predictability of chemical-driven effects at the population level and beyond. Here, perspectives on the challenge to transition experimental data to environmentally relevant scenarios are offered in an attempt to highlight the critical role of molecular ecotoxicology in protecting and supporting ecosystems threatened by chemical pollution.

The impact of repeated pyrethroid pulses on aquatic communities

Pesticides are considered to be one of the main causes of the decline in macroinvertebrate biodiversity in small streams. In particular, pyrethroids are detected in agricultural surface waters worldwide and pose a high risk to aquatic invertebrates. Due to their knock-down effect, even short pyrethroid exposure pulses can have significant short- and long-term effects on macroinvertebrate communities. Therefore, it is necessary to consider more realistic exposure scenarios for the environmental risk assessment of pyrethroids and, consequently, to obtain more realistic effect data by using multi-stressor test systems. In an experimental setup with artificial indoor streams (AIS), four pyrethroid pulses simulated the exposure scenario of heavy rainfall events. Effects of these 12 h-exposures at different concentrations of deltamethrin (0.64 ng/L, 4 ng/L, 16 ng/L, 64 ng/L) with intervening recovery periods of six days were assessed on an aquatic community consisting of Gammarus pulex, Ephemera danica, Lumbriculus variegatus and Potamopyrgus antipodarum with various lethal and sub-lethal endpoints. The mortality rate of G. pulex significantly increased with increasing deltamethrin concentrations, whereas the mean number of offspring significantly decreased (NOECoffspring: 16 ng/L, LOECoffspring: 64 ng/L). The biomass of L. variegatus decreased with increasing deltamethrin concentrations (NOECdry weight: 16 ng/L, LOECdry weight: 64 ng/L). The findings of this study clearly demonstrate that 12 h-deltamethrin pulses at environmentally relevant concentrations adversely affect an aquatic community. Based on the results of this study a RAC value of 5.33 ng/L is assumed, which is below the concentrations measured in rivers of up to 58.8 ng/L. Unacceptable effects on the entire freshwater environment can therefore not be ruled out. The experimental AIS approach is a useful tool for assessing the effects of repeated pulse exposures that occur during surface runoff events.

Unraveling the molecular mechanisms of fish physiological response to freshwater salinization: A comparative multi-tissue transcriptomic study in a river polluted by potash mining

Freshwater salinization is an escalating global environmental issue that threatens freshwater biodiversity, including fish populations. This study aims to uncover the molecular basis of salinity physiological responses in a non-native minnow species (Phoxinus septimaniae x P. dragarum) exposed to saline effluents from potash mines in the Llobregat River, Barcelona, Spain. Employing high-throughput mRNA sequencing and differential gene expression analyses, brain, gills, and liver tissues collected from fish at two stations (upstream and downstream of saline effluent discharge) were examined. Salinization markedly influenced global gene expression profiles, with the brain exhibiting the most differentially expressed genes, emphasizing its unique sensitivity to salinity fluctuations. Pathway analyses revealed the expected enrichment of ion transport and osmoregulation pathways across all tissues. Furthermore, tissue-specific pathways associated with stress, reproduction, growth, immune responses, methylation, and neurological development were identified in the context of salinization. Rigorous validation of RNA-seq data through quantitative PCR (qPCR) underscored the robustness and consistency of our findings across platforms. This investigation unveils intricate molecular mechanisms steering salinity physiological response in non-native minnows confronting diverse environmental stressors. This comprehensive analysis sheds light on the underlying genetic and physiological mechanisms governing fish physiological response in salinity-stressed environments, offering essential knowledge for the conservation and management of freshwater ecosystems facing salinization.

Putting the Asymmetric Response Concept to the test: Modeling multiple stressor exposure and release in a stream food web

Communities in stream ecosystems often respond asymmetrically to increase and release of stressors, as indicated by slow and incomplete recovery. The Asymmetric Response Concept (ARC) posits that this is due to a shift in the relative importance of three mechanisms: tolerance, dispersal, and biotic interactions. In complex natural communities, these mechanisms may produce alternative outcomes through poorly understood indirect effects. To understand how the three mechanisms respond to different temporal stressor scenarios, we studied multiple scenarios using a stream food web model. We asked the following questions: Do groups of species decline as expected on the basis of individual tolerance rankings derived from laboratory experiments when they are embedded in a complex dynamic food web? Does the response of ecosystem function match that of communities? To address these questions, we aggregated data on individual tolerances at the level of functional groups and studied how single and multiple stressors affect food web dynamics and nutrient cycling. Multiple stressor scenarios involved different intensities of salt and temperature increase. Functional groups exhibited a different relative tolerance ranking between the laboratory and dynamic food web contexts. Salt as a single stressor had only minor and transient effects at low level but led to the loss of one or more functional groups at high level. In contrast, high temperature, alone or in combination with salt, caused the loss of functional groups at all tested levels. Patterns often differed between the response of communities and ecosystem function. We discuss our findings with respect to the ARC.

Temporal patterns in multiple stressors shape the vulnerability of overwintering Arctic zooplankton

The Arctic polar nights bring extreme environmental conditions characterised by cold and darkness, which challenge the survival of organisms in the Arctic. Additionally, multiple anthropogenic stressors can amplify the pressure on the fragile Arctic ecosystems during this period. Determining how multiple anthropogenic stressors may affect the survival of Arctic life is crucial for ecological risk assessments and management, but this topic is understudied. For the first time, our study investigates the complex interactions of multiple stressors, exploring stressor temporal dynamics and exposure duration on a key Arctic copepod Calanus glacialis during the polar nights. We conducted experiments with pulse (intermittent) and press (continuous) exposure scenarios, involving microplastics, pyrene and warming in a fully factorial design. We observed significant effects on copepod survival, with pronounced impacts during later stressor phases. We also detected two-way interactions between microplastics and pyrene, as well as pyrene and warming, further intensified with the presence of a third stressor. Continuous stressor exposure for 9 days (press-temporal scenario) led to greater reductions in copepod survival compared to the pulse-temporal scenario, characterised by two 3-day stressor exposure phases. Notably, the inclusion of recovery phases, free from stressor exposure, positively influenced copepod survival, highlighting the importance of temporal exposure dynamics. We did not find behaviour to be affected by the different treatments. Our findings underscore the intricate interactions amongst multiple stressors and their temporal patterns in shaping the vulnerability of overwintering Arctic copepods with crucial implications for managing Arctic aquatic ecosystems under the fastest rate of ongoing climate change on earth.

Studying interactions among anthropogenic stressors in freshwater ecosystems: A systematic review of 2396 multiple-stressor experiments

Understanding the interactions among anthropogenic stressors is critical for effective conservation and management of ecosystems. Freshwater scientists have invested considerable resources in conducting factorial experiments to disentangle stressor interactions by testing their individual and combined effects. However, the diversity of stressors and systems studied has hindered previous syntheses of this body of research. To overcome this challenge, we used a novel machine learning framework to identify relevant studies from over 235,000 publications. Our synthesis resulted in a new dataset of 2396 multiple-stressor experiments in freshwater systems. By summarizing the methods used in these studies, quantifying trends in the popularity of the investigated stressors, and performing co-occurrence analysis, we produce the most comprehensive overview of this diverse field of research to date. We provide both a taxonomy grouping the 909 investigated stressors into 31 classes and an open-source and interactive version of the dataset (https://jamesaorr.shinyapps.io/freshwater-multiple-stressors/). Inspired by our results, we provide a framework to help clarify whether statistical interactions detected by factorial experiments align with stressor interactions of interest, and we outline general guidelines for the design of multiple-stressor experiments relevant to any system. We conclude by highlighting the research directions required to better understand freshwater ecosystems facing multiple stressors.

Timing determines zooplankton community responses to multiple stressors

Human activities and climate change cause abiotic factors to fluctuate through time, sometimes passing thresholds for organismal reproduction and survival. Multiple stressors can independently or interactively impact organisms; however, few studies have examined how they interact when they overlap spatially but occur asynchronously. Fluctuations in salinity have been found in freshwater habitats worldwide. Meanwhile, heatwaves have become more frequent and extreme. High salinity pulses and heatwaves are often decoupled in time but can still collectively impact freshwater zooplankton. The time intervals between them, during which population growth and community recovery could happen, can influence combined effects, but no one has examined these effects. We conducted a mesocosm experiment to examine how different recovery times (0-, 3-, 6-week) between salt treatment and heatwave exposure influence their combined effects. We hypothesized that antagonistic effects would appear when having short recovery time, because previous study found that similar species were affected by the two stressors, but effects would become additive with longer recovery time since fully recovered communities would respond to heatwave similar to undisturbed communities. Our findings showed that, when combined, the two-stressor joint impacts changed from antagonistic to additive with increased recovery time between stressors. Surprisingly, full compositional recovery was not achieved despite a recovery period that was long enough for population growth, suggesting legacy effects from earlier treatment. The recovery was mainly driven by small organisms, such as rotifers and small cladocerans. As a result, communities recovering from previous salt exposure responded differently to heatwaves than undisturbed communities, leading to similar zooplankton communities regardless of the recovery time between stressors. Our research bolsters the understanding and management of multiple-stressor issues by revealing that prior exposure to one stressor has long-lasting impacts on community recovery that can lead to unexpected joint effects of multiple stressors.

Fish predation affects invertebrate community structure of tropical temporary ponds, with downstream effects on phytoplankton that are obscured by pesticide pollution

Aquatic biota of tropical temporary ponds typically experience a wide range of stressors that can drive the structure and dynamics of natural communities. Particularly in regions with intense agricultural activity, aquatic biota may not only experience predation pressure but also stress from pesticides that inadvertently enter the ponds. We increasingly understand how these different sources of stress affect classic model taxa under controlled laboratory conditions, but how predators and pesticides may jointly affect pond invertebrate communities is still unclear, particularly for tropical systems. Here, we conducted an outdoor mesocosm experiment to study how fish predation combined with exposure to an environmentally relevant concentration of the commonly used insecticide cypermethrin (0.8 ng/L) affects the structure of invertebrate communities, and its potential effects on leaf litter decomposition and invertebrate grazing efficiency as measures of ecosystem functioning. A total of seven invertebrate taxa were recorded in the mesocosm communities. Fish predation effectively lowered the number of invertebrate taxa, with fish mesocosms being dominated by high densities of rotifers, associated with lower phytoplankton levels, but only when communities were not simultaneously exposed to cypermethrin. In contrast, cypermethrin exposure did not affect invertebrate community structure, and neither fish predation nor cypermethrin exposure affected our measures of ecosystem functioning. These findings suggest that predation by killifish can strongly affect invertebrate community structure of tropical temporary ponds, and that downstream effects on phytoplankton biomass can be mediated by exposure to cypermethrin. More broadly, we contend that a deeper understanding of (tropical) temporary pond ecology is necessary to effectively manage these increasingly polluted systems.

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.

To Split or Not to Split: Characterizing Chemical Pollution Impacts in Aquatic Ecosystems with Species Sensitivity Distributions for Specific Taxonomic Groups

Bridging applied ecology and ecotoxicology is key to protect ecosystems. These disciplines show a mismatch, especially when evaluating pressures. Contrasting to applied ecology, ecotoxicological impacts are often characterized for whole species assemblages based on Species Sensitivity Distributions (SSDs). SSDs are statistical models describing per chemical across-species sensitivity variation based on laboratory toxicity tests. To assist in the aligning of the disciplines and improve decision-support uses of SSDs, we investigate taxonomic-group-specific SSDs for algae/cyanobacteria/aquatic plants, invertebrates, and vertebrates for 180 chemicals with sufficient test data. We show that splitting improves pollution impact assessments for chemicals with a specific mode of action and, surprisingly, for narcotic chemicals. We provide a framework for splitting SSDs that can be applied to serve in environmental protection, life cycle assessment, and management of freshwater ecosystems. We illustrate that using split SSDs has potentially large implications for the decision-support of SSD-based outputs around the globe.