Multiple Chemical Stressors Research Articles

A perspective on how glyphosate and 2,4-D in wetlands may impact climate change

An increase in herbicide use is occurring due to a growing population and herbicide-resistant crops in agriculture, which has resulted in more herbicide tolerant target species. Glyphosate and 2,4-Dichlorophenoxyacetic acid (2,4-D) are two of the most commonly used herbicides worldwide and are more recently being used in combination in pre-mixed commercial formulas. Subsequently, herbicide contamination of wetlands will increase exposure of microorganisms to multiple chemical stressors. Methane is a potent greenhouse gas naturally emitted from wetlands, but herbicides may disrupt biogeochemical processes leading to an unbalanced methane cycle. We review the impacts of these herbicides on aquatic microbial communities from glyphosate-derived nutrient enrichment and 2,4-D inhibition of methane oxidation, and examine how these altered metabolic processes may lead to increased methane production in wetlands. The response of wetland ecosystems to herbicide contamination will vary across regions, in part due to the complexity of microbial communities, however, this perspective gives a glimpse into the potential global implications of continuing herbicide use on wetlands and demonstrates the importance for research on ecosystem-level co-stressors.

Divergence in heritable life history traits suggests potential for local adaptation and trade-offs associated with a coal ash disposal site.

Globally, human activities have resulted in rapid environmental changes that present unique challenges for wildlife. However, investigations of local adaptation in response to simultaneous exposure to multiple anthropogenic selection pressures are rare and often generate conflicting results. We used an in situ reciprocal transplant design within a quantitative genetic framework to examine how adaptive evolution and phenotypic plasticity contribute to the persistence of an amphibian population inhabiting an environment characterized by high levels of multiple toxic trace elements. We found evidence of phenotypic divergence that is largely consistent with local adaptation to an environment contaminated with multiple chemical stressors, tied to potential trade‐offs in the absence of contaminants. Specifically, the population derived from the contaminated environment had a reduced risk of mortality and greater larval growth and in the contaminated environment, relative to offspring from the naïve population. Further, while survival in the uncontaminated environment was not compromised in offspring from the contaminant‐exposed population, they did show delayed development and reduced growth rates over larval development, relative to the naïve population. We found no evidence of reduced additive genetic variation in the contaminant‐exposed population, suggesting long‐term selection in a novel environment has not reduced the evolutionary potential of that population. We also saw little evidence that past selection in the ASH environment had reduced trait plasticity in the resident population. Maternal effects were prominent in early development, but we did not detect any trends suggesting these effects were associated with the maternal transfer of toxic trace elements. Our results demonstrate the potential for adaptation to multiple contaminants in a wild amphibian population, which may have facilitated long‐term persistence in a heavily impacted environment.

Biological effects of multimetal (Ni, Cd, Pb, Cu, Cr, Zn) mixture in rainbow trout Oncorhynchus mykiss: Laboratory exposure and recovery study

The present study tested the biological consequences of exposure to a multimetal mixture as a multiple chemical stressor on Oncorhynchus mykiss at molecular, cellular, physiological and whole-organism levels and on biomarker responses of this fish during the depuration period. To represent environmentally relevant multiple chemical stressors, in our study, we used the mixture of Zn, Cu, Ni, Cr, Pb and Cd at the concentrations corresponding to Maximum-Permissible-Concentrations (MPCs) acceptable for the EU inland waters. This study was undertaken with a view to elucidate if changes in the MPC of the test mixture components (Ni, Pb, Cd) could cause significantly different biomarker responses in O. mykiss from those previously determined in the carnivorous and omnivorous fishes exposed to the mixture of the same metals but at different MPCs of Ni, Pb and Cd. This study has revealed that exposure to mixtures of metals at MPC produces genotoxic effects in fish blood erythrocytes and a lethargic effect on O. mykiss behaviour, and, also, significantly increases the levels of Cd, Cr and Ni accumulated in the gills tissue. O. mykiss successfully depurated Cr and Ni in less than 28 days, however, the level of Cd decreased by only approximately 40% over the same period. A significant capacity of O. mykiss to restore its DNA integrity (Comet assay) after exposure to metal mixtures was revealed. However, the 28-day recovery period proved to be insufficiently long for erythrocytes with nuclear abnormalities to recover to the unexposed level. In conclusion, changes in the MPCs of Ni, Pb and Cd in the test mixture produce biological effects similar to those previously determined in S. salar, R. rutilus and P. fluviatilis exposed to the mixture of the same metals but at lower MPCs of Ni and Pb and at higher MPC of Cd.

Assessing the chemical contamination dynamics in a mixed land use stream system

Traditionally, the monitoring of streams for chemical and ecological status has been limited to surface water concentrations, where the dominant focus has been on general water quality and the risk for eutrophication. Mixed land use stream systems, comprising urban areas and agricultural production, are challenging to assess with multiple chemical stressors impacting stream corridors. New approaches are urgently needed for identifying relevant sources, pathways and potential impacts for implementation of suitable source management and remedial measures. We developed a method for risk assessing chemical stressors in these systems and applied the approach to a 16-km groundwater-fed stream corridor (Grindsted, Denmark). Three methods were combined: (i) in-stream contaminant mass discharge for source quantification, (ii) Toxic Units and (iii) environmental standards. An evaluation of the chemical quality of all three stream compartments – stream water, hyporheic zone, streambed sediment – made it possible to link chemical stressors to their respective sources and obtain new knowledge about source composition and origin. Moreover, toxic unit estimation and comparison to environmental standards revealed the stream water quality was substantially impaired by both geogenic and diffuse anthropogenic sources of metals along the entire corridor, while the streambed was less impacted. Quantification of the contaminant mass discharge originating from a former pharmaceutical factory revealed that several 100 kgs of chlorinated ethenes and pharmaceutical compounds discharge into the stream every year. The strongly reduced redox conditions in the plume result in high concentrations of dissolved iron and additionally release arsenic, generating the complex contaminant mixture found in the narrow discharge zone. The fingerprint of the plume was observed in the stream several km downgradient, while nutrients, inorganics and pesticides played a minor role for the stream health. The results emphasize that future investigations should include multiple compounds and stream compartments, and highlight the need for holistic approaches when risk assessing these dynamic systems.

Interactive effects of an insecticide and a fungicide on different organism groups and ecosystem functioning in a stream detrital food web

Freshwater ecosystems are often affected by cocktails of multiple pesticides targeting different organism groups. Prediction and evaluation of the ecosystem-level effects of these mixtures is complicated by the potential not only for interactions among the pesticides themselves, but also for the pesticides to alter biotic interactions across trophic levels. In a stream microcosm experiment, we investigated the effects of two pesticides targeting two organism groups (the insecticide lindane and fungicide azoxystrobin) on the functioning of a model stream detrital food web consisting of a detritivore (Ispoda: Asellus aquaticus) and microbes (an assemblage of fungal hyphomycetes) consuming leaf litter. We assessed how these pesticides interacted with the presence and absence of the detritivore to affect three indicators of ecosystem functioning – leaf decomposition, fungal biomass, fungal sporulation – as well as detritivore mortality. Leaf decomposition rates were more strongly impacted by the fungicide than the insecticide, reflecting especially negative effects on leaf processing by detritivores. This result most like reflects reduced fungal biomass and increased detritivore mortality under the fungicide treatment. Fungal sporulation was elevated by exposure to both the insecticide and fungicide, possibly representing a stress-induced increase in investment in propagule dispersal. Stressor interactions were apparent in the impacts of the combined pesticide treatment on fungal sporulation and detritivore mortality, which were reduced and elevated relative to the single stressor treatments, respectively. These results demonstrate the potential of trophic and multiple stressor interactions to modulate the ecosystem-level impacts of chemicals, highlighting important challenges in predicting, understanding and evaluating the impacts of multiple chemical stressors on more complex food webs in situ.

Endocrine and AhR-CYP1A Pathway Responses to the Water-Soluble Fraction of Oil in Zebrafish (Danio rerio Hamilton)

Crude oil is a complex mixture of compounds of which the water-soluble fraction (WSF) is considered to be bioavailable and potentially toxic to aquatic biota. Containing numerous compounds, WSF becomes a source of multiple chemical stressors to wildlife when introduced into the environment. To study the combined effects of WSF components on aquatic biota, the model species zebrafish (Danio rerio Hamilton) was exposed for 24 or 72 h to 10 or 50% WSF solution of known composition, generated from artificially weathered North Sea crude oil. Hepatic expression of genes involved in the aryl hydrocarbon receptor-cytochrome P-450 1A (AhR-CYP1A) pathway (AhR2, AhRR1, CYP1A1) and steroidogenesis (StAR, CYP11A, 3β-HSD, CYP19A, CYP19B) was measured, as well as estrogen receptors ERα and ERβ1. Induction of CYP1A and particularly of AhRR1 was observed while ERα and steroidogenic enzymes CYP11A and 3β-HSD were downregulated. Regression analysis demonstrated a negative relationship between AhR-CYP1A pathway and endocrine transcript levels, although causality remains to be established. These findings indicate that exposure to WSF of oil disrupts steroidogenesis and may therefore constitute a potential risk for reproductive ability of aquatic organisms. In addition, it is proposed that hepatic gene expression of AhRR1 may serve as a novel biomarker of WSF exposure.

INTEGRATING CHEMICAL AND BIOLOGICAL STATUS ASSESSMENT: ASSEMBLING LINES OF EVIDENCE FOR THE EVALUATION OF RIVER ECOSYSTEM RISK

This paper describes various approaches to evaluating ecological risk in rivers affected by multiple chemical stressors, with emphasis on biofilm and invertebrate community responses. Metrics should be considered as different lines of evidence that, when weighted, form an advanced weight of evidence approach to establishing the environmental risk on a basin scale. Combination of field surveys to obtain observational data of communities, in situ experiments, toxicological sediment tests and a good chemical description of the medium (water and sediment) helps give an integrative view of the chemical and biological state of a river ecosystem. The greater and more distinct the variables used, the greater is our ability to identify the effects of major stressors impairing communities. Weight of evidence is an integrative methodology for tackling the challenge of determining causal relations and applying this knowledge in the decision-making processes of river management.

Tolerance of native and non-native fish species to chemical stress: a case study for the River Rhine

Freshwater ecosystems can be impacted by invasive species. Non-native species can become invasive due to their high tolerance to environmental stressors (e.g., pollution and habitat modifications). Yet, tolerance of native and non-native fish species exposed simultaneously to multiple chemical stressors has not been investigated. To quantify tolerance of native and non-native fish species in the Delta Rhine to 21 chemical stressors we derived Species Sensitivity Distributions (SSDs). Differences in tolerance between the two species groups to these stressors were not statistically significant. Based on annual maximum water concentrations of nine chemical stressors in the Delta Rhine the highest contribution to the overall Potentially Affected Fraction (PAF) of both species groups was noted for ammonium, followed by azinphos-methyl, copper, and zinc. PAFs of both groups for metals and ammonium showed a significant linear decrease over the period 1978–2010. Deriving a PAF for each species group was a useful tool for identifying stressors with a relatively highest impact on species of concern and can be applied to water pollution control. Species traits such as tolerance to chemical stress cannot explain the invasiveness of some fish species. For management of freshwater ecosystems potentially affected by non-native species, attention should be given also to temperature, hydrological regimes, and habitat quality.

Ranking ecological risks of multiple chemical stressors on amphibians

Populations of amphibians have been declining worldwide since the late 1960s. Despite global concern, no studies have quantitatively assessed the major causes of this decline. In the present study, species sensitivity distributions (SSDs) were developed to analyze the sensitivity of anurans for ammonium, nitrate, heavy metals (cadmium, copper), pesticides (18 compounds), and acidification (pH) based on laboratory toxicity data. Ecological risk (ER) was calculated as the probability that a measured environmental concentration of a particular stressor in habitats where anurans were observed would exceed the toxic effect concentrations derived from the species sensitivity distributions. The assessment of ER was used to rank the stressors according to their potential risk to anurans based on a case study of Dutch freshwater bodies. The derived ERs revealed that threats to populations of anurans decreased in the sequence of pH, copper, diazinon, ammonium, and endosulfan. Other stressors studied were of minor importance. The method of deriving ER by combining field observation data and laboratory data provides insight into potential threats to species in their habitats and can be used to prioritize stressors, which is necessary to achieve effective management in amphibian conservation.

Maximum Cumulative Ratio (MCR) as a Tool for Assessing the Value of Performing a Cumulative Risk Assessment

Due to the vast number of possible combinations of chemicals to which individuals are exposed and the resource-intensive nature of cumulative risk assessments, there is a need to determine when cumulative assessments are most required. This paper proposes the use of the maximum cumulative ratio (MCR) as a tool for this evaluation. MCR is the ratio of the cumulative toxicity received by an individual from exposure to multiple chemical stressors to the largest toxicity from a single chemical stressor. The MCR is a quantitative measure of the difference in an individual’s toxicity estimated using a chemical-by-chemical approach and using an additive model of toxicity. As such, it provides a conservative estimate of the degree to which individuals’ toxicities could be underestimated by not performing a cumulative risk assessment. In an example application, MCR is shown to be applicable to the evaluation of cumulative exposures involving up to 81 compounds and to provide key insights into the cumulative effects posed by exposures to multiple chemicals. In this example, MCR values suggest that individuals exposed to combinations of chemicals with the largest Hazard Indices were dominated by the contributions of one or two compounds.

Deformities in cane toad (Bufo marinus) populations in Bermuda: Part II. Progress towards characterization of chemical stressors

An alarming high incidence of deformities in mature and newly metamorphosed Bufo marinus was identified in 2002 (29% in adults and 21% in metamorph specimens) in Bermuda. Suspicion of multiple chemical stressors in many of the effected ponds warranted identification of potential causative agents. Short-term embryo-larval, chronic exposure studies through hind limb development, and a battery of toxicity identification studies were performed with B. marinus and the surrogate species, Xenopus laevis, using sediment and water collected from four target ponds and one reference pond in Bermuda. Short-term embryo larval developmental toxicity studies based on a modified FETAX method indicated that samples from each of the target sites were capable of inducing abnormal development. Malformations induced in the early developmental tests were consistent amongst the species and included abnormal development of the craniofacial region, eye, mouth, and gut. Limb reduction deficit defects were identified in the chronic exposure studies with B. marinus and X. laevis. Toxicity characterization studies found that the toxicity of sediment extracts was variably filterable and reduced by C18-SPE, cation exchange, and zeolite treatments. Developmentally toxic samples contained elevated levels of petroleum hydrocarbons, metals (Al, As, Sn, Cd, Cr, Cu, Fe, Pb, Hg, Mn, Ni, and Zn), and ammonia. Sediment spiking studies with reference sediment confirmed that the levels of petroleum hydrocarbons and metals were capable of inducing developmental toxicity individually. Joint mixture interaction studies with sediment extracts prepared from the individually spiked reference sediment suggested however, that these extracts were potentially capable of acting synergistically, producing a greater level of toxicity than would normally be predicted based on the response to the individual spiked samples.

Variation in marine benthic community composition allows discrimination of multiple stressors

Predicting how communities respond to multiple, potentially interacting chemical stressors is inherently difficult because community structure and dynamics, the chemical properties of contaminants, and biological-chemical interactions vary with environmental conditions. Using a field experiment conducted in Antarctica, we tested whether 3 phyla of benthic soft-sediment marine invertebrates—annelids, arthropods, and echinoderms —respond differently to 2 common forms of contamination, organic enrichment and toxic contamination. Based on life history strategies and physiological tolerances to contaminants, we hypothesized that the principal responses of the 3 phyla would be: (1) enhanced abundance of annelids in organically enrichment sediments and (2) decreased abundance of arthropods and echinoderms in toxic metal contamination. Sediment treatments were established in the field experiment with an orthogonal combination of 3 levels of total organic carbon (TOC; 0, 1, and 2% by weight) and copper (Cu; 0, 100, and 500 µg Cu g -1 sedi- ment), and colonization patterns were observed after 1 yr. Densities of annelids (mainly polychaetes) increased with TOC across all levels of Cu. Arthropods and echinoderms decreased with Cu, but responded variably to TOC, based largely on differences in habitat preferences exhibited by epifau- nal and infaunal species. Small subsurface arthropod species (amphipods, isopods, cumaceans, and ostracods) decreased in high organic loading, due to induction of and exposure to hypoxia and hydro- gen sulfide, but large surface deposit-feeding echinoderms (asteroids and echinoids) responded positively to increased carbon food supply. We present a predictive model based on assessment of benthic community structure conducted at the taxonomic level of phyla that could be used to link cause and effect for multiple chemical stressors in marine ecosystems.