Stage-dependent and sex-dependent sensitivity to water-soluble fractions of fresh and weathered oil in the marine copepod Calanus finmarchicus

Do oil droplets and chemical dispersants contribute to uptake of oil compounds and toxicity of crude oil dispersions in cold-water copepods?

Accidental crude oil spills to the marine environment cause dispersion of oil into the water column through the actions of breaking waves, a process that can be facilitated using chemical dispersants. Oil dispersions contain dispersed micron-sized oil droplets and dissolved oil components, and the toxicity of oil dispersions has been assumed to be associated primarily with the latter. However, most hydrophobic, bioaccumulative and toxic crude oil components are retained within the droplets which may interact with marine filter-feeders. We here summarize the findings of 15 years of research using a unique methodology to generate controlled concentrations and droplet size distributions of dispersed crude oil to study effects on the filter-feeding cold-water copepod Calanus finmarchicus. We focus primarily on the contribution of chemical dispersants and micron-sized oil droplets to uptake and toxicity of oil compounds. Oil dispersion exposures cause PAH uptake and oil droplet accumulation on copepod body surfaces and inside their gastrointestinal tract, and exposures to high exposure (mg/L range) reduce feeding activity, causes reproductive impairments and mortality. These effects were slightly higher in the presence of chemical dispersants, possibly due to higher filtration of chemically dispersed droplets. For C. finmarchicus, dispersions containing oil droplets caused more severe toxic effects than filtered dispersions, thus, oil droplets contribute to the observed toxicity. The methodology for generating crude oil dispersion is a valuable tool to isolate impacts of crude oil microdroplets and can facilitate future research on oil dispersion toxicity and produce data to improve oil spill models.

Sublethal effects of microplastic and oil co-exposure on biological rates and lipid profiles of keystone Arctic copepods

Microplastics (MPs) and petroleum hydrocarbons are contaminants of emerging concern in the Arctic, but little is known about their co-exposure effects. In this study, we present the first assessment of the sublethal impacts resulting from combined exposure to microplastics and oil in three key Arctic copepod species. Specifically, we investigated the effects of a 5-day exposure to oil alone (1 μL L-1) and in combination with MPs (polyethylene microspheres, 20 μm, 20 MP mL-1) and dispersant (Corexit 9500, 0.05 μL L-1) on the biological functions and lipid profiles of the planktonic copepods Metridia longa, Calanus finmarchicus, and Calanus glacialis. Exposure to oil alone caused a significant reduction (34-58%) in fecal pellet production, but neither microplastics nor dispersant increased the negative effect of oil on fecal pellet production. C. glacialis and C. finmarchicus exposed to the studied pollutants for 5 days produced eggs with delayed hatching and lower hatching success. The highest hatching inhibition (50%) was observed in eggs of C. glacialis exposed to oil plus MPs and dispersant for five days. This indicates that maternal transfer of oil components to eggs negatively affects embryonic development and hatching. Lipid content and fatty acids profiles varied among the studied copepod species but were not affected by the tested pollutants after five days of exposure. By microscopical observation of fecal pellets, ingestion of small oil droplets and MPs was confirmed in all species, but the estimated ingestion of MPs was low (<25 MPs cop-1 d-1, <0.2% of total offered MPs) suggesting avoidance of MP consumption in copepods. Our results indicate that virgin MPs did not increase the toxicity of oil to the studied Arctic copepods under co-exposure conditions, and dispersants can slightly increase certain adverse effects of oil (hatching). However, environmentally relevant concentrations of oil alone can negatively impact Arctic keystone copepods and potentially the biological carbon pump. These findings emphasize the need to reduce petrogenic pollution and the risk of oil spills in the sensitive Arctic ecosystem.

A simple protocol for estimating the acute toxicity of unresolved polar compounds from field-weathered oils

Crude oil spilled at sea is chemically altered through environmental processes such as dissolution, biodegradation, and photodegradation. Transformation of hydrocarbons to oxygenated species increases water-solubility. Metabolites and oxidation products largely remain uncharacterized by common analytical methods but may be more bioavailable to aquatic organisms. Studies have shown that unresolved (i.e. unidentified) polar compounds ('UPCs') may constitute > 90% of the water-accommodated fraction (WAF) of heavily weathered crude oils, but still there is a paucity of information characterizing their toxicological significance in relation to other oil-derived toxicants. In this study, low-energy WAFs (no droplets) were generated from two field-weathered oils (collected during the 2010 Deepwater Horizon incident) and their polar fractions were isolated through fractionation. To allow establishment of thresholds for acute toxicity (LC50) of the dissolved and polar fraction of field collected oils, we concentrated both WAFs and polar fractions to beyond field-documented concentrations, and the acute toxicity of both to the marine copepod Acartia tonsa was measured and compared to the toxicity of the native WAF (non-concentrated). The difference in toxic units (TUs) between the total of the mixture and of identified compounds of known toxicity (polycyclic aromatic hydrocarbons [PAHs] and alkyl phenols) in both WAF and polar fractions was used to estimate the contribution of the UPC to overall toxicity. This approach identified that UPC had a similar contribution to toxicity as identified compounds within the WAFs of the field-weathered oils. This signifies the relative importance of polar compounds when assessing environmental impacts of spilled and weathered oil.

Cumulative Impacts of Oil Pollution, Ocean Warming, and Coastal Freshening on the Feeding of Arctic Copepods

The Arctic is undergoing rapid changes, and biota are exposed to multiple stressors, including pollution and climate change. Still, little is known about their joint impact. Here, we investigated the cumulative impact of crude oil, warming, and freshening on the copepod species Calanus glacialis and Calanus finmarchicus. Adult females were exposed to ambient conditions (control; 0 °C + 33 psu) and combined warming and freshening: 5 °C + 27 psu (Scenario 1), 5 °C + 20 psu (Scenario 2) for 6 days. All three conditions were tested with and without dispersed crude oil. In Scenario 1, fecal pellet production (FPP) significantly increased by 40-78% and 42-122% for C. glacialis and C. finmarchicus, respectively. In Scenario 2, FPP decreased by 6-57% for C. glacialis, while it fluctuated for C. finmarchicus. For both species, oil had the strongest effect on FPP, leading to a 68-83% reduction. This overshadowed the differences between climatic scenarios. All variables (temperature, salinity, and oil) had significant single effects and several joint effects on FPP. Our results demonstrate that Arctic copepods are sensitive to environmentally realistic concentrations of crude oil and climate change. Strong reductions in feeding can reduce the copepods' energy content with potential large-scale impacts on the Arctic marine food web.

Effects of petrogenic pollutants on North Atlantic and Arctic Calanus copepods: From molecular mechanisms to population impacts

Oil and gas industries in the Northern Atlantic Ocean have gradually moved closer to the Arctic areas, a process expected to be further facilitated by sea ice withdrawal caused by global warming. Copepods of the genus Calanus hold a key position in these cold-water food webs, providing an important energetic link between primary production and higher trophic levels. Due to their ecological importance, there is a concern about how accidental oil spills and produced water discharges may impact cold-water copepods. In this review, we summarize the current knowledge of the toxicity of petroleum on North Atlantic and Arctic Calanus copepods. We also review how recent development of high-quality transcriptomes from RNA-sequencing of copepods have identified genes regulating key biological processes, like molting, diapause and reproduction in Calanus copepods, to suggest linkages between exposure, molecular mechanisms and effects on higher levels of biological organization. We found that the available ecotoxicity threshold data for these copepods provide valuable information about their sensitivity to acute petrogenic exposures; however, there is still insufficient knowledge regarding underlying mechanisms of toxicity and the potential for long-term implications of relevance for copepod ecology and phenology. Copepod transcriptomics has expanded our understanding of how key biological processes are regulated in cold-water copepods. These advances can improve our understanding of how pollutants affect biological processes, and thus provide the basis for new knowledge frameworks spanning the effect continuum from molecular initiating events to adverse effects of regulatory relevance. Such efforts, guided by concepts such as adverse outcome pathways (AOPs), enable standardized and transparent characterization and evaluation of knowledge and identifies research gaps and priorities. This review suggests enhancing mechanistic understanding of exposure-effect relationships to better understand and link biomarker responses to adverse effects to improve risk assessments assessing ecological effects of pollutant mixtures, like crude oil, in Arctic areas.

Drivers of copper sensitivity in copepods: A meta-analysis of LC50s

Copper is both an essential trace element and a potent pesticide. The use of copper as an antifoulant has increased in the last decades in line with the expanding aquaculture and shipping industries. In aquatic environments, it also affects non-target taxa. One of which are copepods, which constitute the central link in the marine food web. Despite their ecological importance, there are no systematic reviews of the lethal concentration range and drivers of copper toxicity in this taxon. Here, we combined literature data from 31 peer-reviewed articles recording the Lethal Concentration 50 (LC50) for copper in copepods and the experiments' respective environmental, developmental, and taxonomic parameters. The LC50 is a traditional endpoint for toxicity testing used in standardized toxicity testing and many ecological studies. In total, we were able to extract 166 LC50 entries. The variability in the metadata allowed for a general analysis of the drivers of copper sensitivity in copepods. Using a generalized additive modeling approach, we find that temperature increases copper toxicity when above approximately 25℃. Counter to our expectations; salinity does not influence copper sensitivity across copepod species. Unsurprisingly, nauplii are more susceptible to copper exposure than adult copepods, and benthos-associated harpacticoids are less sensitive to copper than pelagic calanoids. Our final model can predict sensible specific-specific copper concentrations for future experiments, thus giving an informed analytical approach to range testing in future dose-response experiments. Our model can also potentially improve ecological risk assessment by accounting for environmental differences. The approach can be applied to other toxicants and taxa, which may reveal underlying patterns otherwise obscured by taxonomic and experimental variability.

Size- and sex-related sensitivity differences of aquatic crustaceans to imidacloprid

Field collected aquatic invertebrates are often used as test organisms in the refinement of the standard Tier 1 risk assessment of various pollutants. This approach can provide insights into the effects of pollutants on the natural environment. However, researchers often pragmatically select test organisms of a specific sex and/or size, which may not represent the sensitivity of the whole population. To investigate such intraspecies sensitivity differences, we performed standard acute toxicity and toxicokinetic tests with different size classes and sex of Gammarus pulex and Asellus aquaticus. Furthermore, toxicokinetics and toxicodynamics models were used to understand the mechanism of the intraspecies sensitivity differences. We used neonates, juveniles and male and female adults in separate dedicated experiments, in which we exposed the animals to imidacloprid and its bioactive metabolite, imidacloprid-olefin. For both species, we found that neonates were the most sensitive group. For G. pulex, the sensitivity decreased linearly with size, which can be explained by the size-related uptake rate constant in the toxicokinetic process and size-related threshold value in the toxicodynamic process. For A. aquaticus, female adults were least sensitive to imidacloprid, which could be explained by a low internal biotransformation of imidacloprid to imidacloprid-olefin. Besides, imidacloprid-olefin was more toxic than imidacloprid to A. aquaticus, with differences being 8.4 times for females and 2.7 times for males. In conclusion, we established size-related sensitivity differences for G. pulex and sex-related sensitivity for A. aquaticus, and intraspecies differences can be explained by both toxicokinetic and toxicodynamic processes. Our findings suggest that to protect populations in the field, we should consider the size and sex of focal organisms and that a pragmatic selection of test organisms of equal size and/or sex can underestimate the sensitivities of populations in the field.

Acute and chronic toxicity assessments of 17β-estradiol (E2) and 17α-ethinylestradiol (EE2) on the calanoid copepod Acartia clausi: Effects on survival, development, sex-ratio and reproduction

Estrogens, such as the 17β-estradiol (E2) and the 17α-ethinylestradiol (EE2), have been regarded as a global threat to aquatic ecosystems due to their pseudo-persistence, their high estrogenic activity and their toxicity towards non-target species. Data regarding their ecotoxicological effects on marine calanoid copepods are very scarce. In this study, the calanoid copepod Acartia clausi was used as a model organism for estrogens exposure in marine pelagic ecosystems. Lethal effects of estrogens on A. clausi life-stages (Embryos, one day old nauplii: N1, three day old nauplii: N3, copepodites: C1-C3 and adults: C6) were investigated using 48 h acute tests. Copepods showed stage-specific responses against E2 and EE2 acute exposure. The most resistant life stage was N1 with LC₅₀ values > 1500 μg L^-1 and >5000 μg L^-1, respectively for E2 and EE2. For N3, C1-C3, and C6, sensitivity to estrogens decreased with age and survival was affected at concentrations above those detected in the environment reflecting low estrogens acute toxicity for these life stages. In contrast, embryonic stage revealed high vulnerability to E2 and EE2 acute effects. Embryos showed non-monotonic dose-response and hatching success was significantly reduced at low realistic concentrations of E2 (0.005, 0.5, and 5 μg L^-1) and EE2 (0.05 and 5 μg L^-1). Survival, development and sex ratio of A. clausi to EE2 exposure at 1 and 100 μg L^-1 were also determined during a life cycle experiment. Fitness of the females of the generation F0 was evaluated by measuring lifespan, prosome length and egg production. The main observed effects were the decrease of females' prosome length, the feminization of the population and the reduction of the egg production for the generation F0 at 100 μg L^-1 of EE2. This concentration is above those reported in the environment indicating the tolerance of A. clausi to EE2 at environmentally relevant concentrations.

Oil-mediated oxidative-stress responses in a keystone zooplanktonic species, Calanus finmarchicus

The copepod Calanus finmarchicus is an ecologically important species in the North Atlantic, Norwegian and Barents seas. Accidental or continuous petroleum pollution from oil and gas production in these seas may pose a significant threat to this low trophic level keystone species. Responses related to oxidative stress, protein damage and lipid peroxidation were investigated in C. finmarchicus exposed to a water-accommodated fraction (WAF) of a naphthenic North Atlantic crude oil. The exposure concentration corresponded to 50% of the 96 h LC₅₀, and samples were obtained at 0, 24, 48, 72 and 96 h after exposure initiation. Gene expressions (superoxide dismutase, catalase, glutathione S-transferase, glutathione synthetase, heat shock protein 70 and 90, ubiquitin and cytochrome P-450 330A1), enzyme activities (superoxide dismutase, catalase, glutathione S-transferase) and concentrations of total glutathione and malondialdehyde were analyzed. Gene expression analyses showed no differences between controls and the exposed animals, however significantly higher glutathione S-transferase activity and malondialdehyde concentrations were found in the exposed group, suggests lipid peroxidation as main toxic effect.

Environmental effects of offshore produced water discharges: A review focused on the Norwegian continental shelf

Produced water (PW), a large byproduct of offshore oil and gas extraction, is reinjected to formations or discharged to the sea after treatment. The discharges contain dispersed crude oil, polycyclic aromatic hydrocarbons (PAHs), alkylphenols (APs), metals, and many other constituents of environmental relevance. Risk-based regulation, greener offshore chemicals and improved cleaning systems have reduced environmental risks of PW discharges, but PW is still the largest operational source of oil pollution to the sea from the offshore petroleum industry. Monitoring surveys find detectable exposures in caged mussel and fish several km downstream from PW outfalls, but biomarkers indicate only mild acute effects in these sentinels. On the other hand, increased concentrations of DNA adducts are found repeatedly in benthic fish populations, especially in haddock. It is uncertain whether increased adducts could be a long-term effect of sediment contamination due to ongoing PW discharges, or earlier discharges of oil-containing drilling waste. Another concern is uncertainty regarding the possible effect of PW discharges in the sub-Arctic Southern Barents Sea. So far, research suggests that sub-arctic species are largely comparable to temperate species in their sensitivity to PW exposure. Larval deformities and cardiac toxicity in fish early life stages are among the biomarkers and adverse outcome pathways that currently receive much attention in PW effect research. Herein, we summarize the accumulated ecotoxicological knowledge of offshore PW discharges and highlight some key remaining knowledge needs.

Acute and long-term effects of anionic polyacrylamide (APAM) on different developmental stages of two marine copepod species

The application of synthetic polymers such as anionic polyacrylamides (APAM) in enhanced oil recovery (EOR) may increase in the future. This can lead to environmental release through offshore produced water discharges with so far limited knowledge on impacts in marine ecosystems. We investigated impacts of APAM polymers on two marine copepod species. Acute effects of APAM were studied on different life stages of C. finmarchicus (three molecular sizes: 200 kDa, 2800 kDa and 8000 kDa) and Acartia tonsa (one molecular size: 2800 kDa). Further, effects on development and survival following long-term exposure (spanning over several life stages) to 200 kDa APAM were studied in C. finmarchicus. Results show that none of the APAM molecules caused mortality in acute exposure experiments in adult C. finmarchicus even at high exposure concentrations (≥1000 mg/L). Comparing toxicity of the 2800 kDa APAM between C. finmarchicus and the standard marine toxicity test copepod Acartia tonsa showed that the latter was slightly more sensitive. Early life stages of both copepods were more sensitive compared to later ones, and APAM exposure induced increased mortality and developmental delays. Effects were generally more pronounced for the larger polymers, most likely due to increased viscosity of the test dispersions leading to increased energy expenditures of the animals. However, significant effects were only observed at very high exposure concentrations that are probably higher than concentrations found in the environment.

A Crude Awakening: Effects of Crude Oil on Lipid Metabolism in Calanoid Copepods Terminating Diapause

Calanus finmarchicus and Calanus glacialis are keystone zooplankton species in North Atlantic and Arctic marine ecosystems because they form a link in the trophic transfer of nutritious lipids from phytoplankton to predators on higher trophic levels. These calanoid copepods spend several months of the year in deep waters in a dormant state called diapause, after which they emerge in surface waters to feed and reproduce during the spring phytoplankton bloom. Disruption of diapause timing could have dramatic consequences for marine ecosystems. In the present study, Calanus C5 copepodites were collected in a Norwegian fjord during diapause and were subsequently experimentally exposed to the water-soluble fraction of a naphthenic North Sea crude oil during diapause termination. The copepods were sampled repeatedly while progressing toward adulthood and were analyzed for utilization of lipid stores and for differential expression of genes involved in lipid metabolism. Our results indicate that water-soluble fraction exposure led to a temporary pause in lipid catabolism, suggested by (i) slower utilization of lipid stores in water-soluble fraction-exposed C5 copepodites and (ii) more genes in the β-oxidation pathway being downregulated in water-soluble fraction-exposed C5 copepodites than in the control C5 copepodites. Because lipid content and/or composition may be an important trigger for termination of diapause, our results imply that the timing of diapause termination and subsequent migration to the surface may be delayed if copepods are exposed to oil pollution during diapause or diapause termination. This delay could have detrimental effects on ecosystem dynamics.

Impact of temperature and pyrene exposure on the functional response of males and females of the copepod Calanus finmarchicus

We know very little about the effects of two global stressors, elevated temperature and contaminants, on the grazing of marine copepods. To address this issue, we tested the hypotheses that the individual and combined effects of these two stressors may reduce grazing rates and may depend on food availability and gender. We exposed male and female Calanus finmarchicus copepods to pyrene at two temperatures (10 and 14 °C) and six food concentrations (25-800 μg C Rhodomonas baltica L^-1) and measured fecal pellet size, and grazing rate (GR) from pellet production. Males had smaller fecal pellets and lower GR than did females. Temperature and pyrene exposure had no effect on pellet size. Temperature alone had no effect on GR of males, but females had lower GR at elevated temperature. Pyrene-exposed males and females had lower GR only at the food concentrations of 200-800 μg C R. baltica L^-1 and those patterns were independent of temperature. Pyrene-induced reduction in GR was stronger in females than in males. The negative effects of both elevated temperature and pyrene may reduce the abundance and trophic success of C. finmarchicus in a warmer, more polluted future.

Recommendations for ecotoxicity testing with stygobiotic species in the framework of groundwater environmental risk assessment

As a consequence of the growing global dependence on groundwater resources, environmental risk assessments (ERA) for groundwater are increasingly required and, with that, ecotoxicological studies with groundwater fauna (stygofauna). However, the literature on the ecotoxicological studies with stygobiotic species (i.e. species that complete their life cycle exclusively in groundwater) has not expanded significantly since the first paper published in this field. The limitations regarding the use of stygobiotic species for ecotoxicological testing are clear and consistent across the globe; stygobiotic species are often 1) naturally present in low numbers, 2) difficult to collect, and 3) difficult to culture under laboratory conditions. This paper reviews the methods used in ecotoxicological studies performed with stygobiotic species, and provides ten recommendations for Good Laboratory Practice (GLP) for such tests. The recommendations focused on the following 10 points: 1) the taxonomic identification, the life stage/size and gender of the test organisms; 2) collection methodology of the organisms, including collection location, conditions and methods; 3) holding and acclimation conditions in the laboratory; 4) exposure conditions such as test set up and exposure time, number of replicates and densities of organisms in tests and in test vessels; 5) range-finding test set up and schedule; 6) final test design, including details of controls and treatments, and replication options; 7) incubation conditions, specifying temperature, pH and oxygenation levels throughout the test; 8) test duration; 9) observations and endpoints; 10) test validity criteria and compliance. The recommendations were developed for the purpose of supporting future short-term ecotoxicological testing with stygofauna through providing consistency in the protocols. A discussion of the potential implications for groundwater managers and decision-makers committed to ERA for groundwater is included.

Lethal and behavioral impacts of diesel and fuel oil on the Antarctic amphipod Paramoera walkeri

Toxicity testing with Antarctic species is required for risk assessment of fuel spills in Antarctic coastal waters. The lethal and sublethal (movement behavior) sensitivities of adults and juveniles of the Antarctic amphipod Paramoera walkeri to the water accommodated fractions (WAFs) of 3 fuels were estimated in extended-duration tests at -1 °C to 21 d. Response of P. walkeri for lethal hydrocarbon concentrations was slow, with 50% lethal concentrations (LC50s) first able to be estimated at 7 d for adults exposed to Special Antarctic Blend diesel (SAB), which had the highest hydrocarbon concentrations of the 3 fuel WAFs. Juveniles showed greater response to marine gas oil (MGO) and intermediate residual fuel oil (IFO 180) at longer exposure durations and were most sensitive at 21 d to IFO 180 (LC50 = 12 μg/L). Adults were initially more sensitive than juveniles; at 21 d, however, juveniles were more than twice as sensitive as adults to SAB (LC50 = 153 μg/L and 377 μg/L, respectively). Significant effects on movement behavior were evident at earlier time points and lower concentrations than was mortality in all 3 fuel WAFs, and juveniles were highly sensitive to sublethal effects of MGO. These first estimates of Antarctic amphipod sensitivity to diesel and fuel oils in seawater contribute to the development of ecologically relevant risk assessments for management of hydrocarbon contamination in the region. Environ Toxicol Chem 2017;36:2444-2455. © 2017 SETAC.

Effects of chronic crude oil exposure on early developmental stages of the Northern krill (Meganyctiphanes norvegica)

Rising oil and gas activities in northern high latitudes have led to an increased risk of petroleum pollution in these ecosystems. Further, seasonal high UV radiation at high latitudes may elevate photo-enhanced toxicity of petroleum pollution to marine organisms. Zooplanktons are a key ecological component of northern ecosystems; therefore, it is important to assess their sensitivity to potential pollutants of oil and gas activity. As ontogenetic development may be particularly sensitive, the aim of this study was to examine the impact of chronic exposure to oil water dispersion (OWD) on development and feeding of early life stages of the Northern krill, Meganyctiphanes norvegica. In a range of experiments, embryonic, nonfeeding, and feeding larval stages were exposed to concentrations of between 0.01 and 0.1 mg/L of oil or photo-modified oil for 19 and 21 d. No significant effects on egg respiration, hatching success, development, length and larval survival were observed from these treatments. Similarly, evolution of fatty acid composition patterns during ontogenetic development was unaffected. The results indicates a high degree of resilience of these early developmental stages to such types and concentrations of pollutants. However, feeding and motility in later calyptopis-stage larvae were significantly impaired at exposure of 0.1 mg/L oil. Data indicate that feeding larval stage of krill was more sensitive to OWD than early nonfeeding life stages. This might be attributed to the narcotic effects of oil pollutants, their direct ingestion, or accumulated adverse effects over early development.

Dynamic Links between Lipid Storage, Toxicokinetics and Mortality in a Marine Copepod Exposed to Dimethylnaphthalene

Efficiently assessing and managing the risks of pollution in the marine environment requires mechanistic models for toxic effects. The General Unified Threshold model for Survival (GUTS) provides a framework for deriving toxicokinetic-toxicodynamic (TKTD) models for the end point survival. Two recurring questions in the application of GUTS concern the most appropriate death mechanism, and whether the total body residue is a proper dose metric for toxic effects. We address these questions with a case study for dimethylnaphthalene in the marine copepod Calanus finmarchicus. A detailed analysis revealed that body residues were best explained by representing copepods with two toxicokinetic compartments: separating structural biomass and lipid storage. Toxicity is most likely related to the concentration in structure, which led to identification of "stochastic death" as the most appropriate death mechanism. Interestingly, the parametrized model predicts that lipid content will have only minor influence on short-term toxicity. However, the toxicants stored in lipids may have more substantial impacts in situations not included in our experiments (e.g., during diapause and gonad maturation), and for contaminant transfer to eggs and copepod predators.

Toxic Mixtures in Time-The Sequence Makes the Poison

"The dose makes the poison". This principle assumes that once a chemical is cleared out of the organism (toxicokinetic recovery), it no longer has any effect. However, it overlooks the other process of re-establishing homeostasis, toxicodynamic recovery, which can be fast or slow depending on the chemical. Therefore, when organisms are exposed to two toxicants in sequence, the toxicity can differ if their order is reversed. We test this hypothesis with the freshwater crustacean Gammarus pulex and four toxicants that act on different targets (diazinon, propiconazole, 4,6-dinitro-o-cresol, 4-nitrobenzyl chloride). We found clearly different toxicity when the exposure order of two toxicants was reversed, while maintaining the same dose. Slow toxicodynamic recovery caused carry-over toxicity in subsequent exposures, thereby resulting in a sequence effect-but only when toxicodynamic recovery was slow relative to the interval between exposures. This suggests that carry-over toxicity is a useful proxy for organism fitness and that risk assessment methods should be revised as they currently could underestimate risk. We provide the first evidence that carry-over toxicity occurs among chemicals acting on different targets and when exposure is several days apart. It is therefore not only the dose that makes the poison but also the exposure sequence.

Exposure to crude oil micro-droplets causes reduced food uptake in copepods associated with alteration in their metabolic profiles

Acute oil spills and produced water discharges may cause exposure of filter-feeding pelagic organisms to micron-sized dispersed oil droplets. The dissolved oil components are expected to be the main driver for oil dispersion toxicity; however, very few studies have investigated the specific contribution of oil droplets to toxicity. In the present work, the contribution of oil micro-droplet toxicity in dispersions was isolated by comparing exposures to oil dispersions (water soluble fraction with droplets) to concurrent exposure to filtered dispersions (water-soluble fractions without droplets). Physical (coloration) and behavioral (feeding activity) as well as molecular (metabolite profiling) responses to oil exposures in the copepod Calanus finmarchicus were studied. At high dispersion concentrations (4.1-5.6mg oil/L), copepods displayed carapace discoloration and reduced swimming activity. Reduced feeding activity, measured as algae uptake, gut filling and fecal pellet production, was evident also for lower concentrations (0.08mg oil/L). Alterations in metabolic profiles were also observed following exposure to oil dispersions. The pattern of responses were similar between two comparable experiments with different oil types, suggesting responses to be non-oil type specific. Furthermore, oil micro-droplets appear to contribute to some of the observed effects triggering a starvation-type response, manifested as a reduction in metabolite (homarine, acetylcholine, creatine and lactate) concentrations in copepods. Our work clearly displays a relationship between crude oil micro-droplet exposure and reduced uptake of algae in copepods.

Modelling survival: exposure pattern, species sensitivity and uncertainty

The General Unified Threshold model for Survival (GUTS) integrates previously published toxicokinetic-toxicodynamic models and estimates survival with explicitly defined assumptions. Importantly, GUTS accounts for time-variable exposure to the stressor. We performed three studies to test the ability of GUTS to predict survival of aquatic organisms across different pesticide exposure patterns, time scales and species. Firstly, using synthetic data, we identified experimental data requirements which allow for the estimation of all parameters of the GUTS proper model. Secondly, we assessed how well GUTS, calibrated with short-term survival data of Gammarus pulex exposed to four pesticides, can forecast effects of longer-term pulsed exposures. Thirdly, we tested the ability of GUTS to estimate 14-day median effect concentrations of malathion for a range of species and use these estimates to build species sensitivity distributions for different exposure patterns. We find that GUTS adequately predicts survival across exposure patterns that vary over time. When toxicity is assessed for time-variable concentrations species may differ in their responses depending on the exposure profile. This can result in different species sensitivity rankings and safe levels. The interplay of exposure pattern and species sensitivity deserves systematic investigation in order to better understand how organisms respond to stress, including humans.