Selenium bioaccumulation and maternal transfer in the mayfly Centroptilum triangulifer in a life-cycle, periphyton-biofilm trophic assay

Laboratory and In situ Selenium Bioaccumulation Assessment in the Benthic Macroinvertebrates Hyalella azteca and Chironomus dilutus

Selenium (Se) bioaccumulation and toxicity in aquatic vertebrates have been thoroughly investigated. Limited information is available on Se bioaccumulation at the base of aquatic food webs. In this study, we evaluated Se bioaccumulation in two benthic macroinvertebrates (BMI), Hyalella azteca and Chironomus dilutus raised in the laboratory and caged in-situ to a Canadian boreal lake e (i.e., McClean Lake) that receives continuous low-level inputs of Se (< 1 μg/L) from a uranium mill. Additional Se bioaccumulation assays were conducted in the laboratory with these BMI to (i) confirm field results, (ii) compare Se bioaccumulation in lab-read and native H. azteca populations and (iii) identify the major Se exposure pathway (surface water, top 1 cm and top 2-3 cm sediment layers) leading to Se bioaccumulation in H. azteca. Field and laboratory studies indicated overall comparable Se bioaccumulation and trophic transfer factors (TTFs) in co-exposed H. azteca (whole-body Se 0.9-3.1 µg/g d.w; TTFs 0.6-6.3) and C. dilutus (whole-body Se at 0.7-3.2 µg Se/g d.w.; TTFs 0.7-3.4). Native and lab-reared H. azteca populations exposed to sediment and periphyton from McClean Lake exhibited similar Se uptake and bioaccumulation (NLR, p = 0.003; 4.1 ± 0.8 µg Se/g d.w), demonstrating that lab-reared organisms are good surrogates to assess on-site Se bioaccumulation potential. The greater Se concentrations in H. azteca exposed to the top 1-3 cm sediment layer relative to waterborne exposure, corroborates the importance of the sediment-detrital pathway leading to greater Se bioaccumulation potential to higher trophic levels via BMI.

New insights into pesticide occurrence and multicompartmental monitoring strategies in stream ecosystems using periphyton and suspended sediment

Streams are susceptible to pesticide pollutants which are transported outside of the intended area of application from surrounding agricultural fields. It is essential to monitor the occurrence and levels of pesticides in aquatic ecosystems to comprehend their effects on the aquatic environment. The common sampling strategy used for monitoring pesticides in stream ecosystems is through the collection and analysis of grab water samples. However, grab water sampling may not effectively monitor pesticides due to its limited ability to capture temporal and spatial variability, potentially missing fluctuations and uneven distribution of pesticides in aquatic environments. Monitoring using periphyton and sediment sampling may offer a more comprehensive approach by accounting for accumulative processes and temporal variations. Periphyton are a collective of microorganisms that grow on hard surfaces in aquatic ecosystems. They are responsive to chemical and biological changes in the environment, and therefore have the potential to act as a cost-effective, integrated sampling tool to monitor pesticide exposures in aquatic ecosystems. The objective of this study was to assess pesticides detected through periphyton, suspended sediment, and conventional grab water sampling methods and identify the matrix that offers a more comprehensive characterization of a stream's pesticide exposure profile. Ten streams across Southern Ontario were sampled in 2021 and 2022. At each stream site, water, sediment and periphyton, colonizing both artificial and natural substrates, were collected and analyzed for the presence of ~500 pesticides. Each of the three matrices detected distinctive pesticide exposure profiles. The frequency of detection in periphyton, sediment and water matrices were related to pesticides' log Kow and log Koc (P < 0.05). In addition, periphyton bioconcentrated 22 pesticides above levels observed in the ambient water. The bioconcentration factors of pesticides in periphyton can be predicted from their log Kow (simple linear regressions, P < 0.05). The results demonstrate that sediment and periphyton accumulate pesticides in stream environments. This highlights the importance of monitoring pesticide exposure using these matrices to ensure a complete and comprehensive characterization of exposure in stream ecosystems.

The impact of selenium on insects

Selenium, a naturally occurring metalloid, is an essential trace element for many higher organisms, including humans. Humans primarily become exposed to selenium by ingesting food products containing trace amounts of selenium compounds. Although essential in these small amounts, selenium exhibits toxic effects at higher doses. Previous studies investigating the effects on insects of order Blattodea, Coleoptera, Diptera, Ephemeroptera, Hemiptera, Hymenoptera, Lepidoptera, Odonata, and Orthoptera revealed impacts on mortality, growth, development, and behavior. Nearly every study examining selenium toxicity has shown that insects are negatively affected by exposure to selenium in their food. However, there were no clear patterns of toxicity between insect orders or similarities between insect species within families. At this time, the potential for control will need to be determined on a species-by-species basis. We suspect that the multiple modes of action, including mutation-inducing modification of important amino acids as well as impacts on microbiome composition, influence this variability. There are relatively few studies that have examined the potential effects of selenium on beneficial insects, and the results have ranged from increased predation (a strong positive effect) to toxicity resulting in reduced population growth or even the effective elimination of the natural enemies (more common negative effects). As a result, in those pest systems where selenium use is contemplated, additional research may be necessary to ascertain if selenium use is compatible with key biological control agents. This review explores selenium as a potential insecticide and possible future directions for research.

Physiological and life history responses in a mayfly (Callibaetis floridanus) inhabiting ponds with saltwater intrusion

Freshwater salinity varies in natural systems and plays a role in species distribution. Anthropogenic alterations to freshwater salinity regimes include sea level rise and subsequent intrusion of saline waters to inland habitats. While mayflies are generalized to be sensitive to increasing salinity, we still know remarkably little about the physiological processes (and their plasticity) that determine the performance of species in a changing world. Here, we explored life-history outcomes and physiological plasticity in a population of Callibaetis floridanus (Ephemeroptera: Baetidae) from a coastal pond that routinely experiences saltwater intrusion. We reared naiads from egg hatch to adulthood across a gradient of increasing salinities (113, 5,020, 9,921 μS/cm). Radiotracer flux studies (22Na, 35SO4, and 45Ca) were conducted in naiads reared at each salinity, revealing a positive association between ionic concentration and uptake rates. However, the influence of rearing history on ionic influx rates was apparent when naiads were transferred from their respective rearing water to the other experimental conditions. For example, we observed that naiads reared in the low salinity treatment (113 μS/cm) had 10.8-fold higher Na uptake rates than naiads reared at 9,921 μS/cm and transferred to 113 μS/cm. Additionally, naiads acclimated to the higher salinity water exhibited reduced uptake in ion-rich water relative to those reared in more dilute conditions (e.g., in 9,921 μS/cm water, 113 and 5,020 μS/cm acclimated naiads had 1.5- and 1.1-fold higher Na uptake rates than 9,921 μS/cm acclimated naiads, respectively). We found no significant changes in survival (80 ± 4.4%, mean ± s.e.m.) or naiad development time (24 ± 0.3 days, mean ± s.e.m.) across these treatments but did observe a 27% decrease in subimago female body weight in the most dilute condition. This reduction in female weight was associated with higher oxygen consumption rates in naiads relative to the other rearing conditions. Collectively, these data suggests that saline adapted C. floridanus may be more energetically challenged in dilute conditions, which differs from previous observations in other mayfly species.

Subcellular Accumulation and Depuration of Zinc in Periphytic Algae during Episodic and Continuous Exposures

As the severity of extreme precipitation events increases with global climate change, so will episodic pulses of contamination into lotic systems. Periphytic algae represents bioindicator species in most freshwater systems due to their rapid accumulation of toxicants; therefore, it is vital to understand how accumulation in this group differs across temporally variable exposure regimes. The ability to rapidly accrue contaminants has additional implications for the trophic transfer of metals to primary consumers. While dietary toxicity has been studied in algivorous consumers, techniques used to prepare contaminated periphytic algae for consumption have not been compared. This study used a modified subcellular fractionation method to compare the partitioning of zinc (Zn) in periphyton cultures exposed for various durations (cultured in the presence of Zn and 15 min, 24 h, and 48 h exposures). Three exposure groups were additionally depurated over a period of 24 h in order to compare retention of Zn, an important aspect of preparing diets used in dietary toxicity studies. The results not only provide evidence for increased retention by periphytic algae cultured in the presence of Zn but reveal relationships among treatments and subcellular partitioning that suggest time-dependent accumulation and detoxification. These relationships suggest that episodic exposure of periphytic algae to contaminants may pose a greater risk than that of chronic regimes. Based on these results, we additionally advocate for culturing periphytic algae in the presence of contamination to produce a more reliable diet for dietary exposure testing in algivorous organisms.

Laboratory study of Se speciation in the sediment and oligochaete Lumbriculus variegatus from an aquatic environment

Sediments are the major sink for selenium (Se) in aquatic environments. Se speciation in sediments is crucial for its bioavailability and toxicity in benthos, but this is relatively understudied. In this study, the background levels of Se in the river sediments, fish flakes, and Lumbriculus variegatus were also detected. Then, the dynamic changes of selenium speciation and concentrations in sediments were investigated after adding selenite (Se(IV)) and seleno-L-methionine (Se-Met) in the sediments for 90 and 7 days, and the accumulation and depuration of Se(IV) and Se-Met for 7 days in the oligochaete L. variegatus were also explored. Without the presence of worms, the levels of Se(IV) in the sediments were relatively stable within 7 days but showed a decreasing trend during the 90 days of aging. In contrast, Se-Met in the sediments showed a sharp decrease within 3 days of aging. The LC₅₀-96 h values of Se(IV) and Se-Met in L. variegatus were 372.6 and 9.4 μg/g, respectively. Interestingly, the dominant Se species in Se(IV)- or Se-Met-treated L. variegatus was Se-Met, whose level was increased with time in 7 days of exposure. Se was barely depurated from L. variegatus during the 8 days of the depuration period. This study has provided indispensable data on the levels of total Se in the abiotic and biotic matrices and the biodynamics of Se in a representative benthos, which could better understand the ecological risk of Se to the freshwater benthic communities.

Relative importance of two correlated variables on aquatic macroinvertebrate communities in a Colorado Front Range river: selenium and urbanization

Urbanization and elevated selenium can negatively affect aquatic macroinvertebrate communities. These factors are often highly correlated in streams along the Front Range of the Rocky Mountains, USA. Understanding which factor is the primary driver of macroinvertebrate community health would aid in the development of appropriate management actions to mitigate their influence. Data collected from three sites in the Big Thompson River, Colorado, in seven different years from 2006 to 2016 were used to develop a random forest model to determine the relative importance of an index of urbanization and dissolved selenium levels in predicting the relative status of macroinvertebrate communities as measured by Colorado multi-metric index (MMI) scores. Based on the percent increase in mean square error (%incMSE) variable importance measure, dissolved selenium has a greater influence on MMI scores than a development index (19.5% vs 15.2%). There is a significantly negative relationship between MMI scores and dissolved selenium levels. This relationship predicts a MMI score of 29.2 under the current dissolved selenium standard of 4.6 µg/L and 38.6 under the suggested lower standard of 3.1 µg/L. Although improvements in MMI score may result with any reduction in dissolved selenium, other factors may limit the maximum expected MMI scores in the absence of selenium as the intercept of the relationship is 57.9 on a scale of 0-100.

Selenium Distribution and Trophic Transfer in the Periphyton-Benthic Macroinvertebrate Food Chain in Boreal Lakes Downstream from a Milling Operation

Selenium (Se) is an essential micronutrient with a narrow essentiality-toxicity range known to bioaccumulate in aquatic food webs. Selenium uptake and trophic transfer at the base of aquatic food chains represent a great source of uncertainty for Se risk assessment. The goal of the present study was to investigate Se distribution in water and sediment and its subsequent transfer into the periphyton-benthic macroinvertebrate (BMI) food chain in boreal lakes downstream from a Saskatchewan uranium mill. In particular, the present study aimed to assess potential differences in Se bioaccumulation patterns by BMI taxa to contribute to the current knowledge gap. During summer 2018 and 2019, water, sediment, periphyton, and BMI were sampled at two sites in Vulture Lake, seven sites in McClean Lake east basin, and one reference site in McClean Lake west basin. Periphyton and BMI taxa were sampled with artificial substrates (Hester-Dendy) deployed for 5 weeks in 2018 and 7 weeks in 2019; BMI were sorted into the lowest practical achievable taxonomic level and analyzed for total Se concentrations. At the diluted effluent exposure sites, Se concentrations in BMI ranged from 1.3 to 18.0 µg/g dry weight and from 0.3 to 49.3 µg/g dry weight in 2018 and 2019, respectively, whereas concentrations ranged from 0.01 to 3.5 µg/g dry weight at the reference site. Selenium concentrations in periphyton and some BMI taxa sampled near the effluent diffuser (Se < 1 µg/L) reached levels comparable to higher effluent exposure sites (Se > 2 µg/L). Despite differences in Se bioaccumulation among BMI taxa, an approximately one-to-one trophic transfer ratio was observed for benthic primary consumers and benthic predatory taxa. Environ Toxicol Chem 2022;41:2181-2192. © 2022 SETAC.

The acclimatory response of the mayfly Neocloeon triangulifer to dilute conditions is linked to the plasticity of sodium transport

Relative to a growing body of knowledge about the negative consequences of freshwater salinization, little is known about how aquatic insects respond to progressively ion-poor conditions. Here, we examined life-history and physiological acclimation in Neocloeon triangulifer by rearing nymphs from 1-day post-egg hatch to adulthood across a gradient of decreasing Na concentrations (15, 8, 4, 2 and 1 mg l^-1 Na). We found no significant changes in survival, growth, development time and whole-body Na content across these treatments. Radiotracer data revealed that nymphs acclimated to their dilute exposures by increasing their rates of Na uptake and were able to maintain a relatively narrow range of uptake rates (±s.e.m.) of 38.5 ± 4.2 µg Na g^-1 h^-1 across all treatments. By contrast, the Na uptake rates observed in naive nymphs were much more concentration dependent. This acclimatory response is partially explained by differences in ionocyte counts on the gills of nymphs reared under different salinities. Acclimated nymphs were surprisingly less retentive of their sodium composition when subjected to deionized water challenge. By contrasting our findings with a previous N. triangulifer salinity acclimation study, we show a physiological affinity for dilute conditions in this emerging mayfly model.

Differential selenium uptake by periphyton in boreal lake ecosystems

The largest and most variable step of selenium (Se) assimilation into aquatic ecosystems is the rapid uptake of aqueous Se by primary producers. These organisms can transfer more harmful forms of Se to higher trophic levels via dietary pathways, although much uncertainty remains around this step of Se assimilation due to site-specific differences in water chemistry, hydrological and biogeochemical characteristics, and community composition. Thus, predictions of Se accumulation are difficult, and boreal lake systems are relatively understudied. To address these knowledge gaps, five static-renewal field experiments were performed to examine the bioaccumulation of low, environmentally relevant concentrations of Se, as selenite, by naturally grown periphyton from multiple boreal lakes. Periphyton rapidly accumulated Se at low aqueous Se concentrations, with tissue Se concentrations ranging from 8.0 to 24.9 μg/g dry mass (dm) in the 1-2 μg Se/L treatments. Enrichment functions ranged from 2870 to 12 536 L/kg dm in the 4 μg Se/L treatment, to 11 867-22 653 L/kg dm in the 0.5 μg Se/L treatment among lakes. Periphyton Se uptake differed among the five study lakes, with periphyton from mesotrophic lakes generally accumulating more Se than periphyton from oligotrophic lakes. Higher proportions of charophytes and greater dissolved inorganic carbon in more oligotrophic lakes corresponded to less periphyton Se uptake. Conversely, increased proportions of bacillariophytes and total dissolved phosphorus in more mesotrophic lakes corresponded to greater periphyton Se uptake. Periphyton community composition and water chemistry variables were correlated, limiting interpretation of differences in periphyton Se accumulation among lakes. The results of this research provide insight on the biodynamics of Se assimilation at the base of boreal lake food webs at environmentally relevant concentrations, which can potentially inform ecological risk assessments in boreal lake ecosystems in North America.

Response of Crustacean Zooplankton and Benthic Macroinvertebrate Communities to Selenium Additions in a Boreal Lake

Selenium (Se) is a contaminant of concern in Canada mainly due to its teratogenic effects on fish and birds. However, few studies have assessed the effects of Se on invertebrates in a field setting. The objective of this experiment was to assess potential community-level impacts of Se additions on zooplankton and benthic macroinvertebrates in a boreal lake ecosystem. From June to August 2018, Se (as selenite) was added to six limnocorrals in Lake 239 at the International Institute for Sustainable Development-Experimental Lakes Area, Northwestern Ontario, Canada, to achieve mean measured aqueous concentrations of 0.4, 0.8, 1.6, 3.4, 5.6 and 7.9 µg Se/L, with three untreated limnocorrals serving as controls (background Se = 0.08-0.09 µg/L). Periphyton, phytoplankton, and invertebrates (zooplankton and benthos) were monitored for 63 days. Zooplankton community composition shifted as a function of Se exposure, with Cladocera biomass and density decreasing with increasing Se concentrations. Similarly, cumulative abundance and biomass of Heptageniidae decreased with increasing Se treatment throughout the experimental period. The present study demonstrated that Se can have impacts on aquatic invertebrates at environmentally relevant exposure levels, and that future ecological risk assessments should consider the impacts of Se on both vertebrates and invertebrates. Environ Toxicol Chem 2022;41:95-107. © 2021 SETAC.

Role of Biofilms in Contaminant Bioaccumulation and Trophic Transfer in Aquatic Ecosystems: Current State of Knowledge and Future Challenges

In freshwater environments, microbial assemblages attached to submerged substrates play an essential role in ecosystem processes such as primary production, supported by periphyton, or organic matter decomposition, supported by microbial communities attached to leaf litter or sediments. These microbial assemblages, also called biofilms, are not only involved in nutrients fluxes but also in contaminants dynamics. Biofilms can accumulate metals and organic contaminants transported by the water flow and/or adsorbed onto substrates. Furthermore, due to their high metabolic activity and their role in aquatic food webs, microbial biofilms are also likely to influence contaminant fate in aquatic ecosystems. In this review, we provide (1) a critical overview of the analytical methods currently in use for detecting and quantifying metals and organic micropollutants in microbial biofilms attached to benthic substrata (rocks, sediments, leaf litter); (2) a review of the distribution of those contaminants within aquatic biofilms and the role of these benthic microbial communities in contaminant fate; (3) a set of future challenges concerning the role of biofilms in contaminant accumulation and trophic transfers in the aquatic food web. This literature review highlighted that most knowledge on the interaction between biofilm and contaminants is focused on contaminants dynamics in periphyton while technical limitations are still preventing a thorough estimation of contaminants accumulation in biofilms attached to leaf litter or sediments. In addition, microbial biofilms represent an important food resource in freshwater ecosystems, yet their role in dietary contaminant exposure has been neglected for a long time, and the importance of biofilms in trophic transfer of contaminants is still understudied.

Proton Competition and Free Ion Activities Drive Cadmium, Copper, and Nickel Accumulation in River Biofilms in a Nordic Ecosystem

Biofilms can be used as a biomonitoring tool to determine metal bioavailability in streams affected by mining and other anthropogenic activities. Surface water and biofilm were sampled over two years from rivers located in the vicinity of a mine located in a Nordic ecosystem (Nunavik, Quebec). Biofilm metal content (Cd, Cu, and Ni) as well as a variety of physicochemical properties were determined to examine relationships between metal accumulation and water quality. Among the three metals of interest, copper and nickel had the highest levels of accumulation and cadmium had the lowest. When considering the exposure levels, nickel was the most abundant metal in our sampling sites. Both exposure and accumulation levels were consistent over time. Biofilm metal content was highly correlated to the ambient free metal ion concentration for sites of circumneutral pHs for all three metals. When the surface water pH was below 6, biofilm metal content was much lower than at other sites with similar aqueous metal concentrations of exposure. This apparent protective effect of decreasing pH can be explained by proton competition with dissolved metals for uptake binding sites at the surface of the organisms within the biofilm as described by the Biotic Ligand Model principles. The relationships obtained for Cd and Cu were overlapping those observed in previous publications, indicating strong similarities in metal accumulation processes in biofilms over very large geographical areas. Although more data are needed for Ni, our results show that biofilms represent a promising metal biomonitoring tool.

Transcriptomic and life history responses of the mayfly Neocloeon triangulifer to chronic diel thermal challenge

To better understand the effects of transient thermal stress in an aquatic insect, we first identified static temperatures associated with fitness deficits, and then reared larvae from egg hatch to adulthood under diurnally variable regimens including daily forays into deleterious temperatures. We sampled mature larvae at the coolest and warmest portions of their respective regimens for RNA-seq analysis. Few transcripts (28) were differentially expressed when larvae oscillated between favorable temperatures, while 614 transcripts were differentially expressed when experiencing daily transient thermal stress. Transcripts associated with N-glycan processing were downregulated while those associated with lipid catabolism and chitin turnover were significantly upregulated in heat stressed larvae. An across-regimen comparison of differentially expressed transcripts among organisms sampled at comparable temperatures demonstrated that the effects of daily thermal stress persisted even when larvae were sampled at a more optimal temperature (806 differentially expressed transcripts). The chronically stressed population had reduced expression of transcripts related to ATP synthesis, mitochondrial electron chain functions, gluconeogenesis and glycolytic processes while transcripts associated with cell adhesion, synaptic vesicle transport, regulation of membrane potential and lipid biosynthesis increased. Comparisons of constant vs. variable temperatures revealed that the negative consequences of time spent at stressful temperatures were not offset by more time spent at optimal temperatures.

Biofilms grown in aquatic microcosms affect mercury and selenium accumulation in Daphnia

Experiments examining mercury (Hg) toxicity in Daphnia are usually conducted in highly standardized conditions that prevent the formation of biofilm. Although such standardization has many advantages, extrapolation of results to natural conditions and inference of ecological effects is challenging. This is especially true since biofilms can accumulate metals/metalloids and play a key role in their transfer to higher trophic level organisms. In this study, we experimentally tested the effects of spontaneously appearing biofilm in Daphnia cultures on accumulation of Hg and its natural antagonist selenium (Se) in Daphnia magna. We added Hg (in the form of mercury (II) chloride) at two concentrations (0.2 µg/L and 2 µg/L) to experimental microcosms and measured the uptake of Hg and Se by D. magna in the presence and absence of biofilm. To test for consistent and replicable results, we ran two identical experimental sets one week apart. Biofilm presence significantly reduced the accumulation of Hg, while increasing the tissue Se content in D. magna, and these findings were reproducible across experimental sets. These findings indicate that highly standardized tests may not be adequate to predict the bioaccumulation and potential toxicity of metals/metalloids under natural conditions.

Contaminant Subsidies to Riparian Food Webs in Appalachian Streams Impacted by Mountaintop Removal Coal Mining

Selenium is highly elevated in Appalachian streams and stream organisms that receive alkaline mine drainage from mountaintop removal coal mining compared to unimpacted streams in the region. Adult aquatic insects can be important vectors of waterborne contaminants to riparian food webs, yet pathways of Se transport and exposure of riparian organisms are poorly characterized. We investigated Se concentrations in stream and riparian organisms to determine whether mining extent increased Se uptake in stream biofilms and insects and if these insects were effective Se biovectors to riparian spiders. Biofilm Se concentration increased (p = 0.006) with mining extent, reaching a maximum value of 16.5 μg/g of dw. Insect and spider Se increased with biofilm Se (p = 0.004, p = 0.003), reaching 95 and 26 μg/g of dw, respectively, in mining-impacted streams. Adult insect biomass was not related to mining extent or Se concentrations in biofilm. Even though Se concentrations in aquatic insects were significantly and positively related to mining extent, aquatic insect Se flux was not associated with mining extent because the mass of emerging insects did not change appreciably over the mining gradient. Insect and spider Se concentrations were among the highest reported in the literature, regularly exceeding the bird Se dietary risk threshold of 5 μg/g of dw. Risks of Se exposure and toxicity related to mining are thus not constrained to aquatic systems but extend to terrestrial habitats and food webs.

Uptake, toxicity, and maternal transfer of cadmium in the oribatid soil mite, Oppia nitens: Implication in the risk assessment of cadmium to soil invertebrates

Cadmium (Cd) is a heavy metal of concern in contaminated sites because of its high toxicity to soil biota and humans. Typically, Cd exposure is thought to be dominated by dissolved Cd in soil pore water and, thus, dermal uptake. In this study, we investigated the uptake, toxicity, and maternal transfer of Cd in a standard soil invertebrate, the oribatid mite (Oppia nitens), which is common to boreal and temperate ecozones. We found total soil Cd predicted Cd uptake in adult and juvenile O. nitens with no significant uptake from pore water by juvenile mites. Cadmium significantly inhibited juvenile production and recruitment as well as reduced adult fecundity. Adult O. nitens maternally transferred 39-52% of their Cd body burden to juveniles (tritonymphs) while the maternally-acquired Cd accounted for 41% of the juvenile internal Cd load. Our results suggest that dermal adsorption of metal ions is not important for O. nitens and that maternal transfer of Cd in soil invertebrates has ecological and toxicological implications for populations of soil invertebrates. Maternal transfer should be incorporated as a criterion in setting environmental soil quality guidelines (SQG_E) for cadmium and other non-essential heavy metals.

Selenium in buoyant marine debris biofilm

Marine debris is widespread in all the world's oceans. Currently little is understood about how marine debris affects the chemistry of the surface oceans, particularly trace elements that can adsorb to the surface of marine debris, especially plastic debris, or be taken up by biofilms and algae growing on the surface of marine debris. Selenium (Se) is a micronutrient that is essential to all living organisms. Average seawater Se concentrations in the modern ocean are <1 nM. Here we measure the concentration of Se in surface water and one deep water sample and the concentration of Se found in algae/biofilms growing on the surface of macro-debris collected in October of 2012. Concentrations of Se in biofilm varied more according to the type of biofilm rather than the type of plastic. However, further Se measurements are needed for more conclusive results.

Oppia nitens C.L. Koch, 1836 (Acari: Oribatida): Current Status of Its Bionomics and Relevance as a Model Invertebrate in Soil Ecotoxicology

The oribatid soil mite Oppia nitens C.L. Koch, 1836, is a model microarthropod in soil ecotoxicity testing. This species has a significant role in supporting soil functions and as a suitable indicator of soil contamination. Despite its significance to the environment and to ecotoxicology, however, very little is known of its biology, ecology, and suborganismal responses to contaminants in the soil. In the present review, we present detailed and critical insights into the biology and ecology of O. nitens in relation to traits that are crucial to its adaptive responses to contaminants in soil. We used a species sensitivity distribution model to rank the species sensitivity to heavy metals (cadmium and zinc) and neonicotinoids (imidacloprid and thiacloprid) compared with other standardized soil invertebrates. Although the International Organization for Standardization and Environment and Climate Change Canada are currently standardizing a protocol for the use of O. nitens in soil toxicity testing, we believe that O. nitens is limited as a model soil invertebrate until the molecular pathways associated with its response to contaminants are better understood. These pathways can only be elucidated with information from the mites' genome or transcriptome, which is currently lacking. Despite this limitation, we propose a possible molecular pathway to metal tolerance and a putative adverse outcome pathway to heavy metal toxicity in O. nitens. Environ Toxicol Chem 2019;38:2593-2613. © 2019 SETAC.

The role of the freshwater oligochaete Limnodrilus hoffmeisteri in the distribution of Se in a water/sediment microcosm

Selenite(IV) and selenate(VI) are the major species of Se in the seleniferous aquatic ecosystem. The redistribution of Se in the water/sediment microcosm by bioturbation remains largely unknown. In this study, the redistribution of Se in the water/sediment microcosm by the benthic oligochaete Limnodrilus hoffmeisteri was assessed. The worms were exposed to 2-40 μg/g dry weight of Se(IV) or Se(VI) in the sediment (diet) for 2 months. The changes in the Se levels in different compartments of the microcosm (sediment, overlying water, and worms) were quantified after 2 weeks and 2 months. The subcellular distribution of Se in the worms were also evaluated. Finally, the volatilization of Se from the two Se sources was estimated. The results showed that Se concentration in the overlying water and Se bioaccumulation in the worms were increased with Se levels in the sediments. Approximately 1.6-9.8% of Se was volatilized in the absence of the worms and was intensified in the presence of the worms (2.1-25.7%). The subcellular distribution witnessed high levels of Se in the cell debris (>60%). Se(IV) and Se(VI) differ in their bioaccumulation, redistribution and the effects on the growth of the worms. Our results suggest that the bioturbation by benthos play an essential role in the redistribution of Se in the water/sediment microcosm.

A new and practical Se(IV) removal method using Fe3+ type cation exchange resin

An effective method for removing selenium (Se) from water is required from the viewpoint of environmental preservation. To establish this method, a cation exchange resin that adsorbed ferric ions was applied as an adsorbent. In this study, the adsorption behavior of Se to the adsorbent was examined by both batch and column methods. The batch experiment confirmed that selenite ions (Se(IV)) are effectively adsorbed but selenate ions (Se(VI)) are hardly adsorbed. To elucidate the adsorption mechanism, the Fe in the adsorbent and the Fe in the adsorbent after the adsorption of Se(IV) were characterized by Fe K-edge X-ray absorption spectroscopy and ⁵⁷Fe Mӧssbauer spectroscopy. The analytical result of Se K-edge EXAFS spectra for the Se(IV) adsorbed on the adsorbent suggests that Se(IV) are adsorbed specifically to the adsorbent through the formation of Fe-O-Se bonds. The breakthrough curve obtained by the column experiment showed that Se(IV) in 3 tons of synthetic solution containing 0.1 ppm Se can be efficiently removed using a column in which 12.8 g (10.4 cm³) of the adsorbent was packed.

Selenium oxyanion bioconcentration in natural freshwater periphyton

Selenium (Se) enrichment has been demonstrated to vary by several orders of magnitude among species of planktonic algae. This is a substantial source of uncertainty when modelling Se biodynamics in aquatic systems. In addition, Se bioconcentration data are largely lacking for periphytic species of algae, and for multi-species periphyton biofilms, adding to the challenge of modelling Se transfer in periphyton-based food webs. To better predict Se dynamics in periphyton dominated, freshwater ecosystems, the goal of this study was to assess the relative influence of periphyton community composition on the uptake of waterborne Se oxyanions. Naturally grown freshwater periphyton communities, sampled from five different water bodies, were exposed to environmentally relevant concentrations of selenite [Se(IV)] or selenate [Se(VI)] (nominal concentrations of 5 and 25 μg Se L^-1) under similar, controlled laboratory conditions for a period of 8 days. Unique periphyton assemblages were derived from the five different field sites, as confirmed by light microscopy and targeted DNA sequencing of the plastid 23S rRNA gene in algae. Selenium accumulation demonstrated a maximum of 23.6-fold difference for Se(IV) enrichment and 2.1-fold difference for Se(VI) enrichment across the periphyton/biofilm assemblages tested. The assemblage from one field site demonstrated both high accumulation of Se(IV) and iron, and was subjected to additional experimentation to elucidate the mechanism(s) of Se accumulation. Selenite accumulation (at nominal concentrations of 5 and 25 μg Se L^-1 and mean pH of 7.5 across all treatment replicates) was assessed in both unaltered and heat-killed periphyton, and in periphyton from the same site grown without light to exclude phototrophic organisms. Following an exposure length of 8 days, all periphyton treatments showed similar levels of Se accumulation, indicating that much of the apparent uptake of Se(IV) was due to non-biological processes (i.e., surface adsorption). The results of this study will help reduce uncertainty in the prediction of Se dynamics and food-chain transfer in freshwater environments. Further exploration of the ecological consequences of extracellular adsorption of Se(IV) to periphyton, rather than intracellular absorption, is recommended to further refine predictions related to Se biodynamics in freshwater food webs.

Temperature affects acute mayfly responses to elevated salinity: implications for toxicity of road de-icing salts

Salinity in freshwater ecosystems has increased significantly at numerous locations throughout the world, and this increase often reflects the use or production of salts from road de-icing, mining/oil and gas drilling activities, or agricultural production. When related to de-icing salts, highest salinity often occurs in winter when water temperature is often low relative to mean annual temperature at a site. Our study examined acute (96 h) responses to elevated salinity (NaCl) concentrations at five to seven temperature treatments (5-25°C) for four mayfly species (Baetidae: Neocloeon triangulifer, Procloeon fragile; Heptageniidae: Maccaffertium modestum; Leptophlebiidae: Leptophlebia cupida) that are widely distributed across eastern North America. Based on acute LC50s at 20°C, P. fragile was most sensitive (LC50 = 767 mg l^-1, 1447 µS cm^-1), followed by N. triangulifer (2755 mg l^-1, 5104 µS cm^-1), M. modestum (2760 mg l^-1, 5118 µS cm^-1) and L. cupida (4588 mg l^-1, 8485 µS cm^-1). Acute LC50s decreased as temperature increased for all four species (n = 5-7, R ² = 0.65-0.88, p = 0.052-0.002). Thus, acute salt toxicity is strongly temperature dependent for the mayfly species we tested, which suggests that brief periods of elevated salinity during cold seasons or in colder locations may be ecologically less toxic than predicted by standard 20 or 25°C laboratory bioassays.This article is part of the theme issue 'Salt in freshwaters: causes, ecological consequences and future prospects'.

Are sulfate effects in the mayfly Neocloeon triangulifer driven by the cost of ion regulation?

Elevated major ion concentrations in streams are commonly observed as a consequence of resource extraction, de-icing and other anthropogenic activities. Ecologists report biodiversity losses associated with increasing salinity, with mayflies typically being highly responsive to increases of different major ions. In this study, we evaluated the performance of the mayfly Neocloeon triangulifer reared for its entire larval phase in a gradient of sulfate concentrations. Two natural waters were amended with SO₄ as a blend of CaSO₄ and MgSO₄ and exposures ranged from 5 to 1500 mg l^-1 SO_4. Survival (per cent successful emergence to the subimago stage) was significantly reduced at the highest SO₄ concentration in both waters, while development was significantly delayed at 667 mg l^-1 SO₄ Final sub-adult body weights were consistent across treatments, except at the highest treatment concentration. Despite evidence for sulfate uptake rates increasing with exposure concentrations and not being saturated at even extremely high SO₄ concentrations, total body sulfur changed little in subimagos. Together, these results suggest that elevated SO₄ imposes an energetic demand associated with maintaining homeostasis that is manifested primarily as reduced growth rates and associated developmental delays. We identified two genes related to sulfate transport in N. trianguliferThis article is part of the theme issue 'Salt in freshwaters: causes, ecological consequences and future prospects'.

Selenium dynamics in headwater streams of the central Appalachian coalfield

Coal mining can cause selenium (Se) contamination in US Appalachian streams, but linkages between water-column Se concentrations and Se bioaccumulation within Appalachian headwater streams have rarely been quantified. Using elevated specific conductance (SC) in stream water as an indicator of mining influence, we evaluated relationships between SC and Se concentrations in macroinvertebrates and examined dynamics of Se bioaccumulation in headwater streams. Twenty-three Appalachian streams were categorized into 3 stream types based on SC measurements: 1) reference streams with no coal-mining history; 2) mining-influenced, high-SC streams; and 3) mining-influenced, low-SC streams. Selenium concentrations in macroinvertebrates exhibited strong positive associations with both SC and dissolved Se concentrations in stream water. At 3 streams of each type, we further collected water, particulate matter (sediment, biofilm, leaf detritus), and macroinvertebrates and analyzed them for Se during 2 seasons. Enrichment, trophic transfer, and bioaccumulation factors were calculated and compared among stream types. Particulate matter and macroinvertebrates in mining-influenced streams accumulated high Se concentrations relative to reference streams. Concentrations were found at levels indicating Se to be a potential environmental stressor to aquatic life. Most Se enrichment, trophic transfer, and bioaccumulation factors were independent of season. Enrichment factors for biofilm and sediments and bioaccumulation factors for macroinvertebrate predators varied negatively with water-column Se. Our results increase scientific understanding of Se bioaccumulation processes in Appalachian headwater streams. Environ Toxicol Chem 2018;37:2714-2726. © 2018 SETAC.

Severing Ties: Different Responses of Larval and Adult Aquatic Insects to Atrazine and Selenium

Aquatic insects link aquatic and terrestrial ecosystems through their metamorphosis and subsequent transition from water to land. Chemical stressors in freshwater, such as agricultural contaminants, can potentially disrupt insect life cycles and reduce the number of insects emerging as terrestrial adults, thereby damaging or severing this linkage. Atrazine and selenium, though frequently detected in waterways and often co-occurring, have not been previously studied together in controlled experiments. We conducted a six-week mesocosm experiment to measure the responses of larval and emerging aquatic insects to treatments of atrazine (15 μg/L), selenium (10 μg/L), and a direct combination of the two. Peak adult insect abundance was reduced in all treatments by 35% to 45% relative to the control. Further, cumulative adult emergence in the combined treatment was 33% lower than in the control. However, no reductions in primary production were observed with treatments, and consistent reductions in benthic insect abundance relative to the control were not observed until the end of the experiment, when overall abundance was low. Results suggest that adult insects are more sensitive than larval insects to atrazine and selenium and that the impacts of these contaminants are stronger on the terrestrial than the aquatic ecosystem.

Petroleum hydrocarbon mixture toxicity and a trait-based approach to soil invertebrate species for site-specific risk assessments

Although petroleum hydrocarbons released to the environment typically occur as mixtures, petroleum hydrocarbon remediation guidelines often reflect individual substance toxicity. It is well documented that groups of aliphatic petroleum hydrocarbons act via the same mechanism of action (nonpolar narcosis) and, theoretically, concentration addition mixture toxicity principles apply. To assess this theory, 10 standardized acute and chronic soil invertebrate toxicity tests on a range of organisms (Eisenia fetida, Lumbricus terrestris, Enchytraeus crypticus, Folsomia candida, Oppia nitens, and Hypoaspis aculeifer) were conducted with a refined petroleum hydrocarbon binary mixture. Reference models for concentration addition and independent action were applied to the mixture toxicity data with consideration of synergism, antagonism, and dose level toxicity. Both concentration addition and independent action, without further interactions, provided the best fit with observed response to the mixture. Individual fraction effective concentration values were predicted from optimized, fitted reference models. Concentration addition provided a better estimate than independent action of individual fraction effective concentrations based on comparison with available literature and species trends observed in toxic responses to the mixture. Interspecies differences in standardized laboratory soil invertebrate species responses to petroleum hydrocarbon-contaminated soil was reflected in unique traits. Diets that included soil, large body size, permeable cuticle, low lipid content, lack of ability to molt, and no maternal transfer were traits linked to a sensitive survival response to petroleum hydrocarbon-contaminated soil in laboratory tests. Traits linked to sensitive reproduction response in organisms tested were long life span and small clutch size. By deriving single-fraction toxicity endpoints considerate of mixtures, we can reduce the resources and time required to conduct site-specific risk assessments for the protection of a soil organism's exposure pathway. Environ Toxicol Chem 2018;37:2222-2234. © 2018 SETAC.

Trace element biomineralisation in the carapace in male and female Argulus japonicus

Parasites of fishes have been shown to be effective bioindicators of the aquatic environment. Few investigations have been conducted on ectoparasite models and therefore little is known about the fate of trace elements and metals which they accumulate. In this study trace element sequestration was observed in the carapace of the fish louse, Argulus japonicus and found to relate to the sex of the parasite, as well as, the degree of sclerotization of the carapace. Adults of A. japonicus were collected from cyprinid hosts in the Vaal Dam, South Africa. Parasites were removed and flash frozen in liquid nitrogen before being sectioned with a cryomicrotome. Sections and whole mounts of parasites were prepared and treated with Phen–Green ^TM FL cell–permeant diacetate. Cryosections were assessed for trace elements and metals using a scanning electron microscope equipped with energy dispersive spectroscopy. Results indicated that in both male and female parasites, trace elements become bound to the carapace and produce more intense fluorescence than in soft tissues. Sexual dimorphic differences were further observed between male and female parasites. The intensity of the fluorescence signals was greater in the carapace of male parasites than in females, particularly when comparing the carapace of the ventral side of the thorax. In females, an amorphous layer of material surrounding the eggs was observed and produced an intense fluorescent signal. Levels of trace elements and metals detected were not significantly different between male and female parasites. Results observed serve as a demonstration for the first time of trace element sequestration in a freshwater crustacean parasite and possible mechanisms employed to reduce body burdens of trace elements and metals.

Periphyton and abiotic factors influencing arsenic speciation in aquatic environments

Benthic periphytic biofilms are important food sources at the base of aquatic ecosystems. These biofilms also sit at the interface of oxic waters and hypoxic sediments, and can be influenced by or influence trace element speciation. In the present study, we compared arsenic (As) enrichment in periphyton exposed to arsenate (As[V]) or arsenite (As[III]) (20 μg/L, static renewal, 7 d), and we found similar accumulation patterns of total As (101 ± 27 and 88 ± 22 mg kg^-1 dry wt, respectively). Periphyton As was 6281- and 6684-fold higher than their aqueous exposures and occurred primarily as As(V). When these biofilms were fed to larval mayflies, similar total As tissue concentrations (13.9 and 14.6 mg kg^-1 dry wt, respectively) were observed, revealing significant biodilution (∼ 10% of their dietary concentrations). Finally, we investigated the influence of aeration and periphyton presence on As speciation in solutions and solid phases treated with As(III). Predominantly As(III) solutions were slowly oxidized over a 7-d time period, in the absence of periphyton, and aeration did not strongly affect oxidation rates. However, in the presence of periphyton, solution and solid-phase analyses (by microscale x-ray absorption spectroscopy) showed rapid As(III) oxidation to As(V) and an increasing proportion of organo-As forming over time. Thus periphyton plays several roles in As environmental behavior: 1) decreasing total dissolved As concentrations via abiotic and biotic accumulation, 2) rapidly oxidizing As(III) to As(V), 3) effluxing organo-As forms into solution, and 4) limiting trophic transfer to aquatic grazers. Environ Toxicol Chem 2018;37:903-913. © 2017 SETAC.

Metabolomics reveal physiological changes in mayfly larvae (Neocloeon triangulifer) at ecological upper thermal limits

Aquatic insects play critical roles in freshwater ecosystems and temperature is a fundamental driver of species performance and distributions. However, the physiological mechanisms that determine the thermal performance of species remain unclear. Here we used a metabolomics approach to gain insights into physiological changes associated with a short-term, sublethal thermal challenge in the mayfly Neocloeon triangulifer (Ephemeroptera: Baetidae). Larvae were subjected to a thermal ramp (from 22 to 30°C at a rate of 1°C/h) and metabolomics analysis (both Nuclear Magnetic Resonance (NMR) Spectroscopy and Gas Chromatography coupled Time-of-Flight Mass Spectrometry (GC-TOF-MS)) indicated that processes related to energetics (sugar metabolism) and membrane stabilization primarily differentiated heat treated larvae from controls. Limited evidence of anaerobic metabolism was observed in the heat treated larvae at 30°C, a temperature that is chronically lethal to larvae.

Physiological responses to short-term thermal stress in mayfly (Neocloeon triangulifer) larvae in relation to upper thermal limits

Understanding species' thermal limits and their physiological determinants is critical in light of climate change and other human activities that warm freshwater ecosystems. Here, we ask whether oxygen limitation determines the chronic upper thermal limits in larvae of the mayfly Neocloeon triangulifer, an emerging model for ecological and physiological studies. Our experiments are based on a robust understanding of the upper acute (∼40°C) and chronic thermal limits of this species (&gt;28°C, ≤30°C) derived from full life cycle rearing experiments across temperatures. We tested two related predictions derived from the hypothesis that oxygen limitation sets the chronic upper thermal limits: (1) aerobic scope declines in mayfly larvae as they approach and exceed temperatures that are chronically lethal to larvae; and (2) genes indicative of hypoxia challenge are also responsive in larvae exposed to ecologically relevant thermal limits. Neither prediction held true. We estimated aerobic scope by subtracting measurements of standard oxygen consumption rates from measurements of maximum oxygen consumption rates, the latter of which was obtained by treating with the metabolic uncoupling agent carbonyl cyanide-4-(trifluoromethoxy) pheylhydrazone (FCCP). Aerobic scope was similar in larvae held below and above chronic thermal limits. Genes indicative of oxygen limitation (LDH, EGL-9) were only upregulated under hypoxia or during exposure to temperatures beyond the chronic (and more ecologically relevant) thermal limits of this species (LDH). Our results suggest that the chronic thermal limits of this species are likely not driven by oxygen limitation, but rather are determined by other factors, e.g. bioenergetics costs. We caution against the use of short-term thermal ramping approaches to estimate critical thermal limits (CTmax) in aquatic insects because those temperatures are typically higher than those that occur in nature.

Genetic Characterization of Periphyton Communities Associated with Selenium Bioconcentration and Trophic Transfer in a Simple Food Chain

A major source of uncertainty in predicting selenium (Se) distribution in aquatic food webs lies in the enrichment factor (EF), the ratio of Se bioconcentration in primary producers and microorganisms relative to the concentration of Se in the surrounding water. It has been well demonstrated that EFs can vary dramatically among individual algal taxa, but data are lacking regarding the influence of periphyton community composition on EFs for a given geochemical form of Se. Therefore, the goals of this study were first to assess whether different periphyton communities could be established in aquaria with the same starting inoculum using different light and nutrient regimes, and second, to determine if the periphyton assemblage composition influences the uptake of waterborne Se (as selenite) and subsequent Se transfer to a model macroinvertebrate primary consumer. Periphyton biofilms were grown in aquaria containing filtered pond water (from Saskatoon, SK) spiked with approximately 20 μg Se/L (mean measured concentration 21.0 ± 1.2 μg Se/L), added as selenite. Five different light and nutrient regimes were applied to the aquaria (three replicates per treatment) to influence biofilm community development. After 6 weeks of biofilm maturation, 40 to 80 immature cultured snails (Stagnicola elodes) were added to each aquarium. The bacterial and algal members of the periphyton community were characterized by targeted metagenomic analyses before and after addition of snails to ensure the snails themselves did not significantly alter the biofilm community. Samples were collected for Se analysis of water, periphyton, and whole-body snail. The nutrient and light treatments resulted in substantially different compositions of the periphytic biofilms, with each being relatively consistent across replicates and throughout the study. Although the aqueous concentration of dissolved Se administered to treatments was constant, uptake by the different periphytic biofilms differed significantly. Both the low-light (61.8 ± 12.1 μg Se/g d.w.) and high-light (30.5 ± 4.7 μg Se/g d.w.) biofilms, which were found to have high proportions of cyanobacteria, contained statistically higher concentrations of Se relative to the other treatments. Furthermore, the concentration of Se in bulk periphyton was predictive of Se bioaccumulation in grazing snails but as an inverse relationship, opposite to expectations. The trophic transfer factor was inversely correlated with periphyton enrichment factor (r = -0.841). A number of different bacterial and algal taxa were correlated (either positively or negatively) with Se accumulation in periphyton biofilm and snails. Recent advancements in genetic methods make it possible to conduct detailed characterization of periphyton assemblages and begin to understand the influence that periphyton composition has on Se biodynamics in aquatic systems.

Arsenic (V) bioconcentration kinetics in freshwater macroinvertebrates and periphyton is influenced by pH

Arsenic is an important environmental pollutant whose speciation and mobility in freshwater food webs is complex. Few studies have characterized uptake and efflux rates of arsenic in aquatic benthic invertebrates. Further, we lack a fundamental understanding of how pH influences uptake kinetics in these organisms or how this key environmental variable could alter dietary exposure for primary consumers. Here we used a radiotracer approach to characterize arsenate accumulation dynamics in benthic invertebrates, the influence of pH on uptake in a subset of these organisms, and the influence of pH on uptake of arsenate by periphyton - an important food source at the base of aquatic food webs. Uptake rate constants (K_u) from aqueous exposure were modest, ranging from ∼0.001 L g^-1d^-1 in three species of mayfly to 0.06 L g^-1d^-1in Psephenus herricki. Efflux rate constants ranged from ∼0.03 d^-1 in Corbicula fluminea to ∼0.3 d^-1 in the mayfly Isonychia sp, and were generally high. Arsenate uptake decreased with increasing pH, which may be a function of increased adsorption at lower pHs. A similar but much stronger correlation was observed for periphyton where K_u decreased from ∼3.0 L g^-1d^-1 at 6.5 pH to ∼0.7 L g^-1d^-1 at 8.5 pH, suggesting that site specific pH could significantly alter arsenic exposure, particularly for primary consumers. Together, these findings shed light on the complexity of arsenic bioavailability and help explain observed differences reported in the literature.

Maternal transfer of trace elements in the Atlantic horseshoe crab (Limulus polyphemus)

The maternal transfer of trace elements is a process by which offspring may accumulate trace elements from their maternal parent. Although maternal transfer has been assessed in many vertebrates, there is little understanding of this process in invertebrate species. This study investigated the maternal transfer of 13 trace elements (Ag, As, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Se, and Zn) in Atlantic horseshoe crab (Limulus polyphemus) eggs and compared concentrations to those in adult leg and gill tissue. For the majority of individuals, all trace elements were transferred, with the exception of Cr, from the female to the eggs. The greatest concentrations on average transferred to egg tissue were Zn (140 µg/g), Cu (47.8 µg/g), and Fe (38.6 µg/g) for essential elements and As (10.9 µg/g) and Ag (1.23 µg/g) for nonessential elements. For elements that were maternally transferred, correlation analyses were run to assess if the concentration in the eggs were similar to that of adult tissue that is completely internalized (leg) or a boundary to the external environment (gill). Positive correlations between egg and leg tissue were found for As, Hg, Se, Mn, Pb, and Ni. Mercury, Mn, Ni, and Se were the only elements correlated between egg and gill tissue. Although, many trace elements were in low concentration in the eggs, we speculate that the higher transfer of essential elements is related to their potential benefit during early development versus nonessential trace elements, which are known to be toxic. We conclude that maternal transfer as a source of trace elements to horseshoe crabs should not be overlooked and warrants further investigation.

Environmental exposure of Atlantic horseshoe crab (Limulus polyphemus) early life stages to essential trace elements

This study investigated the accumulation Co, Cu, Fe, Mn, Ni, Se, and Zn in Atlantic horseshoe crab (Limulus polyphemus) early life stages (egg, embryo and larvae) and compared the concentrations to the concentration of each element in sediment, pore water and overlying water for 5 sites across Long Island, NY. For the majority of the sites, all essential trace elements accumulated in the embryos and larvae. However, many of the embryos and larvae at specific sites presented different concentration patterns which had no apparent relationship with the local habitat sediment and water values. Generally, Cu, Fe, and Se sequentially increased from egg stage through larval stages for the majority of sites, while Co, Mn, and Ni only did for a few sites. Zinc also showed an increase across sites from embryo to larval stage, however was the only one to show a decrease in concentration from egg to embryo stage at all sites. Interestingly, Mn at Manhasset Bay presented embryo and larval stages to be 50 fold greater than all other sites while the egg stage showed similar values to other sites; this high degree of uptake could be due to a high concentration in the overlying water. All essential trace elements can be accumulated from the environment but greater concentrations may be influenced by abiotic factors and the predominant uptake route (aqueous versus diet) at each life stage. Future laboratory experiments are required to investigate factors that influence essential trace element accumulation and loss in horseshoe crab early life stages.

A low level of dietary selenium has both beneficial and toxic effects and is protective against Cd-toxicity in the least killifish Heterandria formosa

As an essential element, selenium (Se) is beneficial at low levels yet toxic at high levels. The present study assessed the effects of dietary exposure to Se in the least killifish Heterandria formosa, and investigated how this exposure influences the effects of a subsequent exposure to cadmium (Cd). The fish were pre-exposed to an environmentally relevant concentration (2 μg g(-1) dry wt) of dietary selenite (Se(4+)) or seleno-l-methionine (Se-Met) for 10 d. The same fish were then exposed to 0.5 mg L(-1) of Cd for 5 d. Both Se(IV) and Se-Met rapidly accumulated in H. formosa. Results for the two Se species were generally similar in this study. Fish exposed to Se had lower levels of lipid peroxidation (measured as levels of thiobarbituric acid reactive substances or TBARS) and a higher catalase (CAT) activity. In contrast, their Na(+)/K(+)-ATPase activity was reduced. The Cd exposure resulted in an increase in lipid peroxidation and decreases in the activities of catalase and Na(+)/K(+)-ATPase. The Cd-exposed H. formosa that were pre-exposed to Se had lower Cd body burdens, less lipid peroxidation, and higher catalase activity, than did fish not pre-exposed to Se. The Se exposure did not have a protective effect on the Cd-induced reduction in Na(+)/K(+)-ATPase activity. These results clearly demonstrate that a Se-enriched diet reduces some (but not all) forms of Cd-toxicity and that Se can simultaneously have beneficial and detrimental effects, making it difficult to predict the net outcome of changes in dietary Se levels for fish.

The bioaccumulation and effects of selenium in the oligochaete Lumbriculus variegatus via dissolved and dietary exposure routes

Aquatic organisms take up selenium from solution and from their diets. Many questions remain regarding the relative importance of selenium accumulation from these sources and resulting effects in benthic invertebrates. The present study addressed the toxicity and accumulation of Se via dissolved and dietary exposures to three different Se species, in the freshwater oligochaete Lumbriculus variegatus. Worms were exposed to 20μg/g dry weight of selenite (Se(IV)), selenate (Se(VI)), or seleno-l-methionine (Se-Met) in their diet (sediment) or to 15μg/L dissolved Se in water-only exposures. While the dissolved and sediment Se levels differed greatly, such levels may co-occur at a Se-contaminated site. Se accumulation, worm population growth, lipid peroxidation (as TBARS), and Na(+)/K(+)-ATPase activity were quantified at the end of the 2-week exposure. The sediment Se-Met exposure caused 100% mortality, while worm densities were reduced by the other exposures except the Se(VI) one. Se bioaccumulation was generally higher for the sediment-Se exposure than the dissolved-Se ones, and was higher for Se(IV) than Se(VI) in the dissolved-Se exposure but not the sediment-Se one. The Se accumulation was highest for Se-Met. The oligochaetes that accumulated Se had higher levels of lipid peroxidation and reduced Na(+)/K(+)-ATPase activity. The present study's findings of differences in Se accumulation and toxicity for the three Se species, with effects generally but not exclusively a function of Se body burdens, underscore the need for research on these issues in invertebrates. Moreover, the results imply that the dietary uptake route is the predominant one for Se accumulation in L. variegatus.

Can't take the heat: Temperature-enhanced toxicity in the mayfly Isonychia bicolor exposed to the neonicotinoid insecticide imidacloprid

Neonicotinoid insecticide usage has increased globally in recent decades. Neonicotinoids, such as imidacloprid, are potent insect neurotoxicants that may pose a threat to non-target aquatic organisms, such as aquatic insects. In nature, insects typically live in thermally fluctuating conditions, which may significantly alter both contaminant exposures and affects. Here we investigate the relationship between temperature and time-to-effect for imidacloprid toxicity with the aquatic insect Isonychia bicolor, a lotic mayfly. Additionally, we examined the mechanisms driving temperature-enhanced toxicity including metabolic rate, imidacloprid uptake rate, and tissue bioconcentration. Experiments included acute toxicity tests utilizing sublethal endpoints and mortality, as well as respirometry and radiotracer assays with [(14)C] imidacloprid. Further, we conducted additional uptake experiments with a suite of aquatic invertebrates (including I. bicolor, Neocloeon triangulifer, Macaffertium modestum, Pteronarcys proteus, Acroneuria carolinensis, and Pleuroceridae sp) to confirm and contextualize our findings from initial experiments. The 96h EC50 (immobility) for I. bicolor at 15°C was 5.81μg/L which was approximately 3.2 fold lower than concentrations associated with 50% mortality. Assays examining the impact of temperature were conducted at 15, 18, 21, and 24°C and demonstrated that time-to-effect for sublethal impairment and immobility was significantly decreased with increasing temperature. Uptake experiments with [(14)C] imidacloprid revealed that initial uptake rates were significantly increased with increasing temperature for I. bicolor, as were oxygen consumption rates. Further, in the separate experiment with multiple species across temperatures 15, 20, and 25°C, we found that all the aquatic insects tested had significantly increased imidacloprid uptake with increasing temperatures, with N. triangulifer accumulating the most imidacloprid on a mass-specific basis. Our acute toxicity results highlight the importance of evaluating sublethal endpoints, as profound impairments of motor function were evident far before mortality. Further, we demonstrate that temperature is a powerful modulator of sublethal toxicity within a range of environmentally relevant temperatures, impacting both uptake rates and metabolic rates of I. bicolor. Finally, we show that temperature alters imidacloprid uptake across a range of species, highlighting the physiological variation present within aquatic invertebrate communities and the challenge associated with relying solely on surrogate species. Taken together, this research points to the need to consider the role of temperature in toxicity assessments.

Bioaccumulation Dynamics of Arsenate at the Base of Aquatic Food Webs

Periphyton is an important food source at the base of freshwater ecosystems that tends to bioconcentrate trace elements making them trophically available. The potential for arsenic-a trace element of particular concern due to its widespread occurrence, toxicity, and carcinogenicity-to bioconcentrate in periphyton and thus be available to benthic grazers is less well characterized. To better understand arsenate bioaccumulation dynamics in lotic food webs, we used a radiotracer approach to characterize accumulation in periphyton and subsequent trophic transfer to benthic grazers. Periphyton bioconcentrated As between 3,200-9,700-fold (dry weight) over 8 days without reaching steady state, suggesting that periphyton is a major sink for arsenate. However, As-enriched periphyton as a food source for the mayfly Neocloeon triangulifer resulted in negligible As accumulation in a full lifecycle exposure. Additional studies estimate dietary assimilation efficiency in several primary consumers ranging from 22% in the mayfly N. triangulifer to 75% in the mayfly Isonychia sp. X-ray fluorescence mapping revealed that As was predominantly associated with iron oxides in periphyton. We speculate that As adsorption to Fe in periphyton may play a role in reducing dietary bioavailability. Together, these results suggest that trophic movement of As in lotic food webs is relatively low, though species differences in bioaccumulation patterns are important.

Metal contaminant accumulation in the hive: Consequences for whole-colony health and brood production in the honey bee (Apis mellifera L.)

Metal pollution has been increasing rapidly over the past century, and at the same time, the human population has continued to rise and produce contaminants that may negatively impact pollinators. Honey bees (Apis mellifera L.) forage over large areas and can collect contaminants from the environment. The primary objective of the present study was to determine whether the metal contaminants cadmium (Cd), copper (Cu), lead (Pb), and selenium (Se) can have a detrimental effect on whole-colony health in the managed pollinator A. mellifera. The authors isolated small nucleus colonies under large cages and fed them an exclusive diet of sugar syrup and pollen patty spiked with Cd, Cu, Pb, and Se or a control (no additional metal). Treatment levels were based on concentrations in honey and pollen from contaminated hives around the world. They measured whole-colony health including wax, honey, and brood production; colony weight; brood survival; and metal accumulation in various life stages. Colonies treated with Cd or Cu contained more dead pupae within capped cells compared with control, and Se-treated colonies had lower total worker weights compared to control. Lead had a minimal effect on colony performance, although many members of the hive accumulated significant quantities of the metal. By examining the honey bee as a social organism through whole-colony assessments of toxicity, the authors found that the distribution of toxicants throughout the colony varied from metal to metal, some caste members were more susceptible to certain metals, and the colony's ability to grow over time may have been reduced in the presence of Se. Apiaries residing near metal-contaminated areas may be at risk and can suffer changes in colony dynamics and survival.

Part 2: Sensitivity comparisons of the mayfly Centroptilum triangulifer to Ceriodaphnia dubia and Daphnia magna using standard reference toxicants; NaCl, KCl and CuSO4

Criteria for establishing water quality standards that are protective for 95% of the native species are generally based upon laboratory toxicity tests. These tests utilize common model organisms that have established test methods. However, for invertebrates these species represent mostly the zooplankton community and are not inclusive of all taxa. In order to examine a potential under-representation in emerging aquatic invertebrates the US Environmental Protection Agency has cultured a parthenogenetic mayfly, Centroptilum triangulifer (Ephemeroptera: Baetidae). This study established a 48h acute and a 14-day short-term chronic testing procedure for C. triangulifer and compared its sensitivity to two model invertebrates, Ceriodaphnia dubia and Daphnia magna. Toxicity tests were conducted to determine mortality and growth effects using standard reference toxicants: NaCl, KCl and CuSO4. In 48-h acute tests, the average LC50 for the mayfly was 659mgL(-1) NaCl, 1957mgL(-1) KCl, and 11μgL(-1) CuSO4. IC25 values, using dry weight as the endpoint, were 228mgL(-1) NaCl, 356mgL(-1) KCl and 5μgL(-1) CuSO4. C. triangulifer was the most sensitive species in NaCl acute and chronic growth tests. At KCl concentrations tested, C. triangulifer was less sensitive for acute tests but was equally or more sensitive than C. dubia and D. magna for growth measurements. This study determined C. triangulifer has great potential and benefits for use in ecotoxicological studies.

Part 1: Laboratory culture of Centroptilum triangulifer (Ephemeroptera: Baetidae) using a defined diet of three diatoms

Development of methods for assessing exposure and effects of waterborne toxicants on stream invertebrate species is important to elucidate environmentally relevant information. Current protocols for freshwater invertebrate toxicity testing almost exclusively utilize cladocerans, amphipods or chironomids rather than the more typical aquatic insect taxa found in lotic systems. Centroptilum triangulifer is a parthenogenetic mayfly occurring in depositional habitats of streams and rivers of the Eastern U.S. and Canada. C. triangulifer is an ideal stream insect for toxicity testing under field and laboratory conditions because of its short life cycle, parthenogenetic mode of reproduction, and it represents a group considered sensitive to environmental stressors. In this study, a colony of C. triangulifer was reared using a defined diet of three diatoms, Mayamaea atomus var. permitis, Nitzschia cf. pusilla, and Achnanthidium minutissimum. Percent survival (⩾80%), fecundity measurements (⩾1000 eggs) and pre-egg laying weights were used as indicators of overall colony health and fitness in our laboratory water (Lab-line) and in Moderately Hard Reconstituted Water (MHRW). Lab-line reared C. triangulifer had average survival rate of 92.69% for eleven generations and 82.99% over thirteen generations. MHRW reared C. triangulifer had an average survival rate of 80.65% for four generations and three generations of fecundities greater than 1000 eggs per individual. Pre-egg laying weight and fecundity were highly correlated and a best-fit model equation was derived to estimate egg counts for future generations. Establishment of this culturing protocol provides a more ecologically relevant species for toxicity testing and aids in further stressor identification for stream bioassessments.

Full-life chronic toxicity of sodium salts to the mayfly Neocloeon triangulifer in tests with laboratory cultured food

Although insects occur in nearly all freshwater ecosystems, few sensitive insect models exist for use in determining the toxicity of contaminants. The objectives of the present study were to adapt previously developed culturing and toxicity testing methods for the mayfly Neocloeon triangulifer (Ephemeroptera: Baetidae), and to further develop a method for chronic toxicity tests spanning organism ages of less than 24 h post hatch to adult emergence, using a laboratory cultured diatom diet. The authors conducted 96-h fed acute tests and full-life chronic toxicity tests with sodium chloride, sodium nitrate, and sodium sulfate. The authors generated 96-h median lethal concentrations (LC50s) of 1062 mg Cl/L (mean of 3 tests), 179 mg N-NO3 /L, and 1227 mg SO4 /L. Acute to chronic ratios ranged from 2.1 to 6.4 for chloride, 2.5 to 5.1 for nitrate, and 2.3 to 8.5 for sulfate. The endpoints related to survival and development time were consistently the most sensitive in the tests. The chronic values generated for chloride were in the same range as those generated by others using natural foods. Furthermore, our weight-versus-fecundity plots were similar to those previously published using the food culturing method on which the present authors' method was based, indicating good potential for standardization. The authors believe that the continued use of this sensitive mayfly species in laboratory studies will help to close the gap in understanding between standard laboratory toxicity test results and field-based observations of community impairment.

Elevated major ion concentrations inhibit larval mayfly growth and development

Anthropogenic disturbances, including those from developing energy resources, can alter stream chemistry significantly by elevating total dissolved solids. Field studies have indicated that mayflies (Order Ephemeroptera) are particularly sensitive to high total dissolved solids. In the present study, the authors measured 20-d growth and survivorship of larval Neocloeon triangulifer exposed to a gradient of brine salt (mixed NaCl and CaCl2 ) concentrations. Daily growth rates were reduced significantly in all salt concentrations above the control (363 µS cm(-1) ) and larvae in treatments with specific conductance >812 µS cm(-1) were in comparatively earlier developmental stages (instars) at the end of the experiment. Survivorship declined significantly when specific conductance was >1513 µS cm(-1) and the calculated 20-d 50% lethal concentration was 2866 µS cm(-1) . The present study's results provide strong experimental evidence that elevated ion concentrations similar to those observed in developing energy resources, such as oil and gas drilling or coal mining, can adversely affect sensitive aquatic insect species.

The effects of the pharmaceutical carbamazepine on life history characteristics of flat-headed mayflies (Heptageniidae) and aquatic resource interactions

Pharmaceutical pollutants are commonly detected in freshwater ecosystems around the world and have biological effects on aquatic organisms. However, current understanding of the influence this contaminant class has on freshwater communities and ecosystems is lacking. Recently the scientific community has called for research focusing on certain pharmaceuticals due to their ubiquity and potential toxicity. Carbamazepine is one of these pharmaceuticals. To better understand the effect carbamazepine has on life history characteristics of aquatic organisms and consumer-resource interactions, we quantified the influence of carbamazepine on the development, growth and behavior of mayfly nymphs (Stenonema sp.) and the alterations in food consumer-resource interactions between Stenonema and algae (Chaetophora). Microcosms were assembled in a factorial design containing algae and mayfly nymphs native to central Indiana and dosed with environmentally relevant concentrations of carbamazepine. From this ecotoxicological experiment we were able to infer that carbamazepine at 2,000 ng/L influenced the development and behavior of Stenonema nymphs and the body dimensions of adult individuals. However, it appears that carbamazepine does not influence consumer-resource interactions at concentrations found in surface waters. The pharmaceutical carbamazepine may influence the behavior, growth and development of mayflies, which could have significant consequences at the population, community and ecosystem level.

Bioaccumulation and speciation of selenium in fish and insects collected from a mountaintop removal coal mining-impacted stream in West Virginia

A major contaminant of concern for mountaintop removal/valley fill (MTR/VF) coal mining is selenium (Se), an essential micronutrient that can be toxic to fish. Creek chubs (Semotilus atromaculatus), green sunfish (Lepomis cyanellus), and composite insect samples were collected in March-July, 2011-2013 at two sites within the Mud River, West Virginia. One site (MR7) receives MTR/VF coal mining effluent, while the reference site (LFMR) does not. MR7 water had significantly higher concentrations of soluble Se (p < 0.01) and conductivity (p < 0.005) compared to LFMR. MR7 whole insects contained significantly higher concentrations of Se compared to LFMR insects (p < 0.001). MR7 creek chubs had significantly higher Se in fillets, liver, and ovary tissues compared to LFMR samples (p < 0.0001, p < 0.0001, and p < 0.02, respectively). MR7 green sunfish fillets contained significantly higher Se (p < 0.0001). Histological examination showed LFMR creek chub gills contained a typical amount of parasitic infestations; however MR7 gills contained minimal to no visible parasites. X-ray absorption spectroscopic analyses revealed that MR7 whole insects and creek chub tissues primarily contained organic Se and selenite. These two species of Mud River fish were shown to specifically accumulate Se differently in tissues compartments. Tissue-specific concentrations of Se may be useful in determining potential reproductive consequences of Se exposure in wild fish populations.

Dynamic selenium assimilation, distribution, efflux, and maternal transfer in japanese medaka fed a diet of se-enriched mayflies

Selenium (Se) trafficking in oviparous species remains understudied and a major source of uncertainty in developing sound Se regulations. Here, we utilized (75)Se to follow Se through a simulated natural food chain (water, periphyton, mayflies (Centroptilum triangulifer), fish (Japanese medaka)). We specifically examined Se assimilation efficiency, tissue distribution, efflux rate, and maternal transfer in medaka. Selenium assimilation efficiency (AE) averaged 63.2 ± 8.8% from mayfly diets and was not affected by mayfly [Se] across a dietary range of 5.6-38.7 μg g(-1) (dry wt). However, AE decreased significantly as mayfly larva size increased. Efflux rate constants (ke) were consistent between reproductively inactive (0.066 d(-1)) and spawning females (0.069 d(-1)). Total Se loss rate constant (ke+egg; efflux and egg deposition) was 0.17 d(-1) in spawning females. Interestingly, medaka appeared to rapidly shuttle Se to their eggs directly from their diet via the ovary, as opposed to mobilization from surrounding tissues, resulting in dynamic egg [Se] that was more attributable to recent dietary Se ingestion than female whole body [Se] in this asynchronous spawning fish. Spawning strategy likely plays a large role in the process of fish egg Se deposition and requires further attention to understand risk and toxicity of Se to fish.

Ecological Importance of Insects in Selenium Biogenic Cycling

Selenium is an essential trace element for animal and human beings. Despite the importance of insects in most ecosystems and their significant contribution to the biological cycling of trace elements due to high abundance, population productivity, and diverse ecosystem functions, surprisingly little information is available on selenium bioaccumulation by these arthropods. This review considers selenium essentiality and toxicity to insects as well as insects’ contribution to selenium trophic transfer through the food chains. Data on Se accumulation by insects of the Dniester River Valley with no anthropogenic Se loading reveal typically low Se content in necrophagous insects compared to predators and herbivores and seasonal variations in Se accumulation.