Impacts of Forest Fire Ash on Aquatic Mercury Cycling.

Mercury (Hg) is a ubiquitous contaminant in the environment and its methylated form, methylmercury (MeHg), poses a worldwide health concern for humans and wildlife, primarily through fish consumption. Global production of forest fire ash, derived from wildfires and prescribed burns, is rapidly increasing due to a warming climate, but their interactions with aqueous and sedimentary Hg are poorly understood. Herein, we compared the differences of wildfire ash with activated carbon and biochar on the sorption of aqueous inorganic Hg and sedimentary Hg methylation. Sorption of aqueous inorganic Hg was greatest for wildfire ash materials (up to 0.21 µg g-1 or 2.2 µg g-1 C) among all of the solid sorbents evaluated. A similar Hg adsorption mechanism for activated carbon, biochar made of walnut, and wildfire ash was found that involves the formation of complexes between Hg and oxygen-containing functional groups, especially the -COO group. Notably, increasing dissolved organic matter from 2.4 to 70 mg C L-1 remarkably reduced Hg sorption (up to 40% reduction) and increased the time required to reach Hg-sorbent pseudo-equilibrium. Surprisingly, biochar and wildfire ash, but not activated carbon, stimulated MeHg production during anoxic sediment incubation, possibly due to the release of labile organic matter. Overall, our study indicates that while wildfire ash can sequester aqueous Hg, the leaching of its labile organic matter may promote production of toxic MeHg in anoxic sediments, which has an important implication for potential MeHg contamination in downstream aquatic ecosystems after wildfires.

Environmental Pollution, Management, and Sustainable Development: Strategies for Vietnam and Other Developing Countries.

This editorial introduces the issue of selected papers that were presented at the third International Conference on Environmental Pollution, Restoration, and Management in Quy Nhon, Vietnam on 6-10 March 2017. While environmental problems caused by diverse municipal, industrial, and other economic development activities continue to increase in many Asian countries, public awareness about environmental management for public health and the environment remain at levels that favor accepting environmental degradation and impacts for tradeoffs with economic values. This special issue resulted from a conference that was organized to bringing scientists from developed and developing countries to Vietnam to share experiences, discuss environmental problems, and enhance future collaborations for research and training in support of better management plans for the environment and health. Papers published in this issue present original results from diverse research on current environmental management challenges in Vietnam and other Asian countries. The research areas include environmental contamination in groundwater and diet that affect human health, waste composting, and effects of wastewater effluent, which is one of the greatest challenges in most Asian countries. In addition, impacts of hazardous chemical emissions and related environmental management efforts, and sustainable development approaches are included.

Origin, Reactivity, and Bioavailability of Mercury in Wildfire Ash.

Wildfires are expected to become more frequent and intensive at the global scale due to climate change. Many studies have focused on the loss of mercury (Hg) from burned forests; however, little is known about the origins, concentration, reactivity, and bioavailability of Hg in residual ash materials in postfire landscapes. We examine Hg levels and reactivity in black ash (BA, low burn intensity) and white ash (WA, high burn intensity) generated from two recent northern California wildfires and document that all ash samples contained measurable, but highly variable, Hg levels ranging from 4 to 125 ng/g dry wt. ( n = 28). Stable Hg isotopic compositions measured in select ash samples suggest that most Hg in wildfire ash is derived from vegetation. Ash samples had a highly variable fraction of Hg in recalcitrant forms (0-75%), and this recalcitrant Hg pool appears to be associated with the black carbon fraction in ash. Both BA and WA were found to strongly sequester aqueous inorganic Hg but not gaseous elemental Hg under controlled conditions. During anoxic ash incubation with natural surface water, we find that Hg in most ash samples had a minimal release and low methylation potential. Thus, the formation of wildfire ash can sequester Hg into relatively nonbioavailable forms, attenuating the potentially adverse effects of Hg erosion and transport to aquatic environments along with eroded wildfire ash.

Effects of contaminated St. Lucie River saltwater sediments on an amphipod (Ampelisca abdita) and a hard-shell clam (Mercenaria mercenaria).

The St. Lucie estuary (SLE) ecosystem in South Florida has been shown to be contaminated with metals and pesticides. Our earlier studies also showed that aquatic organisms, especially benthic species in the SLE ecosystem, might be potentially at high risk from copper (Cu) exposure. The objectives of this study were to conduct studies with separate groups of organisms exposed to seven field-collected sediment samples from the St. Lucie River according to standard procedures to evaluate toxicity and tissue concentrations of Cu and zinc (Zn). Short term and longer term whole sediment acute toxicity studies were performed with Ampelisca abdita and Mercenaria mercenaria. Analysis of sediment chemical characteristics showed that Cu and Zn are of most concern because their concentrations in 86 % of the sediments were higher than the threshold effect concentrations for Florida sediment quality criteria and the National Oceanic and Atmospheric Administration Screening Quick Reference Tables (SQuiRTs) sediment values. There was no significant effect on survival of the tested organisms. However, increased Cu and Zn concentrations in the test organisms were found. Dry weight of the tested organisms was also inversely related to Cu and Zn concentrations in sediments and organisms. The effects on organism weight and Cu and Zn uptake raise concerns about the organism population dynamics of the ecosystem because benthic organisms are primary food sources in the SLE system and are continuously exposed to Cu- and Zn-contaminated sediments throughout their life cycle. The results of the present study also indicate that Cu and Zn exposures by way of sediment ingestion are important routes of exposure.

Microplastic accumulation in the gastrointestinal tracts of nestling and adult migratory birds.

This study examines the abundance and types of microplastic (MP) content in the digestive system of different bird species to help us better understand MP transfer to birds from their environments. The lower GI tracts of six bird species (Tree Swallow (Tachycineta bicolor), Ovenbird (Seiurus aurocapilla), Dark-eyed Junco (Junco hyemalis), Hermit Thrush (Catharus guttatus), Tennessee Warbler (Leiothlypis peregrine), White-throated Sparrow (Zonotrichia albicollis)) were collected for MP analysis. Tree Swallows were collected from nine locations along three drainages in the Milwaukee area and at a remote lake in Wisconsin, USA and consisted of nestlings not yet fledged. The five other bird species were adult migratory bird fatalities from window strikes at the Loyola University Chicago's Lake Shore Campus. Results of plastic analysis showed that the lower GI tracts of all bird species contained different types of MPs (i.e., fibers, fragment, beads). Fiber was the most dominant, followed by fragment and beads. Microplastics were polyethylene, polyethylene terephthalate, nylon, and polyvinyl base materials. There was no statistically significant difference in the site average MP concentration for nestling Tree Swallows or in the species average concentration for adult migratory birds. However, except for Ovenbird, species of adult migratory birds had a species average MP concentration (20.1) that was statistically higher than the site-average MP concentration for Tree Swallow nestlings (5.9). The presence of MPs in the lower GI tracts of unfledged swallows could suggest that MPs had been transferred to the birds from the environment via their diet, such as flying insects-the food source of the birds. Further study should be conducted to investigate the potential transfer of MPs from aquatic insects to birds.

Characterization of Metal Pulsed Exposure to Aquatic Organisms with Post-Exposure Monitoring: Implications for Stormwater Exposure Assessment.

Freshwater organisms are often exposed to contaminants such as heavy metals from stormwater discharges, which are dependent on rainfall duration and intensity. Therefore, standardized (48- or 96-h) continuous exposure methods developed for whole effluent toxicity (WET) testing might not always accurately convey the effects of stormwater and runoff contaminants. The present study characterized the acute toxicity of copper (Cu), zinc (Zn), and cadmium (Cd) to freshwater amphipods (Hyalella azteca) and cadmium (Cd) to water fleas (Ceriodaphnia dubia) using a modified exposure design that integrated relevant pulsed durations and included post-exposure monitoring. Less than 24-h-old C. dubia and 7 to 8-day-old H. azteca were exposed to water spiked with Cu, Zn, or Cd using 6-, 12-, 26-, or 96-h durations under standard laboratory conditions and monitored for cumulative mortality and reproduction (C. dubia only). Lethal effect (LC10s, LC25s, LC50s) and reproductive effect (EC25s, EC50s) were determined based on either mortality or reproduction of organisms at the end of each pulse (6, 12, or 26 h) and at the end of their respective tests (96 h). For all metals exposed to each organism, acute toxicity was found to be highest for the (96 h) continuous exposures. For pulsed exposures, mortality continued to increase following transfer to clean water for post-exposure monitoring. These results indicate a latent effect of Cu, Zn, and Cd to H. azteca and Cd to C. dubia. The present study concluded that using the continuous (48- or 96-h) WET exposure method overestimates the effects of stormwater and runoff contaminants. However, pulsed exposures without post-exposure monitoring also underestimate the toxicity of contaminants. The proposed pulsed exposure design provides a compromise that is more realistic than current WET methods to assess impacts from episodic events and accounts for potential latent effects that may be overlooked without monitoring post-exposure. Environ Toxicol Chem 2022;41:2488-2499. © 2022 SETAC.

Bioaccumulation and toxicity of copper in outdoor freshwater microcosms.

This study characterizes the effects of copper (Cu) on Florida apple snails (Pomacea paludosa) and mosquito fish (Gambusia affinis) using a replicated outdoor microcosm design. Soils used in this study were collected from two Cu-enriched citrus agricultural sites in South Florida (Agler property (AGLR) in St. Lucie County and Sunrise Boys property (SRB) in Palm Beach County) and a reference site (Equus property) in St. Lucie County. The study included a 5-week aging phase, an 11 month exposure phase, and a 3 month post-treatment (exposure) phase. The aging phase was initiated by flooding agricultural soils with rainwater in 4 m(3) fiberglass microcosm tanks. Introducing juvenile apple snails (≤7 d old) and mosquito fish (2-3 cm) into the microcosm tanks initiated the exposure phase. Survival, growth, and reproduction of apple snails and fish, and Cu uptake in apple snails, fish, and periphyton were determined in this study. Water chemistry (e.g., dissolved Cu concentration, dissolved organic carbon and dissolved oxygen concentrations, pH, hardness, alkalinity, etc.) was measured daily or weekly during the study. Initial soil Cu concentrations in Equus, SRB, and AGLR microcosms were 7, 55, and 99 mg/kg dw, respectively. Dissolved Cu concentrations in Equus, SRB and AGLR microcosms at the beginning of the study were 3, 82, and 43 µg/L, respectively and decreased to low saturation levels of about ≤9 µg/L Cu after the first 3 months of the study. The decrease of dissolved Cu concentrations was likely due to the dilution of rainwater. Snail and fish mortality appeared to be higher in SRB microcosms than in Equus and AGLR microcosms. There was no significant difference in growth of the snails between treatments. Snail growth data followed the von Bertalanffy Model. The maximum shell length, shell height, and shell width of the snails calculated by the von Bertalanffy Model (L(∞)) were 2.76, 2.05, and 2.18 cm, respectively. The maximum wet weight was 9.38 g. Growth rate (k) of the snails increased in order of shell height (0.459), shell length (0.550), and shell weight (0.598). There was no reproduction in the snails in any treatments including the reference during the exposure phase. However, Cu did not affect reproduction of fish during this period. Copper concentrations in periphyton from Equus, SRB, and AGLR microcosms ranged from 2 to 62, 31 to 371, and 13 to 478 mg/kg, respectively. Copper concentrations in fish at the beginning, days 30 and 150 of the study ranged from 3.19 to 7.53 mg/kg and were not significantly different from the different treatments. Average Cu concentrations in the soft tissue of dead snails from SRB and AGLR microcosms were 4602 mg/kg dw (ranged from 2913 to 8370 mg/kg dw) and 2824 mg/kg dw (ranged from 2118 to 3600 mg/kg dw), respectively. The Cu concentrations in the soft tissue of dead snails found in this study were higher than the tissue Cu concentrations in live aquatic organisms reported in the literature. These high Cu concentrations in edible apple snail soft tissue might pose a risk to Florida apple snail predators, including the snail kite. The post-exposure phase, with snails exposed to only water (i.e., no soils) showed depuration of copper from apple snails and reproduction in all treatments.

Acute toxicity and effects analysis of endosulfan sulfate to freshwater fish species.

Endosulfan sulfate is a persistent environmental metabolite of endosulfan, an organochlorine insecticide-acaricide presently registered by the United States Environmental Protection Agency. There is, however, limited acute fish toxicity data for endosulfan sulfate. This study determines the acute toxicity (LC50s and LC10s) of endosulfan sulfate to three inland Florida native fish species (mosquitofish [Gambusia affinis]; least killifish [Heterandria formosa]; and sailfin mollies [Poecilia latipinna]) as well as fathead minnows (Pimephales promelas). Ninety-six-h acute toxicity tests were conducted with each fish species under flow-through conditions. For all of the above-mentioned fish species, 96-h LC50 estimates ranged from 2.1 to 3.5 µg/L endosulfan sulfate. The 96-h LC10 estimates ranged from 0.8 to 2.1 µg/L endosulfan sulfate. Of all of the fish tested, the least killifish appeared to be the most sensitive to endosulfan sulfate exposure. The above-mentioned data were combined with previous acute toxicity data for endosulfan sulfate and freshwater fish for an effects analysis. The effects analysis estimated hazardous concentrations expected to exceed 5, 10, and 50% of the fish species' acute LC50 or LC10 values (HC5, HC10, and HC50). The endosulfan sulfate freshwater-fish acute tests were also compared with the available freshwater-fish acute toxicity data for technical endosulfan. Technical endosulfan is a mixture of α- and ß-endosulfan. The LC50s had a wider range for technical endosulfan, and their distribution produced a lower HC10 than for endosulfan sulfate. The number of freshwater-fish LC50s for endosulfan sulfate is much smaller than the number available for technical endosulfan, reflecting priorities in examining the toxicity of the parent compounds of pesticides. The toxicity test results and effects analyses provided acute effect values for endosulfan sulfate and freshwater fish that might be applied in future screening level ecologic risk assessments. The effects analyses also discussed several deficiencies in conventional methods for setting water-quality criteria and determining ecologic effects from acute toxicity tests.

Effects of Zinc in an Outdoor Freshwater Microcosm System.

A long-term exposure outdoor microcosm study was conducted to evaluate the effects of zinc (Zn) on zooplankton, phytoplankton, and periphyton in a freshwater system. Five Zn treatment concentrations (nominal: 8, 20, 40, 80, and 160 µg/L Zn) and an untreated control with 3 replicates each were used. Various physical and chemical characteristics of the microcosms and biological assessment endpoints (e.g., total abundance, group abundance, species richness, chlorophyll a, etc.) were measured to determine the effects of Zn over time. In general, physical and chemical characteristics (e.g., total dissolved solids, total suspended solids, dissolved oxygen, pH, dissolved organic carbon) of water fluctuated over time, but they were not significantly different within treatments and controls during the study. Zinc significantly affected the population dynamics and community structure of plankton. The effects occurred 7 d after initial treatment exposures began and continued to the end of the treatment phase, especially at the high treatment concentrations. Total and group abundance, species richness, the Shannon index, and chlorophyll a concentrations for high Zn treatment concentrations were significantly lower than the controls during the treatment phase. The no-observed-effect, lowest-observed-effect, and median effect concentrations were generally lower than the literature-reported results from single-species toxicity tests for fish and invertebrates, suggesting that plankton are more sensitive to Zn than planktivores. Although primary producers play an important role in the ecosystem, they have not been consistently incorporated into numerical environmental quality criteria for freshwater organisms, at least in the United States. The results of the present study are useful for development of environmental quality guidelines for freshwater ecosystems and ecological risk assessment. Environ Toxicol Chem 2021;40:2053-2072. © 2021 SETAC.

Survival time analysis of least killifish (Heterandria formosa) and mosquitofish (Gambusia affinis) in acute exposures to endosulfan sulfate.

Single-species flow-through toxicity tests were conducted to determine the times-to-death of two indigenous fish to South Florida--least killifish (Heterandria formosa) and mosquitofish (Gambusia affinis)--from acute exposure to endosulfan sulfate. Mortalities were recorded within 8-h periods from test initiation to termination at 96 h. The 96-h LC(50)s for least killifish and mosquitofish estimated using the trimmed-Spearman-Karber method were 2.0 and 2.3 microg/l, respectively. An accelerated failure time model was used to estimate times to death at selected concentrations. Data were fit to log-normal, log-logistic, and Weibull distributions. Acute toxicity data fit to the Weibull distribution produced a better relative fit than log-normal or log-logistic distributions for both toxicity tests. The survival-time profiles and associated statistics illustrate the benefit of considering exposure duration as well as concentration when predicting acute risk to species' populations. Both toxicity tests had similar outcomes from exposure to endosulfan sulfate, with least killifish being slightly more likely to die at lower concentrations and shorter time periods than mosquitofish. From the models generated by the toxicity tests, times-to-death for least killifish and mosquitofish were estimated for environmentally relevant concentrations of total endosulfan at a site of concern in South Florida. When the results from the current toxicity tests were compared to environmental concentrations from previous screening-level ecological risk assessments, the durations necessary to potentially kill 10% or more of the populations of the two native south Florida fish species were estimated to be 77 and 96 h for least killifish and mosquitofish, respectively. However, the exposure values included the alpha and beta isomers as well as endosulfan sulfate; therefore, an understanding of their toxicity might be important in understanding the survival dynamics of fish species in endosulfan sulfate-contaminated sites.

Microplastic consumption and excretion by fathead minnows (Pimephales promelas): Influence of particles size and body shape of fish.

The present study characterizes the dependence of microplastic consumption and excretion on particle size and body shape of fathead minnow (Pimephales promelas) over time that has not been studied. Specifically, the study is to answer four important questions: 1) how do P. promelas consume microplastic particles at different size ranges over time? 2) how long does it take for P. promelas to excrete microplastic particles after consumption? 3) do P. promelas reconsume microplastic particles after excretion? 4) are microplastic consumption and excretion by P. promelas dependent on the body shape? To answer these questions, larval P. promelas were exposed to polyethylene microbeads (PMBs) at two different consumable size ranges of 63-75 µm and 125-150 µm in moderately hard water. The experiments were designed to allow and to not allow fish to reconsume the particles they excreted. Results of the present study showed that P. promelas consumed significant amount of PMBs after 1 h of exposure to PMBs regardless particle size. The number of consumed PMBs per fish at smaller size range was up to 10 times higher than that at larger size range. When expressing the consumption in µg PMBs/fish, this difference was approximately 1.3 times, suggesting the importance of the measurement unit. After consuming, fish excreted PMBs over time and reconsumed excreted PMBs if reconsumption was allowed. Interestingly, it took longer for bent body fish to excrete PMBs than regular straight body fish. Our observation showed that excreted PMBs were likely coated with intestinal fluid that is denser than water, resulting in aggregation and deposition of PMBs. This result suggests that in the natural environment, the consumption and excretion of plastics by fish would enhance the movement of plastics from the water column to the waterbed and make it available for benthic organisms.

Effects of sublethal chronic copper exposure on the growth and reproductive success of the Florida apple snail (Pomacea paludosa).

Florida apple snails (Pomacea paludosa) were exposed to three concentrations of copper (Cu), in water (8 microg/L, 16 microg/L, 24 microg/L), for one generation to examine uptake and the effects on survival, growth, and reproduction of the F(0) generation and survival, growth, and whole body Cu of the F(1) generation. During a 9-month Cu exposure, apple snails exposed to 8-16 microg/L Cu had high Cu accumulation (whole body, foot, viscera, and shell) and significantly reduced clutch production (8-16 microg/L) and egg hatching (16 microg/L). Apple snails exposed to the 24 microg/L Cu had low survival and the treatment was therefore terminated. Concentrations of minerals (Na(+), K(+), Mg(2+), Ca(2+)) in tissues were maintained regardless of Cu exposure, but the distribution of Cu in the body of snails differed, depending on exposure concentrations. Higher exposure concentrations resulted in a greater percentage of Cu accumulated in the viscera of the snail. Copper exposure to the F(0) generation did not affect the survival, growth, or whole body Cu concentrations in the F(1) generation. These finding are significant, given the importance of the Florida apple snail in the Everglades food chain. Changes in the abundance of apple snail populations, as a result of Cu exposure, could ultimately affect foraging success of predators.

Effects of copper in flooded Florida agricultural soils on Hyalella azteca.

This study examined the uptake and effects of copper (Cu) from flooded agricultural soils to epibenthic amphipods (Hyalella azteca) using 10-day sediment toxicity tests. Soils were collected from 10 citrus agricultural sites in South Florida. One sediment toxicity test was conducted with one flooding of the 10 soils, and based on the results of this test a second sediment toxicity test was conducted with 4 of the soils, after four 14-day flooding and four 14-day drying intervals over 4 months. Sediment toxicity tests were conducted under flow-through conditions using U.S. EPA methodology. Effects on survival, dry weight, and whole-body Cu concentrations of H. azteca were determined. Cu concentrations in overlying water and sediment of both sediment toxicity tests exceeded regulatory criteria for aquatic organisms. Although survival of H. azteca was not consistently affected from the first to the second sediment toxicity tests, dry weight was consistently reduced and related to Cu concentrations in soil, overlying water, and pore water. Furthermore, whole-body tissue Cu concentrations were significantly higher in H. azteca in all 10 soil-water treatments in the first sediment toxicity test and in all 4 soil-water treatments in the second sediment toxicity test compared to controls. Whole-body tissue concentrations and effects on dry weight were related to Cu exposures in soil, overlying water, and pore water. In these managed soil-water systems, small fish consuming H. azteca with high concentrations of Cu may be at risk.

Copper desorption in flooded agricultural soils and toxicity to the Florida apple snail (Pomacea paludosa): implications in Everglades restoration.

Copper (Cu) desorption and toxicity to the Florida apple snail were investigated from soils obtained from agricultural sites acquired under the Comprehensive Everglades Restoration Plan. Copper concentrations in 11 flooded soils ranged from 5 to 234 mg/kg on day 0 and from 6.2 to 204 mg/kg on day 28 (steady-state). The steady-state Cu concentration in overlying water ranged from 9.1 to 308.2 microg/L. In a 28-d growth study, high mortality in snails occurred within 9 to 16 d in two of three soil treatments tested. Growth of apple snails over 28 d was affected by Cu in these two treatments. Tissue Cu concentrations by day 14 were 12-23-fold higher in snails exposed to the three soil treatments compared to controls. The endangered Florida snail kite and its main food source, the Florida apple snail, may be at risk from Cu exposure in these managed agricultural soil-water ecosystems.

Characterizing the toxicity of pulsed selenium exposure to Daphnia magna.

The acute toxicity of selenium (Se) to aquatic biota has been studied extensively for decades. However, most studies have used a constant concentration aqueous exposure of Se to an invertebrate species. Since constant concentration exposure of toxicants to invertebrates is unusual in the environment, episodic exposure or pulsed exposures may represent true risk to aquatic biota more accurately. This research was designed to characterize the toxicity effects of pulsed Se exposure to Daphnia magna. Selenium exposure was varied during a 21-d chronic toxicity test to examine the effects of exposure concentration, duration, and recovery on survival, growth, and reproduction of D. magna. While D. magna did not die during exposures, latent mortality was observed. Latent mortality increased with exposure concentration and duration. Hence, standard toxicity test using continuous exposures would underestimate Se toxicity. Risk assessment method using results of continuous exposure would underestimate risk of Se to biota. For double-pulse exposures, cumulative mortality on day 21 was higher when time interval between pulses was shorter. With the same total exposure time, continuous exposure caused higher toxicity than did pulsed exposures due to recovery and tolerance development in D. magna after earlier pulses. Growth and reproduction of surviving D. magna were not affected by pulsed Se exposure due to recovery of D. magna after removal of the pulses. Based on these results, risk assessment for Se should take latent effects and the effect of recovery in to account.

Responses of Daphnia magna to chronic exposure of cadmium and nickel mixtures.

The present study assessed the chronic toxicity of cadmium (Cd) and nickel (Ni) mixtures to Daphnia magna. Using a titration design, Ni concentrations of 20, 40, 80, 100, 120, 140, and 160 µg/L were tested alone and simultaneously titrated in increments against a constant concentration of 1.5 µg/L Cd. The results demonstrated that Cd at 1.5 µg/L was highly toxic to D. magna, and Ni alone concentrations ≥80 µg/L were toxic to D. magna survival, reproduction, and growth. No Ni alone concentration was found to induce a toxic effect on undeveloped embryos and the time to first brood. Only the Ni alone treatment containing 200 µg/L affected the reproductive rates of D. magna. For CdNi mixtures, Ni concentrations of 20, 40, and 80 µg/L were found to strongly protect D. magna from Cd toxicity at the survival and growth endpoints, resulting in less-than-additive effects, but not on the reproductive endpoint. At higher concentrations, Ni exceeded the necessary concentration needed to protect D. magna, and appeared to contribute to the toxicity. Overall, the results of metal uptake support the competitive binding mechanism at the biotic ligand and explain the less-than-additive effects observed in the CdNi mixtures concentration. The embryonic effects of CdNi mixtures are not explained by the competitive binding mechanism at the biotic ligand. More research is needed to determine the mechanisms that produce embryonic impairment when cellular metals interact. Overall, the results of the present study are relevant for the development of improved environmental quality guidelines for metal mixtures.

Microplastic ingestion by Daphnia magna and its enhancement on algal growth.

The rapid increase in plastic use over the last few decades has resulted in plastic pollution in freshwater and marine ecosystems. However, more attention has been paid to plastic pollution in marine ecosystems than to freshwater ecosystems. This research determined microplastic ingestion by Daphnia magna and the potential effect of microplastics on the organism's survival and reproduction. The study also examined the potential of microplastics to enhance algal growth in support of understanding effects of microplastic ingestion on the organism. When exposed to 25, 50, and 100mg/L fluorescent green polyethylene microbeads at size of 63-75µm, D. magna ingested significant amount of plastic microbeads. The number of ingested beads increased with increasing particle concentration and exposure time. However, no significant effect on survival and reproduction was observed although the gut of D. magna was filled with plastic microbeads. In the algal experiment, Raphidocelis subcapitata grew more in the exposure media with the present of plastic microbeads than without plastic microbeads. This result suggests that plastic microbeads could serve as substrates for R. subcapitata to grow. Raphidocelis subcapitata then could be transferred to the organism's gut and provided energy for survival and reproduction. Results of the present study add to the literature of microplastic ingestion by aquatic organisms. Caution should be taken when interpreting hazards of microplastics based on ingestion, such as the measurement unit and the presence of algae in the environment.

Chronic effects of lead exposure on topsmelt fish (Atherinops affinis): Influence of salinity, organism age, and relative sensitivity to other marine species.

The aim of the present study was to determine the influence of salinity and organism age on the chronic toxicity of waterborne lead (Pb) to Atherinops affinis and to compare the relative Pb sensitivity of A. affinis with other marine species. Chronic Pb exposure experiments were conducted in a water flow-through testing system. Survival, standard length, dry weight, and tissue Pb concentration were measured and lethal concentrations (LCs), effect concentrations (ECs), and bioconcentration factors (BCFs) were calculated. In general, increasing salinity and organism age decreased Pb toxicity. The LC50s for larval fish at 14 and 28 ppt salinity were 15.1 and 79.8 µg/L dissolved Pb, respectively; whereas, the LC50 for juvenile fish was 167.6 µg/L dissolved Pb at 28 ppt salinity. Using standard length data, the EC10 values for larval fish were 16.4 and 82.4 µg/L dissolved Pb at 14 and 28 ppt salinity, respectively. The dry weight EC25s for low and high salinity were 15.6 and 61.84 µg/L dissolved Pb, respectively. The BCFs were higher with the lower salinity study (1703) in comparison to the higher salinity study (654). Results of Pb speciation calculation showed higher fraction of Pb2+ in water with lower salinity, explaining the higher observed toxicity of Pb in lower salinity water than higher salinity water. Atherinops affinis is more sensitive to Pb than several other marine species. Evidence of abnormal swimming and skeletal deformities were observed in Pb exposure treatments. Results of the present study are useful for marine biotic ligand modeling and support ecological risk assessment and deriving Pb environmental quality criteria for marine environments. Environ Toxicol Chem 2018;37:2705-2713. © 2018 SETAC.

Influence of organism age on metal toxicity to Daphnia magna.

Aquatic organisms living in surface water experience contaminant exposure at different life stages. While some investigators have examined the influence of organism age on the toxicity of pollutants, the general assumption in toxicology has been that young organisms were more sensitive than older organisms. In fact, some standardized toxicity tests call for the use of organisms less than 24 h old. This research characterized the age sensitivity of the water flea Daphnia magna to copper, zinc, selenium, and arsenic. During 21-d toxicity tests, organisms were exposed to a single 12-h pulse of either 70 microg/L Cu, 750 microg/L Zn, 1000 microg/L Se, or 5000 microg/L As at different ages ranging from 3 h to 10 d old. Mortality and reproduction were compiled over 21 d. During the juvenile stage, mortality increased and cumulative reproduction decreased with age, respectively. However, mortality decreased and cumulative reproduction increased with age when organisms became adult. Peak sensitivity occurred in 4-d-old organisms exposed to Cu and Zn, while 2- to 3-d-old organisms were most sensitive to As and Se. Growth of D. magna over 21 d was not affected by the 12-h pulse of Cu, Zn, Se, or As given at any organism age. This indicates the recovery of the organisms after exposure termination.

An integrated model describing the toxic responses of Daphnia magna to pulsed exposures of three metals.

Some toxicology research in which toxicant exposures are continual (pulsed) rather than continuous have been reported. A number of toxicity models have been developed for pulsed and continuous exposures. Most of these models were developed based on one- or two-compartment, first-order toxicokinetics and were calibrated with organic compounds. In the present study, the relationship between mortality (after 21 d) of Daphnia magna in response to pulsed and continuous exposures to Cu, Zn, and Se was used to develop a model that integrated the effects of single and multiple pulsed metal exposures based on first-order uptake and depuration kinetics. Mortality was a function of exposure concentration, duration, and recovery time between exposures. The model was successfully validated using an independent data set. It is applicable to risk assessment and, potentially, may be incorporated with other models (e.g., the biotic ligand model) to predict the toxicity of pulsed metal exposures under a range of environmental conditions.