Relative acute toxicity of three per- and polyfluoroalkyl substances on nine species of larval amphibians

Per- and polyfluoroalkyl substances (PFAS) are widespread, persistent environmental pollutants known to elicit a wide range of negative effects on wildlife species. There is scarce information regarding the toxicity of PFAS on amphibians, but amphibians may be highly susceptible because of their permeable skin and dependence on fresh water. Acute toxicity studies are a first step toward understanding responses to PFAS exposure, providing benchmarks for species-specific tolerances, informing ecological risk assessment (ERA), and designing chronic toxicity studies. We conducted standardized 96-h lethal concentration (LC50) toxicity tests for perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) with 9 widely distributed amphibian species native to eastern and central North America. We also conducted LC50 tests with perfluorohexane sulfonate (PFHxS) for 2 species and determined whether toxicity of PFOS and PFOA varied between life stages for 3 species. Acute toxicity varied among PFAS and species and between developmental stages within species. Across all species, toxicity of PFOS was more than 8× higher than PFOA. Salamanders in the genus Ambystoma were generally more sensitive to PFOS than were anurans (frogs and a toad). Toxicity of PFOA was highest for small-mouthed salamanders and gray tree frogs and lowest for Jefferson salamanders, American bullfrogs, green frogs, and wood frogs. Although only 2 species were exposed to PFHxS, survival was lower for green frogs than for American bullfrogs. Toxicity of PFAS also varied between developmental stages of larvae. Gray tree frogs were more sensitive at later developmental stages, and small-mouthed salamanders were more sensitive at earlier developmental stages. Our study is one of the first to report species-, developmental stage-, and compound-specific differences in sensitivity to PFAS across a wide range of amphibian species. The benchmarks for toxicity we determined can inform conservation and remediation efforts, guide chronic toxicity studies, and help predict influences on amphibian communities, thereby informing future ERAs for PFAS. Integr Environ Assess Manag 2021;17:684-689. © 2021 SETAC.

Contrasting patterns of 2-methylisoborneol (MIB) vs. geosmin across depth in a drinking water reservoir are mediated by cyanobacteria and actinobacteria

Taste and odor episodes caused by off-flavor secondary metabolites, such as 2-methylisoborneol (MIB) and geosmin, pose one of the greatest challenges for drinking water utilities around the world. The prevalence of these compounds is predicted to increase in the future as a function of nutrient enrichment and elevated temperatures of surface drinking water sources. We conducted a manipulative field experiment in a drinking water reservoir to elucidate patterns for two taste and odor compounds, MIB and geosmin, as well as two taxa known to produce these compounds, phytoplankton (more specifically, cyanobacteria) and actinobacteria, across different depths in response to nutrient enrichment with two common dissolved nitrogen forms, organic urea or inorganic nitrate. In general, we found that MIB levels increased by greater than 250% with nutrient enrichment mediated by increased phytoplankton biomass. However, the effect of the fertilization treatments on MIB decreased with depth with a 35% reduction at 7 m versus 1.5 m. In contrast, geosmin levels reached a maximum at the lowest measured depth (7 m), were unaffected by the fertilization treatments, and followed a similar pattern to the abundance of actinobacteria. Thus, our data suggest that the positive response of phytoplankton (e.g., cyanobacteria, such as Oscillatoria species) to the fertilization treatments is likely responsible for increased MIB, while geosmin concentrations may be a function of actinobacteria-mediated decomposition in the hypolimnion in our study system.

Acute and chronic effects of perfluoroalkyl substance mixtures on larval American bullfrogs (Rana catesbeiana)

Discovery of elevated concentrations of perfluoroalkyl substances (PFAS) in ground and surface waters globally has heightened concern over their potential adverse health effects. The effects of PFAS are known largely from acute toxicity studies of single PFAS compounds in model organisms, while little is understood concerning effects of mixtures on wildlife. To address this gap, we examined the acute and chronic effects of two of the most common PFAS (perfluorooctanesulfonic acid [PFOS] and perfluorooctanoic acid [PFOA]) and their mixtures on survival, growth, and development of American bullfrog (Rana catesbeiana) tadpoles. In 96 h acute toxicity tests, PFOS was 10X more toxic than PFOA and effects of the two chemicals in combination appeared additive. The effects of PFOS, PFOA, and their interaction varied by the sublethal endpoint under consideration in a 72 d exposure. Effects of PFAS on tadpole mass and developmental stage were largely driven by PFOS and there was no evidence of interactions suggesting deviations from additivity. However, for snout-vent length, reductions in length in mixture treatments were greater than expected based on the effects of the two chemicals independently (i.e. non-additivity). Further, effects on snout-vent length in single chemical exposures were only observed with PFOA. Our results highlight the importance of assessing combined effects of PFAS co-occurring in the environment and suggest caution in extrapolating the effects of acute toxicity studies to more environmentally relevant exposures. Future studies examining effects of environmentally relevant mixtures on wildlife will be essential for effective environmental risk assessment and management.

Healthy but smaller herds: Predators reduce pathogen transmission in an amphibian assemblage

Predators and pathogens are fundamental components of ecological communities that have the potential to influence each other via their interactions with victims and to initiate density- and trait-mediated effects, including trophic cascades. Despite this, experimental tests of the healthy herds hypothesis, wherein predators influence pathogen transmission, are rare. Moreover, no studies have separated effects mediated by density vs. traits. Using a semi-natural mesocosm experiment, we investigated the interactive effects of predatory dragonfly larvae (caged or lethal [free-ranging]) and a viral pathogen, ranavirus, on larval amphibians (grey treefrogs and northern leopard frogs). We determined the influence of predators on ranavirus transmission and the relative importance of density- and trait-mediated effects on observed patterns. Lethal predators reduced ranavirus infection prevalence by 57%-83% compared to no-predator and caged-predator treatments. The healthy herds effect was more strongly associated with reductions in tadpole density than behavioural responses to predators. We also assessed whether ranavirus altered the responses of tadpoles to predators. In the absence of virus, tadpoles reduced activity levels and developed deeper tails in the presence of predators. However, there was no evidence that virus presence or infection altered responses to predators. Finally, we compared the magnitude of trophic cascades initiated by individual and combined natural enemies. Lethal predators initiated a trophic cascade by reducing tadpole density, but caged predators and ranavirus did not. The absence of a virus-induced trophic cascade is ostensibly the consequence of limited virus-induced mortality and the ability of infected individuals to continue interacting within the community. Our results provide support for the healthy herds hypothesis in amphibian communities. We uniquely demonstrate that density-mediated effects of predators outweigh trait-mediated effects in driving this pattern. Moreover, this study was one of the first to directly compare trophic cascades caused by predators and pathogens. Our results underscore the importance of examining the interactions between predators and pathogens in ecology.

Consumer adaptation mediates top-down regulation across a productivity gradient

Humans have artificially enhanced the productivity of terrestrial and aquatic ecosystems on a global scale by increasing nutrient loading. While the consequences of eutrophication are well known (e.g., harmful algal blooms and toxic cyanobacteria), most studies tend to examine short-term responses relative to the time scales of heritable adaptive change. Thus, the potential role of adaptation by organisms in stabilizing the response of ecological systems to such perturbations is largely unknown. We tested the hypothesis that adaptation by a generalist consumer (Daphnia pulicaria) to toxic prey (cyanobacteria) mediates the response of plankton communities to nutrient enrichment. Overall, the strength of Daphnia's top-down effect on primary producer biomass increased with productivity. However, these effects were contingent on prey traits (e.g., rare vs. common toxic cyanobacteria) and consumer genotype (i.e., tolerant vs sensitive to toxic cyanobacteria). Tolerant Daphnia strongly suppressed toxic cyanobacteria in nutrient-rich ponds, but sensitive Daphnia did not. In contrast, both tolerant and sensitive Daphnia genotypes had comparable effects on producer biomass when toxic cyanobacteria were absent. Our results demonstrate that organismal adaptation is critical for understanding and predicting ecosystem-level consequences of anthropogenic environmental perturbations.

Pond bank access as an approach for managing toxic cyanobacteria in beef cattle pasture drinking water ponds

Forty-one livestock drinking water ponds in Alabama beef cattle pastures during were surveyed during the late summer to generally understand water quality patterns in these important water resources. Since livestock drinking water ponds are prone to excess nutrients that typically lead to eutrophication, which can promote blooms of toxigenic phytoplankton such as cyanobacteria, we also assessed the threat of exposure to the hepatotoxin, microcystin. Eighty percent of the ponds studied contained measurable microcystin, while three of these ponds had concentrations above human drinking water thresholds set by the US Environmental Protection Agency (i.e., 0.3 μg/L). Water quality patterns in the livestock drinking water ponds contrasted sharply with patterns typically observed for temperate freshwater lakes and reservoirs. Namely, we found several non-linear relationships between phytoplankton abundance (measured as chlorophyll) and nutrients or total suspended solids. Livestock had direct access to all the study ponds. Consequently, the proportion of inorganic suspended solids (e.g., sediment) increased with higher concentrations of total suspended solids, which underlies these patterns. Unimodal relationships were also observed between microcystin and phytoplankton abundance or nutrients. Euglenoids were abundant in the four ponds with chlorophyll concentrations > 250 μg/L (and dominated three of these ponds), which could explain why ponds with high chlorophyll concentrations would have low microcystin concentrations. Based on observations made during sampling events and available water quality data, livestock-mediated bioturbation is causing elevated total suspended solids that lead to reduced phytoplankton abundance and microcystin despite high concentrations of nutrients, such as phosphorus and nitrogen. Thus, livestock could be used to manage algal blooms, including toxic secondary metabolites, in their drinking water ponds by allowing them to walk in the ponds to increase turbidity.

Eutrophication mediates a common off-flavor compound, 2-methylisoborneol, in a drinking water reservoir

Off-flavors, such as 2-methylisoborneol (MIB) and geosmin, cause drinking water to have earthy or musty tastes and odors. Humans can detect such compounds at minute concentrations (10 and 30 ng/L for MIB and geosmin, respectively), and, although not a health risk, off-flavors can promote consumer distrust. Removal of these compounds is costly and often unreliable or only suitable under certain conditions. Minimizing off-flavor production at the watershed-scale may be more cost-effective in addition to improving ecosystem health and aesthetics. Cyanobacteria are considered to be the primary drivers of off-flavors in freshwater systems. Due to their ability to produce toxins, cyanobacteria have been under particular scrutiny, and environmental factors promoting cyanobacterial blooms are relatively well-studied. Using this body of literature, we conducted a seven-week, limnocorral experiment where we manipulated nitrogen and nitrogen-to-phosphorus concentrations to influence phytoplankton community structure and off-flavor production. The addition of a single nutrient across broad ranges (nitrogen or phosphorus) had no effect on MIB. However, the addition of both nitrogen and phosphorus promoted high concentrations of MIB relative to treatments that received no nutrients (448% increase) or only nitrogen or phosphorus (722% increase). Interestingly, cyanobacteria waned during the experiment and were replaced by diatoms, which were the dominant taxa by the end of the experiment. Our findings clearly show that eutrophication affects MIB production, but mechanisms leading to the production of this compound may differ from what has been previously predicted.

Cylindrospermopsis raciborskii dominates under very low and high nitrogen-to-phosphorus ratios

In freshwater ecosystems, a variety of factors mediate phytoplankton community structure, including herbivore community structure, light availability, temperature, mixing, and absolute and relative nutrient concentrations (total nitrogen (TN), total phosphorus (TP)). Ecological stoichiometry examines how the nutrient content of organisms and their environment may mediate population-, community-, and ecosystem-level processes. The manipulation of N:P ratios is a widely regarded tool for managing phytoplankton species composition given that nitrogen-fixing cyanobacteria should dominate algal communities under relatively low N:P (<64:1, by atoms) given their ability to convert dissolved dinitrogen gas into organic nitrogen. However, due to the physiological expense of nitrogen fixation, diazotrophs should be outcompeted by non-nitrogen fixing phytoplankton under higher N:P when other environmental factors are similar. We tested this hypothesis in a field experiment using 2500-L limnocorrals installed in a eutrophic lake (ambient N:P ∼40:1 (by atoms); TN ∼1360 μgL(-1); TP ∼75 μgL(-1)). At the start of the experiment, we randomly assigned limnocorrals among the ambient (40:1) and low (7:1) or high (122:1) N:P treatments (n = 4 replicates/treatment), which were established by adding P or N at the start of the experiment, respectively. The phytoplankton community in the enclosures at the start of the experiment was diverse (i.e., 18 phytoplankton genera) and dominated by chlorophytes (including Coelastrum and Scenedesmus (30% and 13% of total biomass, respectively)) and cyanobacteria (including Anabaena and Cylindrospermopsis (23% and 17% of total biomass, respectively)). In contrast to predictions based on ecological stoichiometry, the phytoplankton community in all N:P treatments increased in abundance and was almost entirely composed of the nitrogen-fixing cyanobacterium, Cylindrospermopsis raciborskii, by the conclusion of the study. Moreover, concentrations of the cyanobacterial neurotoxin, saxitoxin, were enhanced under the two highest N:P conditions. The ability of C. raciborskii to dominate phytoplankton communities under such extreme N:P shows that short-term management of nutrient stoichiometry through fertilization is not likely to be effective for controlling blooms of this noxious cyanobacterium and may help to explain the rapid expansion of this invasive species to temperate latitudes.

Large effects of consumer offense on ecosystem structure and function

Study of the role of within-species adaptation in ecological dynamics has focused largely on prey adaptations that reduce consumption risk (prey defense). Few, if any, studies have examined how consumer adaptations to overcome prey defenses (consumer offense) affect ecosystem structure and function. We manipulated two sets of genotypes of a planktonic herbivore (Daphnia pulicaria) in a highly productive ecosystem with abundant toxic prey (cyanobacteria). The two sets of consumer genotypes varied widely in their tolerance of toxic cyanobacteria in the diet (i.e., sensitive vs. tolerant). We found a large effect of tolerant D. pulicaria on phytoplankton biomass and gross primary productivity but no effect of sensitive genotypes, this result stemming from genotype-specific differences in population growth in the presence of toxic prey. The former effect was as large as effects seen in previous Daphnia manipulations at similar productivity levels. Thus, we demonstrated that the effect of consumer genotypes with contrasting offensive adaptations was as large as the effect of consumer presence/absence.

Do high concentrations of microcystin prevent Daphnia control of phytoplankton?

Toxin-producing cyanobacteria have frequently been hypothesized to limit the ability of herbivorous zooplankton (such as Daphnia) to control phytoplankton biomass by inhibiting feeding, and in extreme cases, causing zooplankton mortality. Using limnocorral experiments in hyper-eutrophic ponds located in Alabama and Michigan (U.S.A.), we tested the hypothesis that high levels of cyanobacteria and microcystin, a class of hepatotoxins produced by several cyanobacterial genera, prevent Daphnia from strongly reducing phytoplankton abundance. At the start of the first experiment (Michigan), phytoplankton communities were dominated by toxic Microcystis and Anabaena (∼96% of total phytoplankton biomass), and concentrations of microcystin were ∼3 μg L⁻¹. Two weeks after adding Daphnia pulicaria from a nearby eutrophic lake, microcystin levels increased to ∼6.5 μg L⁻¹, yet Daphnia populations increased exponentially (r = 0.24 day⁻¹). By the third week, Daphnia had suppressed phytoplankton biomass by ∼74% relative to the no Daphnia controls and maintained reduced phytoplankton biomass until the conclusion of the five-week experiment. In the second experiment (Alabama), microcystin concentrations were greater than 100 μg L⁻¹, yet a mixture of three D. pulicaria clones from eutrophic lakes in southern MI increased and again reduced phytoplankton biomass, in this case by over 80%. The ability of Daphnia to increase in abundance and suppress phytoplankton biomass, despite high initial levels of cyanobacteria and microcystin, indicates that the latter does not prevent strong control of phytoplankton biomass by Daphnia genotypes that are adapted to environments with abundant cyanobacteria and associated cyanotoxins.

Blinded by the stink: Nutrient enrichment impairs the perception of predation risk by freshwater snails

The acquisition of sensory information is central to all species interactions. Most aquatic organisms use chemical cues to assess predation risk and other key ecological factors, but chemoreception may be disrupted in systems with elevated pH. Elevated pH in lakes and rivers is often associated with eutrophication. We used laboratory and mesocosm experiments to test whether elevated pH impairs perception of predation risk by the freshwater snails Physa acuta and Helisoma trivolvis. In one set of experiments, nutrients were added to outdoor mesocosms, resulting in mid-afternoon pH values of 8.5-9.7. Both snail species moved to avoid fish in water with pH < 9.0 but showed no avoidance at higher pH. In a laboratory study, we used buffers to establish six pH treatments ranging from 7.5 to 10.0. At lower pH Physa acuta responded to fish cues by moving into safer habitats, but avoidance became impaired at a pH of 9.4. Helisoma trivolvis also responded to fish at lower pH, and their avoidance behavior became impaired at a pH of 8.8. Given the diversity of aquatic organisms that depend on reception of chemical cues and the broad extent of eutrophication, chemosensory impairment is likely a common occurrence in nature.

Dragonfly predators influence biomass and density of pond snails

Studies in lakes show that fish and crayfish predators play an important role in determining the abundance of freshwater snails. In contrast, there are few studies of snails and their predators in shallow ponds and marshes. Ponds often lack fish and crayfish but have abundant insect populations. Here we present the results of field surveys, laboratory foraging trials, and an outdoor mesocosm experiment, testing the hypothesis that insects are important predators of pulmonate snails. In laboratory foraging trials, conducted with ten species of insects, most insect taxa consumed snails, and larval dragonflies were especially effective predators. The field surveys showed that dragonflies constitute the majority of the insect biomass in fishless ponds. More focused foraging trials evaluated the ability of the dragonflies Anax junius and Pantala hymenaea to prey upon different sizes and species of pulmonate snails (Helisoma trivolvis, Physa acuta, and Stagnicola elodes). Anax junius consumed all three species up to the maximum size tested. Pantala hymenaea consumed snails with a shell height of 3 mm and smaller, but did not kill larger snails. P. acuta were more vulnerable to predators than were H. trivolvis or S. elodes. In the mesocosm experiment, conducted with predator treatments of A. junius, P. hymenaea, and the hemipteran Belostoma flumineum, insect predators had a pronounced negative effect on snail biomass and density. A. junius and B. flumineum reduced biomass and density to a similar degree, and both reduced biomass more than did P. hymenaea. Predators did not have a strong effect on species composition. A model suggested that A. junius and P. hymenaea have the largest effects on snail biomass in the field. Given that both pulmonate snails and dragonfly nymphs are widespread and abundant in marshes and ponds, snail assemblages in these water bodies are likely regulated in large part by odonate predation.