Metal accumulation by stream bryophytes, related to chemical speciation

A method for studying the influence of Mn oxyhydroxides on the trace element content of aquatic bryophytes

The main objective of this study was to develop and test a method of separating externally deposited Mn oxyhydroxides and co-precipitated elements from samples of aquatic moss (the moss Fontinalis antipyretica). The method, which uses 0.1 M hydroxylamine to dissolve the oxyhydroxides, was tested with samples collected in rivers with slightly acidic, well‑oxygenated waters, where high rates of Mn precipitation occur. The method was effective (it extracted up to 84 % of the Mn) and selective (Fe oxyhydroxides were not extracted). The elements Ba, Cd, Zn and Ni were associated with the Mn oxyhydroxides, while Al, As, Cr, Cu, Fe, Hg and Pb were not. Deposition of Mn therefore increased the concentration of some elements in the moss samples. However, as Mn precipitation depends on Eh and pH, which are independent of the concentrations of the elements in water, the relationship between water and moss element concentrations is not clear (i.e. the data are noisy). This is a problem in biomonitoring studies, which assume a close relationship between element concentrations in moss and water. The value of the proposed extraction method is that it can be used to correct the effect of Mn deposition. We present an example of this correction applied to the Cd concentrations in the test data. We found that the noise introduced by the Mn, including age-related effects (observed by comparing concentrations in 0-2.5 and 2.2-5.0 cm sections from the shoot apex), can be reduced. Additionally, the correction revealed recent increases in Cd concentrations in one site that were not observed in the uncorrected data. Another finding of interest was the low content of total Mn and different extractability (of most elements) observed in moss samples collected in alkaline waters. Finally, we discuss how future studies designed for different environmental scenarios can benefit from application of the proposed method.

Mechanisms of Copper Toxicity and Tolerance in the Aquatic Moss Taxiphyllum barbieri

Aquatic habitats are very frequently polluted with different kinds of xenobiotics, including heavy metals. For biomonitoring studies of aquatic pollution, algae are frequently used, as they do not contain protective cuticle on the surface of their thalli and can accumulate pollutants over the whole surface of thalli. However, this is a feature of most cryptogams. For this reason, we assessed the sensitivity of the aquatic moss Taxiphyllum barbieri (Java moss) to copper excess in a short-term study. Moss T. barbieri belongs to the common aquatic plants originating from Southeast Asia. To test the sensitivity (or tolerance) of the moss to excess Cu, selected concentrations (50, 250 and 500 µM) were employed in our 24 h studies. Total and intracellular Cu accumulation positively correlated with Cu availability in the water. This total and intracellular Cu accumulation was negatively correlated with decreased intracellular K content. Excess Cu negatively affected the composition of assimilation pigments and soluble proteins. Cu caused increased peroxidation of membrane lipids assessed using TBARS assay. Excess Cu decreased GSH to GSSG ratio and ascorbic acid content. We did not observe phytochelatin synthesis in this moss. The roles of selected amino acids, their intermediates and derivatives, as well as S-containing nucleosides and phenolic acids in Cu homeostasis and toxicity or tolerance were evaluated. We assume that this moss has potential for future employment in water quality evaluation.

Unravelling the metal uptake process in mosses: Comparison of aquatic and terrestrial species as air pollution biomonitors

Transplanted mosses have been widely shown to be excellent tools for biomonitoring air pollution; however, it is not clear how the functional groups present on their surfaces affect the uptake of metal cations. In the present study, we examined differences in trace metal accumulation in two terrestrial and one aquatic moss species, and investigated whether the differences depended on their physico-chemical characteristics. In the laboratory, we determined C, N and H contents in their tissues and obtained the ATR-FTIR spectra (to identify the presence of functional groups). We also conducted surface acid-base titrations and metal adsorption assays with Cd, Cu and Pb. In the field, we exposed transplants of each species near different air-polluting industries, and determined the mosses enrichment of Al, Cd, Co, Cr, Cu, Fe, Ni, Pb and V. Laboratory results demonstrated higher metal uptake capacity in the terrestrial mosses Sphagnum palustre and Pseudoscleropodium purum, compared to that in the aquatic moss Fontinalis antipyretica, which can be attributed to a greater abundance of acidic functional groups (i.e. negatively charged binding sites) on the surface of the terrestrial mosses. The affinity of moss for certain elements depends on the abundance and nature of surface functional groups. Accordingly, the metal concentrations generally reached higher levels in S. palustre transplants compared to the other species, except for the uptake of Hg, which was higher in F. antipyretica. However, the findings also suggest an interaction between the type of environment (terrestrial or aquatic) and the moss characteristics that may influence the abovementioned trend. Thus, irrespective of the physico-chemical characteristics, metal uptake varied depending on the environment of origin of the mosses "i.e. atmospheric or aquatic". In other words, the findings suggest that species that accumulate more metals in terrestrial environments will accumulate lower amounts of metals in aquatic environments and vice versa.

WHAM-FTOXβ - An aquatic toxicity model based on intrinsic metal toxic potency and intrinsic species sensitivity

We developed a model that quantifies aquatic cationic toxicity by a combination of the intrinsic toxicities of metals and protons and the intrinsic sensitivities of the test species. It is based on the WHAM-F_TOX model, which combines the calculated binding of cations by the organism with toxicity coefficients (α_H, α_M) to estimate the variable F_TOX, a measure of toxic effect; the key parameter α_M,max (applying at infinite time) depends upon both the metal and the test species. In our new model, WHAM-F_TOXβ, values of α_M,max are given by the product α_M* × β, where α_M* has a single value for each metal, and β a single value for each species. To parameterise WHAM-F_TOXβ, we assembled a set of 2182 estimates of α_M,max obtained by applying the basic model to laboratory toxicity data for 76 different test species, covering 15 different metals, and including results for metal mixtures. Then we fitted the log₁₀α_M,max values with α_M* and β values (a total of 91 parameters). The resulting model accounted for 72% of the variance in log₁₀α_M,max. The values of α_M* increased markedly as the chemical character of the metal changed from hard (average α_M* = 4.4) to intermediate (average α_M* = 25) to soft (average α_M* = 560). The values of log₁₀β were normally distributed, with a 5-95 percentile range of -0.73 to +0.56, corresponding to β values of 0.18 to 3.62. The WHAM-F_TOXβ model entails the assumption that test species exhibit common relative sensitivity, i.e. the ratio α_M,max / α_M* is constant across all metals. This was tested with data from studies in which the toxic responses of a single organism towards two or more metals had been measured (179 examples for the most-tested metals Ni, Cu, Zn, Ag, Cd, Pb), and statistically-significant (p < 0.003) results were obtained.

Is there any direct link between hazardous trace metals and the allantoin content in some moss species?

The accumulation of allantoin and trace metals (TMs) in nine moss species was examined after the exposure to stress conditions. Both the environmental anthropopressure effect and laboratory-simulated stress conditions were monitored. Moss samples were collected from different locations, i.e. a non-TM contaminated area, an urban area, and a metalliferous area. The effect of Cd, Pb, Hg, Ni, Zn, salinity, and an acidic environment on the allantoin content was tested. Principal component analysis was performed to reveal the relationship between samples of different origin. Large differences in the metal and allantoin accumulation capability of mosses were noted between samples harvested from the different locations. Seven species were considered as potential metal accumulators, as they exhibited tolerance to elevated levels of heavy metals. The observed TM effect on the allantoin accumulation indicated TM pollution as an important environmental factor that can significantly influence the content of this compound in mosses. Further studies on the contribution of various environmental factors and individual characteristics of plant species are highly expected to recognize the trend in the accumulation of specialized metabolites and TMs in response to hazardous growth conditions.

Responses to Cadmium in Early-Diverging Streptophytes (Charophytes and Bryophytes): Current Views and Potential Applications

Several transition metals are essential for plant growth and development, as they are involved in various fundamental metabolic functions. By contrast, cadmium (Cd) is a metal that can prove extremely toxic for plants and other organisms in a dose-dependent manner. Charophytes and bryophytes are early-diverging streptophytes widely employed for biomonitoring purposes, as they are able to cope with high concentrations of toxic metal(loid)s without showing any apparent heavy damage. In this review, we will deal with different mechanisms that charophytes and bryophytes have evolved to respond to Cd at a cellular level. Particular attention will be addressed to strategies involving Cd vacuolar sequestration and cell wall immobilization, focusing on specific mechanisms that help achieve detoxification. Understanding the effects of metal(loid) pollution and accumulation on the morpho-physiological traits of charophytes and bryophytes can be in fact fundamental for optimizing their use as phytomonitors and/or phytoremediators.

The use of WHAM-FTOX, parameterized with laboratory data, to simulate zooplankton species richness in acid- and metal- contaminated lakes

The WHAM-F_TOX model quantifies cation toxicity towards freshwater organisms, assuming an additive toxic response to the amounts of protons and metals accumulated by an organism. We combined a parameterization of the model, using data from multi-species laboratory toxicity tests, with a fitted field species sensitivity distribution, to simulate the species richness (n_sp) of crustacean zooplankton in acid- and metal-contaminated lakes near Sudbury, Ontario over several decades, and also in reference (uncontaminated) lakes. A good description of variation in toxic response among the zooplankton species was achieved with a log-normal distribution of a new parameter, β, which characterizes an organism's intrinsic sensitivity towards toxic cations; the greater is β, the more sensitive is the species. The use of β assumes that while species vary in their sensitivity, the relative toxicities of different metals are the same for each species (common relative sensitivity). Unbiased agreements between simulated and observed n_sp were obtained with a high correlation (r² = 0.81, p < 0.0001, n = 217). Variations in zooplankton species richness in the Sudbury lakes are calculated to be dominated by toxic responses to H, Al, Cu and Ni, with a small contribution from Zn, and negligible effects of Cd, Hg and Pb. According to the model, some of the Sudbury lakes were affected predominantly by acidification (H and Al), while others were most influenced by toxic heavy metals (Ni, Cu, Zn); for lakes in the latter category, the relative importance of heavy metals, compared to H and Al, has increased over time. The results suggest that, if common relative sensitivity operates, n_sp can be modelled on the basis of a single set of parameters characterizing the average toxic effects of different cations, together with a species sensitivity distribution.

Systematic analysis of freshwater metal toxicity with WHAM-FTOX

We used the WHAM chemical speciation model and the WHAM-F_TOX toxicity model to analyse the published results of laboratory toxicity experiments covering 52 different freshwater biological test species and 24 different metals, a total of 2037 determinations of EC₅₀ with accompanying data on solution composition. The key extracted parameter was α_M, the parameter in WHAM-F_TOX that characterises the toxic potency of a metal on the basis of its estimated metabolically active body burden. For 16 data sets applying to metal-test species pairs with appreciable variations in solution composition, values of EC₅₀ back-calculated from averaged values of α_M showed significantly (p < 0.001) less deviation from the measured EC₅₀ values than did the simple average EC₅₀, confirming that the modelling calculations could account for some of the dependence of toxicity on chemical speciation. Data for different exposure times permitted a simple parameterisation of temporal effects, enabling values of α_M,max (values at infinite exposure time) to be obtained, and the effects of different exposure times to be factored out for further analysis. Comparison of averaged values of α_M,max for different metals showed little difference among major taxa (invertebrates, plants, and vertebrates). For Cd, Cu, Ni and Zn (the four metals with most data) there were significant differences among α_M,max values for different species, but within-species variabilities were greater. Reasonably similar species sensitivity distributions of standardised α_M,max applied to Cd, Cu, Ni and Zn. The average values, over all species, of α_M,max increased in the order Al < lanthanides < Zn ∼ UO₂ < Ni ∼ Cu < Pb < Cd < Ag. Considering all the α_M,max values, there was a strong dependence (r² = 0.56, p < 0.001) on Pearson's hardness-softness categories, and a slightly stronger relationship (r² = 0.59) if ionic radius was included in the statistical model, indicating that softer, larger cations are the most effective toxicants.

Bryophyte Communities along a Tropical Urban River Respond to Heavy Metal and Arsenic Pollution

Aquatic and rheophilous bryophytes can indicate water pollution as they bioaccumulate toxic water elements. We evaluated (1) bioaccumulation of eight heavy metals and arsenic by Marchantia polymorpha L., and (2) changes in bryophyte community structure, as responses to urban pollution in southern Ecuador. To this end, we registered presence/absence and coverage of submerged bryophytes in 120 quadrats across three zones of the Zamora river inside Loja city, and a control zone in a nearby forest. We found that the concentrations of five (Al, Cd, Cu, Fe, and Zn) of the eight chemical elements and arsenic were highest in urban M. polymorpha. Moreover, bryophyte species richness decreased in urban zones. Bryophyte community structure also differed between control and city zones, but no differences were found among city zones. The control zone was composed by a more distinct set of bryophyte species, e.g., an indicator species analysis showed that 16 species had high and significant indicator values for control zone, but only 11 species were indicators of at least one of the three urban zones. We concluded that bryophytes, in general, and M. polymorpha, in particular, can be suitable biomonitors of water quality in tropical urban rivers.

Phytotoxicity of individual and binary mixtures of rare earth elements (Y, La, and Ce) in relation to bioavailability

Rare earth elements (REEs) are typically present as mixtures in the environment, but a quantitative understanding of mixture toxicity and interactions of REEs is still lacking. Here, we examined the toxicity to wheat (Triticum aestivum L.) of Y, La, and Ce when applied individually and in combination. Both concentration addition (CA) and independent action (IA) reference models were used for mixture toxicity analysis because the toxicity mechanisms of REEs remain obscure. Upon single exposure, the EC50s of Y, La, and Ce, expressed as dissolved concentrations, were 1.73 ± 0.24 μM, 2.59 ± 0.23 μM, and 1.50 ± 0.22 μM, respectively. The toxicity measured with relative root elongation followed La < Y ≈ Ce, irrespective of the dose descriptors. The use of CA and IA provided similar estimates of REE mixture interactions and toxicity. When expressed as dissolved metal concentrations, nearly additive effects were observed in Y-La and La-Ce mixtures, while antagonistic interactions were seen in Y-Ce mixtures. When expressed as free metal activities, antagonistic interactions were found for all three binary mixtures. This can be explained by a competitive effect of REEs ions for binding to the active sites of plant roots. The application of a more elaborate MIXTOX model in conjunction with the free ion activities, which incorporates the non-additive interactions and bioavailability-modifying factors, well predicted the mixture toxicity (with >92% of toxicity variations explained). Our results highlighted the importance of considering mixture interactions and subsequent bioavailability in assessing the joint toxicity of REEs.

Responses of aquatic macrophytes to anthropogenic pressures: comparison between macrophyte metrics and indices

Macrophyte responses to anthropogenic pressures in two rivers of Central Spain were assessed to check if simple metrics can exhibit a greater discriminatory and explanatory power than complex indices at small spatial scales. Field surveys were undertaken during the summer of 2014 (Duraton River) and the spring of 2015 (Tajuña River). Aquatic macrophytes were sampled using a sampling square (45 × 45 cm). In the middle Duraton River, macrophytes responded positively to the presence of a hydropower dam and a small weir, with Myriophyllum spicatum and Potamogeton pectinatus being relatively favored. Index of Macrophytes (IM) was better than Macroscopic Aquatic Vegetation Index (MAVI) and Fluvial Macrophyte Index (FMI) in detecting these responses, showing positive and significant correlations with total coverage, species richness, and species diversity. In the upper Tajuña River, macrophytes responded both negatively and positively to the occurrence of a trout farm effluent and a small weir, with Leptodictyum riparium and Veronica anagallis-aquatica being relatively favored. Although IM, MAVI, and FMI detected both negative and positive responses, correlations of IM with total coverage, species richness, and species diversity were higher. Species evenness was not sensitive enough to detect either positive or negative responses of aquatic macrophytes along the study areas. Overall, traditional and simple metrics (species composition, total coverage, species richness, species diversity) exhibited a greater discriminatory and explanatory power than more recent and complex indices (IM, MAVI, FMI) when assessing responses of aquatic macrophytes to anthropogenic pressures at impacted specific sites.

Evaluating water quality and ecotoxicology assessment techniques using data from a lead and zinc effected upland limestone catchment

Point and diffuse sources associated with historical metal ore mining are major causes of metal pollution. The understanding of metal behaviour and fate has been improved by the integration of water chemistry, metal availability and toxicity. Efforts have been devoted to the development of efficient methods of assessing and managing the risk posed by metals to aquatic life and meeting national water quality standards. This study focuses on the evaluation of current water quality and ecotoxicology techniques for the metal assessment of an upland limestone catchment located within a historical metal (lead ore) mining area in northern England. Within this catchment, metal toxicity occurs at circumneutral pH (6.2-7.5). Environmental Quality Standards (EQSs) based on a simple single concentration approach like hardness based EQS (EQS-H) are more overprotective, and from sixteen sites monitored in this study more than twelve sites (>75%) failed the EQSs for Zn and Pb. By increasing the complexity of assessment tools (e.g. bioavailability-based (EQS-B) and WHAM-F_TOX), less conservative limits were provided, decreasing the number of sites with predicted ecological risk to seven (44%). Thus, this research supports the use of bioavailability-based approaches and their applicability for future metal risk assessments.

Effect of temperature on chronic toxicity of copper, zinc, and nickel to Daphnia magna

Few studies have considered the effect of temperature on the chronic sensitivity of Daphnia magna to other stressors. The present study investigated the effect of temperature on chronic metal toxicity and whether this effect differed among 4 different D. magna clones. Life table experiments were performed with copper, zinc, and nickel at 15 °C, 20 °C, and 25 °C. General linear modeling indicated that chronic Cu, Zn, and Ni toxicity to D. magna were all significantly affected by temperature. When averaged across clones, our results suggest that chronic metal toxicity to D. magna was higher at 15 °C than at 20 °C, which is the temperature used in standard toxicity tests. At 15 °C, the 21-d median effect concentrations (EC50s) of Cu, Zn, and Ni were 1.4 times, 1.1 times, and 1.3 times lower than at 20 °C, respectively. At 25 °C, chronic Cu and Zn toxicity did not change in comparison with 20 °C, but chronic Ni toxicity was lower (21-d EC50 of nickel at 25 °C was 1.6 times higher than at 20 °C). The same trends were observed for Cu and Ni when the 21-d 10% and 20% effect concentrations were considered as the effect estimator, but not for Zn, which warns against extrapolating temperature effects on chemical toxicity across effect sizes. Overall, however, chronic metal toxicity was generally highest at the lowest temperature investigated (15 °C), which is in contrast with the usually observed higher acute metal toxicity at higher temperatures. Furthermore, the effect of temperature on chronic Ni toxicity depended significantly on the clone. This warns against extrapolating results about effect of temperature on chemical toxicity from single clone studies to the population level. Environ Toxicol Chem 2017;36:1909-1916. © 2016 SETAC.

Does specific parameterization of WHAM improve the prediction of copper competitive binding and toxicity on plant roots?

We aimed at assessing whether the binding and rhizotoxicity of metal cations such as copper that exhibit high affinity for plant roots could be adequately predicted using the Windermere Humic Aqueous Model (WHAM) default parameterization. Accordingly, we first compared the ability of the default parameterization of WHAM and a specific parameterization for terrestrial higher plants (WHAM-THP) to model the competitive binding of copper on wheat (Triticum aestivum L.) and tomato (Solanum lycopersicum L.) roots. Secondly, in an external dataset, we evaluated the ability of WHAM-THP to predict the copper concentration and toxicity to pea (Pisum sativum L.) roots relative to WHAM. WHAM-THP estimates generated a slightly better fit for the competitive binding of copper on wheat and tomato roots (log10 of the root-mean-square error, RMSE = 0.15) than WHAM estimates (RMSE = 0.24). WHAM-THP estimates slightly better fitted the copper concentration in pea roots (RMSE ≤ 0.49) than WHAM estimates (RMSE ≤ 0.67) at low copper exposure and pH ≤ 5. However, WHAM-THP did not at all improve the prediction of copper toxicity to pea roots (RMSE = 13% as also for WHAM). We thus conclude that, although the default parameterization of WHAM does not neatly predict the binding of metal cations on roots, it could however be used with confidence in predictive ecotoxicology for terrestrial higher plants without any specific parameterization.

Effect of Ocean Acidification on Organic and Inorganic Speciation of Trace Metals

Rising concentrations of atmospheric carbon dioxide are causing acidification of the oceans. This results in changes to the concentrations of key chemical species such as hydroxide, carbonate and bicarbonate ions. These changes will affect the distribution of different forms of trace metals. Using IPCC data for pCO2 and pH under four future emissions scenarios (to the year 2100) we use a chemical speciation model to predict changes in the distribution of organic and inorganic forms of trace metals. Under a scenario where emissions peak after the year 2100, predicted free ion Al, Fe, Cu, and Pb concentrations increase by factors of up to approximately 21, 2.4, 1.5, and 2.0 respectively. Concentrations of organically complexed metal typically have a lower sensitivity to ocean acidification induced changes. Concentrations of organically complexed Mn, Cu, Zn, and Cd fall by up to 10%, while those of organically complexed Fe, Co, and Ni rise by up to 14%. Although modest, these changes may have significance for the biological availability of metals given the close adaptation of marine microorganisms to their environment.

Incorporating bioavailability into toxicity assessment of Cu-Ni, Cu-Cd, and Ni-Cd mixtures with the extended biotic ligand model and the WHAM-F(tox) approach

There are only a limited number of studies that have developed appropriate models which incorporate bioavailability to estimate mixture toxicity. Here, we explored the applicability of the extended biotic ligand model (BLM) and the WHAM-F(tox) approach for predicting and interpreting mixture toxicity, with the assumption that interactions between metal ions obey the BLM theory. Seedlings of lettuce Lactuca sativa were exposed to metal mixtures (Cu-Ni, Cu-Cd, and Ni-Cd) contained in hydroponic solutions for 4 days. Inhibition to root elongation was the endpoint used to quantify the toxic response. Assuming that metal ions compete with each other for binding at a single biotic ligand, the extended BLM succeeded in predicting toxicity of three mixtures to lettuce, with more than 82% of toxicity variation explained. There were no significant differences in the values of f(mix50) (i.e., the overall amounts of metal ions bound to the biotic ligand inducing 50% effect) for the three mixture combinations, showing the possibility of extrapolating these values to other binary metal combinations. The WHAM-F(tox) approach showed a similar level of precision in estimating mixture toxicity while requiring fewer parameters than the BLM-f(mix) model. External validation of the WHAM-F(tox) approach using literature data showed its applicability for other species and other mixtures. The WHAM-F(tox) model is suitable for delineating mixture effects where the extended BLM also applies. Therefore, in case of lower data availability, we recommend the lower parameterized WHAM-F(tox) as an effective approach to incorporate bioavailability in quantifying mixture toxicity.

Delineating the dynamic uptake and toxicity of Ni and Co mixtures in Enchytraeus crypticus using a WHAM-FTOX approach

Uptake and toxicity of Ni, Co and their mixtures in Enchytraeus crypticus after different exposure times (4, 7, 10 and 14d) were predicted using the WHAM-FTOX model, which incorporates the effects of metal speciation, affinity and competition of metals for binding sites. The combined toxicity of metals was quantified by the toxicity function (FTOX), a linear combination of the amount of metal binding to non-specific ligand sites (vi) and a toxicity coefficient (αi). Observed body concentrations of Ni and Co in the animals correlated well with the WHAM-calculated amounts binding to humic acid, supporting the use of humic acid as a surrogate for metal binding sites of E. crypticus. The toxicity of metals at different exposure times was well predicted by the WHAM-FTOX model. The derived αNi increased with time and reached equilibrium after approximately 14d, while αCo remained almost independent of time. This suggests for Ni more time is needed than for Co to reach equilibrium of body concentrations, so the toxicity of Ni is much more time-dependent. The WHAM-FTOX model provides a new tool for evaluating the potential mixture toxicity of metals to soil organisms in a dynamic environment. However, as αi varied with exposure time, caution is warranted when using the parameters estimated from acute toxicity experiments for predicting the chronic toxicity of metal mixtures.

Human health risk assessment related to contaminated land: state of the art

Exposure of humans to contaminants from contaminated land may result in many types of health damage ranging from relatively innocent symptoms such as skin eruption or nausea, on up to cancer or even death. Human health protection is generally considered as a major protection target. State-of-the-art possibilities and limitations of human health risk assessment tools are described in this paper. Human health risk assessment includes two different activities, i.e. the exposure assessment and the hazard assessment. The combination of these is called the risk characterization, which results in an appraisal of the contaminated land. Exposure assessment covers a smart combination of calculations, using exposure models, and measurements in contact media and body liquids and tissue (biomonitoring). Regarding the time frame represented by exposure estimates, biomonitoring generally relates to exposure history, measurements in contact media to actual exposures, while exposure calculations enable a focus on exposure in future situations. The hazard assessment, which is different for contaminants with or without a threshold for effects, results in a critical exposure value. Good human health risk assessment practice accounts for tiered approaches and multiple lines of evidence. Specific attention is given here to phenomena such as the time factor in human health risk assessment, suitability for the local situation, background exposure, combined exposure and harmonization of human health risk assessment tools.

Testing WHAM-FTOX with laboratory toxicity data for mixtures of metals (Cu, Zn, Cd, Ag, Pb)

The Windermere humic aqueous model using the toxicity function (WHAM-FTOX ) describes cation toxicity to aquatic organisms in terms of 1) accumulation by the organism of metabolically active protons and metals at reversible binding sites, and 2) differing toxic potencies of the bound cations. Cation accumulation (νi , in mol g(-1) ) is estimated through calculations with the WHAM chemical speciation model by assuming that organism binding sites can be represented by those of humic acid. Toxicity coefficients (αi ) are combined with νi to obtain the variable FTOX (= Σ αi νi ) which, between lower and upper thresholds (FTOX,LT , FTOX,UT ), is linearly related to toxic effect. Values of αi , FTOX,LT , and FTOX,LT are obtained by fitting toxicity data. Reasonable fits (72% of variance in toxic effect explained overall) were obtained for 4 large metal mixture acute toxicity experiments involving daphnids (Cu, Zn, Cd), lettuce (Cu, Zn, Ag), and trout (Zn, Cd, Pb). Strong nonadditive effects, most apparent in results for tests involving Cd, could be explained approximately by purely chemical competition for metal accumulation. Tentative interpretation of parameter values obtained from these and other experimental data suggests the following order of bound cation toxicity: H < Al < (Cu Zn Pb UO2 ) < (Cd Ag). Another trend is a strong increase in Cd toxicity relative to that of Zn as organism complexity increases (from bacteria to fish).

Modelling metal accumulation using humic acid as a surrogate for plant roots

Metal accumulation in roots was modelled with WHAM VII using humic acid (HA) as a surrogate for root surface. Metal accumulation was simulated as a function of computed metal binding to HA, with a correction term (E(HA)) to account for the differences in binding site density between HA and root surface. The approach was able to model metal accumulation in roots to within one order of magnitude for 95% of the data points. Total concentrations of Mn in roots of Vigna unguiculata, total concentrations of Ni, Zn, Cu and Cd in roots of Pisum sativum, as well as internalized concentrations of Cd, Ni, Pb and Zn in roots of Lolium perenne, were significantly correlated to the computed metal binding to HA. The method was less successful at modelling metal accumulation at low concentrations and in soil experiments. Measured concentrations of Cu internalized in L. perenne roots were not related to Cu binding to HA modelled and deviated from the predictions by over one order of magnitude. The results indicate that metal uptake by roots may under certain conditions be influenced by conditional physiological processes that cannot simulated by geochemical equilibrium. Processes occurring in chronic exposure of plants grown in soil to metals at low concentrations complicate the relationship between computed metal binding to HA and measured metal accumulation in roots.

Dissolved trace metal speciation in estuarine and coastal waters: comparison of WHAM/Model VII predictions with analytical results

The authors apply the chemical speciation model WHAM/Model VII to investigate the distribution of metal species of Fe(III) and the divalent cations of Ni, Cu, Zn, Cd, Hg, and Pb, in the water column of estuaries and coastal areas. The authors compare, for the same locations, measured and modeled free ion and organically bound metal concentrations. The modeled free ion calculations show varying levels of agreement with experimental measurements. Where only natural organic matter is considered as the organic ligand, for Ni, Cd, and Pb, agreement within 1 order of magnitude is found in 122 of 128 comparisons. For Fe and Zn comparisons 12 of 34 (Fe) and 10 of 18 (Zn) agree to within 1 order of magnitude, the remaining modeled values being over 1 order of magnitude higher than measurements. Copper measurements agree within 1 order of magnitude of modeled values in 314 of 533 (59%) cases and are more than 1 order of magnitude lower than modeled values in 202 cases. There is a general tendency for agreement between modeled and measured values to improve with increasing total metal concentrations. There are substantial variations among different analysis techniques but no systematic bias from the model is observed across techniques. It would be beneficial to cross-validate the different analytical methods, in combination with further modeling. The authors also assessed the effect of including an anthropogenic organic ligand (ethylenediamine tetraacetic acid (EDTA)) in the modeling, given its known presence in some coastal environments. Except for Cd, all metals were sensitive to the presence of EDTA, even at a low concentration of 50 nM.

Relating metal exposure and chemical speciation to trace metal accumulation in aquatic insects under natural field conditions

The present study investigated to what extent measured dissolved metal concentrations, WHAM-predicted free metal ion activity and modulating water chemistry factors can predict Ni, Cu, Zn, Cd and Pb accumulation in various aquatic insects under natural field conditions. Total dissolved concentrations and accumulated metal levels in four taxa (Leuctra sp., Simuliidae, Rhithrogena sp. and Perlodidae) were determined and free metal ion activities were calculated in 36 headwater streams located in the north-west part of England. Observed invertebrate body burdens were strongly related to free metal ion activities and competition among cations for uptake in the biota. Taking into account competitive effects generally provided better fits than considering uptake as a function of total dissolved metal levels or the free ion alone. Due to the critical importance and large range in pH (4.09 to 8.33), the H(+) ion activity was the most dominant factor influencing metal accumulation. Adding the influence of Na(+) on Cu(2+) accumulation improved the model goodness of fit for both Rhithrogena sp. and Perlodidae. Effects of hardness ions on metal accumulation were limited, indicating the minor influence of Ca(2+) and Mg(2+) on metal accumulation in soft-water streams (0.01 to 0.94 mM Ca; 0.02 to 0.39 mM Mg). DOC levels (ranging from 0.6 to 8.9 mg L(-1)) significantly affected Cu body burdens, however not the accumulation of the other metals. Our results suggest that 1) uptake and accumulation of free metal ions are most dominantly influenced by competition of free H(+) ions in low-hardness headwaters and 2) invertebrate body burdens in natural waters can be predicted based on the free metal ion activity using speciation modelling and effects of H(+) competition.

Lead toxicity to Lemna minor predicted using a metal speciation chemistry approach

In the present study, predictive measures for Pb toxicity and Lemna minor were developed from bioassays with 7 surface waters having varied chemistries (0.5-12.5 mg/L dissolved organic carbon, pH of 5.4-8.3, and water hardness of 8-266 mg/L CaCO3 ). As expected based on water quality, 10%, 20%, and 50% inhibitory concentration (IC10, IC20, and IC50, respectively) values expressed as percent net root elongation (%NRE) varied widely (e.g., IC20s ranging from 306 nM to >6920 nM total dissolved Pb), with unbounded values limited by Pb solubility. In considering chemical speciation, %NRE variability was better explained when both Pb hydroxides and the free lead ion were defined as bioavailable (i.e., f{OH} ) and colloidal Fe(III)(OH)3 precipitates were permitted to form and sorb metals (using FeOx as the binding phase). Although cause and effect could not be established because of covariance with alkalinity (p = 0.08), water hardness correlated strongly (r(2)  = 0.998, p < 0.0001) with the concentration of total Pb in true solution ([Pb]T_True solution ). Using these correlations as the basis for predictions (i.e., [Pb]T_True solution vs water hardness and %NRE vs f{OH} ), IC20 and IC50 values produced were within a factor of 2.9 times and 2.2 times those measured, respectively. The results provide much needed effect data for L. minor and highlight the importance of chemical speciation in Pb-based risk assessments for aquatic macrophytes.

Metal and proton toxicity to lake zooplankton: a chemical speciation based modelling approach

The WHAM-FTOX model quantifies the combined toxic effects of protons and metal cations towards aquatic organisms through the toxicity function (FTOX), a linear combination of the products of organism-bound cation and a toxic potency coefficient for each cation. We describe the application of the model to predict an observable ecological field variable, species richness of pelagic lake crustacean zooplankton, studied with respect to either acidification or the impacts of metals from smelters. The fitted results give toxic potencies increasing in the order H(+) < Al < Cu < Zn < Ni. In general, observed species richness is lower than predicted, but in some instances agreement is close, and is rarely higher than predictions. The model predicts recovery in agreement with observations for three regions, namely Sudbury (Canada), Bohemian Forest (Czech Republic) and a subset of lakes across Norway, but fails to predict observed recovery from acidification in Adirondack lakes (USA).

Predicting the toxicity of metal mixtures

The toxicity of single and multiple metal (Cd, Cu, Pb, and Zn) solutions to trout is predicted using an approach that combines calculations of: (1) solution speciation; (2) competition and accumulation of cations (H, Ca, Mg, Na, Cd, Cu, Pb, and Zn) on low abundance, high affinity and high abundance, low affinity biotic ligand sites; (3) a toxicity function that accounts for accumulation and potency of individual toxicants; and (4) biological response. The approach is evaluated by examining water composition from single metal toxicity tests of trout at 50% mortality, results of theoretical calculations of metal accumulation on fish gills and associated mortality for single, binary, ternary, and quaternary metal solutions, and predictions for a field site impacted by acid rock drainage. These evaluations indicate that toxicity of metal mixtures depends on the relative affinity and potency of toxicants for a given aquatic organism, suites of metals in the mixture, dissolved metal concentrations and ratios, and background solution composition (temperature, pH, and concentrations of major ions and dissolved organic carbon). A composite function that incorporates solution composition, affinity and competition of cations for two types of biotic ligand sites, and potencies of hydrogen and individual metals is proposed as a tool to evaluate potential toxicity of environmental solutions to trout.

Metal mixture toxicity to aquatic biota in laboratory experiments: application of the WHAM-FTOX model

The WHAM-FTOX model describes the combined toxic effects of protons and metal cations towards aquatic organisms through the toxicity function (FTOX), a linear combination of the products of organism-bound cation and a toxic potency coefficient (αi) for each cation. Organism-bound, metabolically-active, cation is quantified by the proxy variable, amount bound by humic acid (HA), as predicted by the WHAM chemical speciation model. We compared published measured accumulations of metals by living organisms (bacteria, algae, invertebrates) in different solutions, with WHAM predictions of metal binding to humic acid in the same solutions. After adjustment for differences in binding site density, the predictions were in reasonable line with observations (for logarithmic variables, r(2)=0.89, root mean squared deviation=0.44), supporting the use of HA binding as a proxy. Calculated loadings of H(+), Al, Cu, Zn, Cd, Pb and UO2 were used to fit observed toxic effects in 11 published mixture toxicity experiments involving bacteria, macrophytes, invertebrates and fish. Overall, WHAM-FTOX gave slightly better fits than a conventional additive model based on solution concentrations. From the derived values of αi, the toxicity of bound cations can tentatively be ranked in the order: H<Al<(Zn-Cu-Pb-UO2)<Cd. The WHAM-FTOX analysis indicates much narrower ranges of differences amongst individual organisms in metal toxicity tests than was previously thought. The model potentially provides a means to encapsulate knowledge contained within laboratory data, thereby permitting its application to field situations.

The use of invertebrate body burdens to predict ecological effects of metal mixtures in mining-impacted waters

The present study investigated whether invertebrate body burdens can be used to predict metal-induced effects on aquatic invertebrate communities. Total dissolved metal levels and four invertebrate taxa (Leuctra sp., Simuliidae, Rhithrogena sp. and Perlodidae) were sampled in 36 headwater streams located in the north-west part of England. Using the River Invertebrate Prediction and Classification System (RIVPACS) taxonomic completeness of invertebrate communities was assessed. Quantile regression was used to relate invertebrate body burdens to a maximum (90th quantile) ecological response, both for all metals separately and in mixtures. Significant relations between Cu, Zn and Pb burdens in Leuctra sp. (Zn, Pb), Simuliidae (Zn, Pb), Rhithrogena sp. (Cu, Zn, Cu+Zn) and Perlodidae (Zn) and both taxonomic completeness (O/E taxa) and Biological Monitoring Working Party index scores (O/E BMWP) were observed. Correspondingly the obtained Cu-Zn mixture model an acceptable impact of 5% change in taxonomic completeness is expected at Rhithrogena sp. body burdens of 1.9μmolg(-1) Cu (121 μg g(-1) Cu) in case of low Zn bioavailability (Rhithrogena sp. Zn body burden of 2.9 μmol g(-1) or 190 μg g(-1)), which will drop to 0.30 μmol g(-1) Cu (19.1 μg g(-1) Cu) in case of higher Zn bioavailability (Zn body burden of 72.6 μmol g(-1) or 4747 μg g(-1)). For Zn, 5% change in taxonomic completeness is expected at Rhithrogena sp. body burdens of 76.4 μmol g(-1) Zn (4995 μg g(-1) Zn) in case of low Cu bioavailability (Cu body burden of 0.19 μmol g(-1) or 12.1 μg g(-1)), which will drop to 6.6 μmol g(-1) Zn (432 μg g(-1) Zn) at higher Cu bioavailability (Cu body burden of 1.74 μmol g(-1) or 111 μg g(-1)). Overall, the present study concludes that invertebrate body burdens can be used to (1) predict metal-induced ecological effects and (2) to derive critical burdens for the protection of aquatic invertebrate communities.

Comparison of different predictors of exposure for modeling impacts of metal mixtures on macroinvertebrates in stream microcosms

Knowledge about which predictors of metal exposure are best to model the impacts of metal mixtures on river macroinvertebrates remains uncertain. A new predictor based on the amount of metals binding to humic acid, which is assumed to be a proxy of non-specific biotic ligand sites, has been proposed. The amount can be calculated using Windermere Humic Aqueous Model (WHAM), which we will refer to as the WHAM-HA approach. Here, we tested the hypothesis that the predictor based on the WHAM-HA approach provided a better estimate of metal effects observed in microcosm experiments than three other measures: total metal concentrations, free metal ion concentrations, and the cumulative criterion unit (CCU) which is a measure of the ratios of measured metal concentrations relative to the U.S. Environmental Protection Agency hardness adjusted criterion values. For this evaluation, we used nine macroinvertebrate metrics of abundance and richness obtained from microcosm experiments conducted with metal mixtures (Zn alone, Zn+Cd, and Zn+Cd+Cu). For each of the four predictors, we performed multiple linear regression with variables corresponding to the three metal concentrations or CCU and selected the best model based on Akaike's information criterion corrected for small sample sizes. For all of the macroinvertebrate metrics affected by metals, the WHAM-HA approach was selected as the best among the four predictors, followed by the model with total metal concentration. In most of best models, Zn and Cu or Cu alone was responsible for reductions in invertebrate metrics, even though the highest concentrations of Cd exceeded 100 times the hardness-adjusted criterion value. Either of the models with free metal ion concentration and CCU was the third ranked model. Our results suggest that the estimated amount of metals binding to humic acid is a better predictor for the effects on macroinvertebrate richness and abundance observed in microcosm experiments than total or free ion concentrations of metals and CCU.

Testing copper-speciation predictions in freshwaters over a wide range of metal-organic matter ratios

The harsh chemical conditions involved in the isolation of fulvic acids (FA) and humic acids (HA) have been identified as a possible contributing factor to the significant mismatch between in situ measurements and model predictions of trace metal speciation in freshwaters, resulting from the use of isolated FA and HA in model calibration. A set of experimental assays were developed to enable Cu binding to DOM to be measured over the full range of [Cu]/[DOC] ratios (∼1-460 μmol g(-1)) observed in surface freshwaters. They were applied to the widely used and traditionally isolated Suwannee River HA and FA and to DOM isolated from headwater streams by a mild procedure using minimal chemical treatment. Good agreement was observed between measured free ion activities and those predicted using both WHAM/Model VII and NICA-Donnan speciation models for both traditionally and mildly isolated DOM. Agreement to within a factor of 2 for WHAM/Model VII contrasts with 100-fold differences previously reported between in situ Cu(2+) measurements and model predictions for a wide range of conditions. The results demonstrate that (a) existing speciation models are capable of accurately predicting Cu-humic binding in natural waters at environmentally realistic [Cu]/[DOC] ratios, under equilibrium conditions, and (b) that the isolation procedures traditionally used for HA and FA do not appreciably affect their binding characteristics.

Copper toxicity to Lemna minor modelled using humic acid as a surrogate for the plant root

Humic acids are chemically analogous to plant root cell walls in that their surface sites are principally comprised of carboxylic and phenolic acids which bind both metals and protons. Based on this analogy, we developed a biotic-ligand type of model to predict Cu toxicity to Lemna minor, using particulate humic acid (HA(part)) of the Windermere Humic Aqueous Model (WHAM), and 7d static-renewal exposures with five surface waters and one nutrient media which varied in DOC (1-10 mg L(-1)), pH (6.9-8.7), and water hardness (35-236 mg equivalent CaCO(3)L(-1)). Although the range of waters tested resulted in a 36-fold variation in 50% inhibitory concentration (IC50) values, the calculated concentration of Cu bound to HA(part) using this framework was highly correlated with pooled percent net root elongation (%NRE) (R(2)=0.95). Ten and fifty percent IC values based on [Cu-HA(part)] were additionally within a factor of ±1.5 and ±1.4, respectively, inclusive of 95% confidence limits. This model construct, which defines the free metal ion and the first hydrolysis product (but not metal carbonate complexes) as being bioavailable, provides an alternative means of defining the binding surface in bioavailability models, whereby a heterogeneous mixture of ligands collectively influence root-metal sorption and toxicity.

Estimating safe concentrations of trace metals from inter-continental field data on river macroinvertebrates

We derived safe concentrations (SCs) of copper, zinc, cadmium, and manganese using river macroinvertebrate surveys at over 400 individual sites on three continents represented by the UK, USA, and Japan. We related a standardized measure of EPT (Ephemeroptera, Plecoptera, and Trichoptera) taxon richness to dissolved metal concentrations and identified SCs as the thresholds at which effects became apparent. Estimated SCs (and 95% confidence interval, μg/L) for copper, zinc, cadmium, and manganese were 6.6 (1.2-14.2), 34 (11-307), 0.11 (0.06-0.49), and 7.1 (1.4-20.5), respectively. These values for copper, zinc, and cadmium overlapped closely with laboratory-derived SCs available from water quality criteria/standards in the USA/UK and also predicted no effect concentrations from European Union risk assessments. Such laboratory-derived SCs for manganese are unavailable. These results not only add considerable confidence to the application of existing metal standards, but illustrate also how standard values might be widely transportable geographically.

Similar cation exchange capacities among bryophyte species refute a presumed mechanism of peatland acidification

Fen-bog succession is accompanied by strong increases of carbon accumulation rates. We tested the prevailing hypothesis that living Sphagna have extraordinarily high cation exchange capacity (CEC) and therefore acidify their environment by exchanging tissue-bound protons for basic cations in soil water. As Sphagnum invasion in a peatland usually coincides with succession from a brown moss-dominated alkaline fen to an acidic bog, the CEC of Sphagna is widely believed to play an important role in this acidification process. However, Sphagnum CEC has never been compared explicitly to that of a wide range of other bryophyte taxa. Whether high CEC directly leads to the ability to acidify the environment in situ also remains to be tested. We screened 20 predominant subarctic bryophyte species, including fen brown mosses and bog Sphagna for CEC, in situ soil water acidification capacity (AC), and peat acid neutralizing capacity (ANC). All these bryophyte species possessed substantial CEC, which was remarkably similar for brown mosses and Sphagna. This refutes the commonly accepted idea of living Sphagnum CEC being responsible for peatland acidification, as Sphagnum's ecological predecessors, brown mosses, can do the same job. Sphagnum AC was several times higher than that of other bryophytes, suggesting that CE (cation exchange) sites of Sphagna in situ are not saturated with basic cations, probably due to the virtual absence of these cations in the bog water. Together, these results suggest that Sphagna can not realize their CEC in bogs, while fen mosses can do so in fens. The fen peat ANC was 65% higher than bog ANC, indicating that acidity released by brown mosses in the CE process was neutralized, maintaining an alkaline environment. We propose two successional pathways indicating boundaries for a fen-bog shift with respect to bryophyte CEC. In neither of them is Sphagnum CE an important factor. We conclude that living Sphagnum CEC does not play any considerable role in the fen-bog shift. Alternatively, we propose that exclusively indirect effects of Sphagnum expansion such as peat accumulation and subsequent blocking of upward alkaline soil water transport are keys to the fen-bog succession and therefore for bog-associated carbon accumulation.

Toxicity of proton-metal mixtures in the field: linking stream macroinvertebrate species diversity to chemical speciation and bioavailability

Understanding metal and proton toxicity under field conditions requires consideration of the complex nature of chemicals in mixtures. Here, we demonstrate a novel method that relates streamwater concentrations of cationic metallic species and protons to a field ecological index of biodiversity. The model WHAM-F(TOX) postulates that cation binding sites of aquatic macroinvertebrates can be represented by the functional groups of natural organic matter (humic acid), as described by the Windermere Humic Aqueous Model (WHAM6), and supporting field evidence is presented. We define a toxicity function (F(TOX)) by summing the products: (amount of invertebrate-bound cation) x (cation-specific toxicity coefficient, α(i)). Species richness data for Ephemeroptera, Plecoptera and Trichoptera (EPT), are then described with a lower threshold of F(TOX), below which all organisms are present and toxic effects are absent, and an upper threshold above which organisms are absent. Between the thresholds the number of species declines linearly with F(TOX). We parameterised the model with chemistry and EPT data for low-order streamwaters affected by acid deposition and/or abandoned mines, representing a total of 412 sites across three continents. The fitting made use of quantile regression, to take into account reduced species richness caused by (unknown) factors other than cation toxicity. Parameters were derived for the four most common or abundant cations, with values of α(i) following the sequence (increasing toxicity) H+ < Al < Zn < Cu. For waters affected mainly by H+ and Al, F(TOX) shows a steady decline with increasing pH, crossing the lower threshold near to pH 7. Competition effects among cations mean that toxicity due to Cu and Zn is rare at lower pH values, and occurs mostly between pH 6 and 8.

In situ speciation measurements of trace metals in headwater streams

Concentrations of Al, Fe, Mn, Ni, Cu, Cd, Pb, and Zn were measured using DGT (diffusive gradients in thin-films) devices deployed in situ in 34 headwater streams in Northern England. Mean values of filtered samples analyzed by ICP-MS (inductively coupled plasma mass spectrometry) were used, along with DOC (dissolved organic carbon), pH and major ions, to calculate the distribution of metal species using the speciation code WHAM. DGT-measured concentrations, [Me]DGT, of Zn and Cd were generally similar to concentrations in filtered samples, [Me]filt. For the other metals, [Me]DGT was similar to or lower than [Me]filt. Calculation of the maximum dynamic metal from the speciation predicted using WHAM showed that most of the lower values of [Cu]DGT could be attributed to the dominance of Cu-fulvic acid complexes, which diffuse more slowly than simple inorganic species. Similar calculations for Al, Pb, and Mn were consistent with appreciable proportions of these metals being present as colloids that are not simple complexes with humic substances. Differences between WHAM predictions and the measured [Ni]DGT indicated that WHAM used with the default binding parameters underestimates Ni binding to natural organic matter. Plots of [Me]DGT versus the ratio of bound metal to DOC provided slight evidence of heterogeneous binding of Pb and Cu, while results for Mn, Cd, and Zn were consistent with weak binding and complete lability.