Trophic dynamics of selenium in a boreal lake food web.

Selenium (Se) is both an essential micronutrient and a contaminant of concern that is of particular interest in mining-influenced waterbodies in Canada. The objective of this research was to characterize the trophic dynamics of selenium along a gradient of exposure concentrations in a Canadian boreal lake ecosystem. From June 20 to August 22, 2018, six limnocorrals (littoral, ∼3000 L enclosures) were spiked with mean measured concentrations of 0.4, 0.8, 1.6, 3.4, 5.6 and 7.9 µg Se/L as selenite, and three limnocorrals served as untreated controls (background aqueous Se = 0.08-0.09 µg/L). Total Se (TSe) concentrations in water, periphyton, phytoplankton, sediment, benthic macroinvertebrates, zooplankton and female finescale dace (Phoxinus neogaeus; added on day 21 of the experiment) were measured throughout and at the end of the experiment. Total Se bioaccumulation by organisms was generally non-linear. Greater uptake by phytoplankton than periphyton was observed. Taxonomic differences in accumulation of TSe by invertebrates (Heptageniidae = Chironomidae > zooplankton) were observed as well. Fish muscle and ovary tissue TSe bioaccumulation was more variable than that at lower trophic levels and uptake patterns indicated that fish did not reach steady state concentrations. This research provides field-derived models for the uptake of Se by algae and invertebrates, and contributes to a better understanding of the dynamics of TSe bioaccumulation over a gradient of exposure concentrations in cold-water lentic systems.

Effects of selenium on benthic macroinvertebrates and fathead minnow (Pimephales promelas) in a boreal lake ecosystem.

Selenium (Se) is a contaminant of concern in many aquatic ecosystems due to its narrow range between essentiality and toxicity in oviparous (yolk-bearing) vertebrates. The objective of the present study was to determine the effects of Se, experimentally added to in situ limnocorrals as selenite, on invertebrate communities and fathead minnow (Pimephales promelas) at environmentally realistic Se concentrations. Nine limnocorrals were deployed in a mesotrophic lake at the International Institute for Sustainable Development - Experimental Lakes Area in Ontario, Canada in May 2017. From June 1 to August 17, 2017, selenite was added to six enclosures to attain mean measured aqueous Se concentrations of 1.0 ±â€¯0.10 or 8.9 ±â€¯2.7 µg/L Se (in triplicate) and three limnocorrals were untreated controls (background mean aqueous Se = 0.12 ±â€¯0.03 µg/L). Benthic macroinvertebrates were collected throughout and at the end of the exposure period using artificial substrates to determine density, dry biomass, diversity, and taxa richness at the family level. Reproductively mature female fathead minnows (added on d 33 of the study) were collected throughout and at the end of the exposure period. After 77 d, Chironomidae and Gammaridae densities and biomass were significantly lower in the 8.9 µg/L Se treatment relative to the 1.0 µg/L Se treatment and the control. Invertebrate diversity (measured as Shannon's and Simpson's indices) significantly declined in the 1.0 µg/L and 8.9 µg/L Se treatments relative to the control (0.12 µg/L Se group). Fulton's condition factor for fathead minnow was significantly less in the 8.9 µg/L treatment compared to 0.12 and 1.0 µg/L Se experimental groups. The results of this study indicated that exposure to relatively low aqueous selenite concentrations can negatively affect invertebrate density and biomass, as well as fish condition. More research is necessary to characterize the risk of selenite exposure to aquatic invertebrates under realistic field conditions, and future risk assessments may need to consider reduced food availability as a factor that may impair the health of higher trophic level organisms in areas with elevated selenite.

Vanadium and thallium exhibit biodilution in a northern river food web.

Trophic transfer of contaminants dictates concentrations and potential toxic effects in top predators, yet biomagnification behaviour of many trace elements is poorly understood. We examined concentrations of vanadium and thallium, two globally-distributed and anthropogenically-enriched elements, in a food web of the Slave River, Northwest Territories, Canada. We found that tissue concentrations of both elements declined with increasing trophic position as measured by δ15N. Slopes of log [element] versus δ15N regressions were both negative, with a steeper slope for V (-0.369) compared with Tl (-0.099). These slopes correspond to declines of 94% with each step in the food chain for V and 54% with each step in the food chain for Tl. This biodilution behaviour for both elements meant that concentrations in fish were well below values considered to be of concern for the health of fish-eating consumers. Further study of these elements in food webs is needed to allow a fuller understanding of biomagnification patterns across a range of species and systems.

Distribution of Experimentally Added Selenium in a Boreal Lake Ecosystem.

Human activities have increased the release of selenium (Se) to aquatic environments, but information about the trophic transfer dynamics of Se in Canadian boreal lake systems is limited. In the present study, Se was added as selenite to limnocorrals (2-m-diameter, 3000-L in situ enclosures) in a boreal lake in northwestern Ontario to reach nominal concentrations of 1 and 10 µg Se/L in triplicate each for 77 d, and 3 additional limnocorrals were controls with no Se added. Total Se concentrations were determined in water, sediment, periphyton, benthic macroinvertebrates, zooplankton, and reproductively mature female fathead minnows (Pimephales promelas; added on day 33) collected throughout (and at the end of) the exposure period. Mean measured water Se concentrations in the control, 1-, and 10-µg/L treatments were 0.12, 1.0, and 8.9 µg/L. At the end of exposure (day 77), enrichment functions ranged from 7772 L/kg dry mass in the 8.9-µg/L treatment to 23 495 L/kg dry mass in the 0.12-µg/L treatment, and trophic transfer factors for benthic macroinvertebrates ranged from 0.49 for Gammaridae to 2.3 for Chironomidae. Selenium accumulated in fathead minnow ovaries to concentrations near or above the current US Environmental Protection Agency criterion (15.1 µg/g dry mass for fish ovary/egg) in the 1.0- and 8.9-µg/L treatments, suggesting that, depending on aqueous Se speciation, such exposures have the potential to cause Se accumulation in fish to levels of concern in cold-water, boreal lake systems. Environ Toxicol Chem 2019;38:1954-1966. © 2019 SETAC.

Selenium oxyanion bioconcentration in natural freshwater periphyton.

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

Combining High-Throughput Sequencing of sedaDNA and Traditional Paleolimnological Techniques To Infer Historical Trends in Cyanobacterial Communities.

Freshwaters worldwide are under increasing pressure from anthropogenic activities and changing climate. Unfortunately, many inland waters lack sufficient long-term monitoring to assess environmental trends. Analysis of sedimentary ancient DNA ( sedaDNA) is emerging as a means to reconstruct the past occurrence of microbial communities of inland waters. The purpose of this study was to assess a combination of high-throughput sequencing (16S rRNA) of sedaDNA and traditional paleolimnological analyses to explore multidecadal relationships among cyanobacterial community composition, the potential for cyanotoxin production, and paleoenvironmental proxies. DNA was extracted from two sediment cores collected from a northern Canadian Great Plains reservoir. Diversity indices illustrated significant community-level changes since reservoir formation. Furthermore, higher relative abundances in more recent years were observed for potentially toxic cyanobacterial genera including Dolichospermum. Correlation-based network analysis revealed this trend significantly and positively correlated to abundances of the microcystin synthetase gene ( mcyA) and other paleoproxies (nutrients, pigments, stanols, sterols, and certain diatom species), demonstrating synchrony between molecular and more standard proxies. These findings demonstrate a novel approach to infer long-term dynamics of cyanobacterial diversity in inland waters and highlight the power of high-throughput sequencing to reconstruct trends in environmental quality and inform lake and reservoir management and monitoring program design.

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

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

Open-water and under-ice seasonal variations in trace element content and physicochemical associations in fluvial bed sediment.

Across the circumpolar world, intensive anthropogenic activities in the southern reaches of many large, northward-flowing rivers can cause sediment contamination in the downstream depositional environment. The influence of ice cover on concentrations of inorganic contaminants in bed sediment (i.e., sediment quality) is unknown in these rivers, where winter is the dominant season. A geomorphic response unit approach was used to select hydraulically diverse sampling sites across a northern test-case system, the Slave River and delta (Northwest Territories, Canada). Surface sediment samples (top 1 cm) were collected from 6 predefined geomorphic response units (12 sites) to assess the relationships between bed sediment physicochemistry (particle size distribution and total organic carbon content) and trace element content (mercury and 18 other trace elements) during open-water conditions. A subset of sites was resampled under-ice to assess the influence of season on these relationships and on total trace element content. Concentrations of the majority of trace elements were strongly correlated with percent fines and proxies for grain size (aluminum and iron), with similar trace element grain size/grain size proxy relationships between seasons. However, finer materials were deposited under ice with associated increases in sediment total organic carbon content and the concentrations of most trace elements investigated. The geomorphic response unit approach was effective at identifying diverse hydrological environments for sampling prior to field operations. Our data demonstrate the need for under-ice sampling to confirm year-round consistency in trace element-geochemical relationships in fluvial systems and to define the upper extremes of these relationships. Whether contaminated or not, under-ice bed sediment can represent a "worst-case" scenario in terms of trace element concentrations and exposure for sediment-associated organisms in northern fluvial systems. Environ Toxicol Chem 2017;36:2916-2924. © 2017 SETAC.

Bridging science and traditional knowledge to assess cumulative impacts of stressors on ecosystem health.

Cumulative environmental impacts driven by anthropogenic stressors lead to disproportionate effects on indigenous communities that are reliant on land and water resources. Understanding and counteracting these effects requires knowledge from multiple sources. Yet the combined use of Traditional Knowledge (TK) and Scientific Knowledge (SK) has both technical and philosophical hurdles to overcome, and suffers from inherently imbalanced power dynamics that can disfavour the very communities it intends to benefit. In this article, we present a 'two-eyed seeing' approach for co-producing and blending knowledge about ecosystem health by using an adapted Bayesian Belief Network for the Slave River and Delta region in Canada's Northwest Territories. We highlight how bridging TK and SK with a combination of field data, interview transcripts, existing models, and expert judgement can address key questions about ecosystem health when considerable uncertainty exists. SK indicators (e.g., bird counts, mercury in fish, water depth) were graded as moderate, whereas TK indicators (e.g., bird usage, fish aesthetics, changes to water flow) were graded as being poor in comparison to the past. SK indicators were predominantly spatial (i.e., comparing to other locations) while the TK indicators were predominantly temporal (i.e., comparing across time). After being populated by 16 experts (local harvesters, Elders, governmental representatives, and scientists) using both TK and SK, the model output reported low probabilities that the social-ecological system is healthy as it used to be. We argue that it is novel and important to bridge TK and SK to address the challenges of environmental change such as the cumulative impacts of multiple stressors on ecosystems and the services they provide. This study presents a critical social-ecological tool for widening the evidence-base to a more holistic understanding of the system dynamics of multiple environmental stressors in ecosystems and for developing more effective knowledge-inclusive partnerships between indigenous communities, researchers and policy decision-makers. This represents new transformational empirical insights into how wider knowledge discourses can contribute to more effective adaptive co-management governance practices and solutions for the resilience and sustainability of ecosystems in Northern Canada and other parts of the world with strong indigenous land tenure.

Stable sulfur isotopes identify habitat-specific foraging and mercury exposure in a highly mobile fish community.

Tracking the uptake and transfer of toxic chemicals, such as mercury (Hg), in aquatic systems is challenging when many top predators are highly mobile and may therefore be exposed to chemicals in areas other than their location of capture, confounding interpretation of bioaccumulation trends. Here we show how the application of a less commonly used ecological tracer, stable sulfur isotope ratios (34S/32S, or δ34S), in a large river-delta-lake complex in northern Canada allows differentiation of resident from migrant fishes, beyond what was possible with more conventional 13C/12C and 15N/14N measurements. Though all large fishes (n=105) were captured in the river, the majority (76%) had δ34S values that were indicative of the fish having been reared in the lake. These migrant fishes were connected to a food chain with greater Hg trophic magnification relative to the resident fish of the river and delta. Yet, despite a shallower overall trophic magnification slope, large river-resident fish had higher Hg concentrations owing to a greater biomagnification of Hg between small and large fishes. These findings reveal how S isotopes can trace fish feeding habitats in large freshwater systems and better account for fish movement in complex landscapes with differential exposure pathways and conditions.

Reconstructing the ecological impacts of eight decades of mining, metallurgical, and municipal activities on a small boreal lake in northern Canada.

As a result of long-term metal mining and metallurgical activities, the sediment of Ross Lake (Flin Flon, MB, Canada) is highly contaminated with metals and other elements. Although the effluents likely were discharged into Ross Lake as early as the late 1920s, lake biophysical data were not collected until 1973, more than 4 decades after the onset of mining and municipal activities. The early influence of these activities on the ecology of Ross Lake is unknown, as are the effects of improvements to metallurgical effluent quality and discontinuation of municipal wastewater discharge into the lake's north basin. To address this knowledge gap, analyses typical of paleolimnological investigations were applied to cores of sediment collected in 2009 from the south basin of Ross Lake. Stratigraphic analyses of physicochemical sediment characteristics (e.g., the concentrations of metals and other elements, organic C, total N, and δ(13)C and δ(15)N values) and subfossil remains (diatoms, Chironomidae, Chaoborus, and Cladocera) were used to infer historical biological and chemical changes in Ross Lake. With the onset of mining activities, concentrations of various elements (e.g., As, Cr, Cu, Zn, and Se) increased dramatically in the sediment profile, eventually declining with improved tailings management. Nevertheless, concentrations of metals in recent sediments remain elevated compared with pre-industrial sediments. Constrained cluster analyses demonstrated distinct pre-industrial and postindustrial communities for both the diatoms and chironomids. The biodiversity of the postindustrial diatom assemblages were much reduced compared with the pre-industrial assemblages. The postindustrial chironomid assemblage was dominated by Chironomus and to a lesser extent by Procladius, suggesting that Ross Lake became a degraded environment. Abundances of Cladocera and Chaoborus were severely reduced in the postindustrial era, likely because of metals toxicity. Overall, improvements to the management of both metallurgical and municipal effluent are reflected in the physicochemical sediment record; nevertheless, the ecology of Ross Lake remains impaired and shows minimal signs of returning to a pre-industrial state. Recommendations are made regarding possible future investigations at this site and the need for a framework to help assess causation using paleolimnological and other site data.

Bioavailability, toxicity and biotransformation of selenium in midge (Chironomus dilutus) larvae exposed via water or diet to elemental selenium particles, selenite, or selenized algae.

Elemental selenium (Se) is generally considered to be biologically inert due to its insolubility in water. It is a common form of Se in sediment near uranium mining and milling operations in northern Saskatchewan, Canada. Nanosized particles of many materials exhibit different properties compared with their bulk phases, in some cases posing health and ecological risks. Here we investigated the bioavailability and toxicity of Se nanoparticles (SeNPs) using 10-day waterborne and dietary exposures to larvae of Chironomus dilutus, a common benthic invertebrate. For comparison, larvae were also exposed to waterborne dissolved selenite and to dietary selenomethionine as selenized algae. Larval Se accumulation was evaluated using graphite furnace atomic absorption spectroscopy or inductively coupled plasma mass spectroscopy for total Se and X-ray absorption spectroscopy for Se chemical speciation. Exposure to nanoparticulate Se resulted in Se bioaccumulation, at high concentrations, inhibiting larval growth in both waterborne and dietary exposures; larvae predominantly accumulated selenomethionine-like species regardless of uptake route or form of Se tested. Despite the observed Se accumulation, our findings suggest there is little risk of direct SeNP toxicity to benthic invertebrates in Se-contaminated sediments in northern Saskatchewan. Nevertheless, elemental Se in sediments may be biologically available and may contribute directly or indirectly to the risk of Se toxicity to egg-laying vertebrates (fish and piscivorous birds) in Se-contaminated aquatic systems. It thus may be necessary to include elemental Se as a source of potential Se exposure in ecological risk assessments.

Toxicity of uranium, molybdenum, nickel, and arsenic to Hyalella azteca and Chironomus dilutus in water-only and spiked-sediment toxicity tests.

A series of laboratory spiked-sediment toxicity tests with the amphipod Hyalella azteca and the midge Chironomus dilutus were undertaken to determine acute and chronic toxicity thresholds for uranium (U), molybdenum (Mo), nickel (Ni), and arsenic (As) based on both whole-sediment (total) and pore water exposure concentrations. Water-only toxicity data were also generated from separate experiments to determine the toxicities of these metals/metalloids under our test conditions and to help evaluate the hypothesis that pore water metal concentrations are better correlated with sediment toxicity to benthic organisms than whole-sediment metal concentrations. The relative toxicity of the four elements tested differed depending on which test species was used and whether whole-sediment or pore water metal concentrations were correlated with effects. Based on measured whole-sediment concentrations, Ni and As were the two most acutely toxic elements to H. azteca with 10-d LC50s of 521 and 532 mg/kg d.w., respectively. Measured pore water concentrations indicated that U and Ni were the two most acutely toxic elements, with 10-d LC50s to H. azteca of 2.15 and 2.05 mg/L, respectively. Based on pore water metal concentrations, the no-observed-effect concentrations (NOECs) for growth were (H. azteca and C. dilutus, respectively) 0.67 and 0.21 mg/L for U, <0.37 and 0.60 mg/L for Ni, and 16.43 and <0.42 mg/L for As. Pore-water lowest-observed-effect concentrations (LOECs) for growth were (H. azteca and C. dilutus, respectively) 2.99 and 0.48 mg/L for U, 0.37 and 2.33 mg/L for Ni, and 58.99 and 0.42 mg/L for As. For U and Ni, results from 96-h water-only acute toxicity tests correlated well with pore water metal concentrations in acutely toxic metal-spiked sediment. This was not true for As where metalloid concentrations in overlying water (diffusion from sediment) may have contributed to toxicity. The lowest whole-sediment LOEC reported here for As was 6.6- and 4-fold higher than the Canadian Council of Ministers of the Environment interim sediment quality guideline and the Canadian Nuclear Safety Commission (CNSC) lowest effect level (LEL), respectively. The lowest whole-sediment LOECs reported here for Ni, U and Mo were 4-, 17.5-, and >260-fold higher, respectively, than the CNSC LELs for these metals/metalloids. Data on pore water metal concentrations in toxic sediment would be a useful addition to future Guidelines documents.

An assessment of Hyalella azteca burrowing activity under laboratory sediment toxicity testing conditions.

Burrowing of the freshwater amphipod Hyalella azteca was evaluated under laboratory conditions similar to those recommended for standard sediment toxicity testing in Canada (EPS 1/RM/33; Environment Canada, 1997) and the United States (EPA/600/R-99/064; US EPA, 2000). Sediment type, time of day (light versus dark), size of animal, and the presence or absence of food were varied to assess their effects on burrowing activity. Hyalella azteca were found to burrow rapidly in fine, organic-rich sediments, but were slower to burrow in a sandy sediment. There was no increase in the number of animals occupying the sediment surface of a fine, organic-rich sediment after 4h of darkness compared to the previous 4h of light. Over a 9- to 10-d duration, a higher percentage of animals occupied the surface of the sandy sediment. The addition of food promoted burrowing in sandy sediment, as did using smaller animals. Overall, longer-duration tests involving older animals and coarse sediments may require formal observation to confirm burrowing and ensure adequate sediment exposure. The addition of food during a test may promote the burrowing of larger animals in coarse sediments, but may not be necessary in field-collected sediments that are not excessively sandy.

Nickel speciation in the presence of different sources and fractions of dissolved organic matter.

This study evaluated nickel (Ni) speciation in the presence of different fractions (humic acid (HA), fulvic acid (FA)) and sources (natural sediment, Suwannee River, peat moss) of dissolved organic matter (DOM) at Ni concentrations toxicologically relevant to the freshwater amphipod, Hyalella azteca. The free Ni ion, Ni(2+), was measured in reconstituted water (with or without DOM) using a miniaturized ion-exchange technique (IET). Ni speciation from these experiments was compared to calculated results obtained from equilibrium modelling (WHAM, Model VI). While it is known that Ni will complex with DOM, it was found that under acutely toxic Ni exposure concentrations ([Ni(Total)]=5mg/L, or 85.1 microM) representative surface-water DOC concentrations ( approximately 10mg/L) played little or no role in Ni speciation. Conversely, at sublethal Ni exposure concentrations ([Ni(Total)]=0.2 and 0.5 microg/L, or 3.4 and 8.51 microM, respectively) DOM significantly affected Ni speciation with [Ni(2+)] decreasing with increasing concentration of DOM. It was found that for similar concentrations of DOC (same fraction, different sources), the measured Ni(2+) concentrations were reduced (relative to the control), but similar to one another. Conversely, at similar DOC concentrations, the HA fraction reduced Ni(2+) levels to a greater extent than the associated FA fraction. Overall, this study provides proof of principle that Suwannee River and peat humic substances are suitable analogues for natural sediment pore-water DOM when evaluating Ni bioavailability in freshwater.

Nickel partitioning in formulated and natural freshwater sediments.

A natural, field-collected sediment high in organic carbon (OC) and low in acid-volatile sulfide (AVS) was used to evaluate nickel (Ni) complexation to organic matter (OM) over a range of pH under anoxic conditions. It was found that OM strongly influenced Ni partitioning and that Ni complexation to OM was significantly influenced by pH, with complexation increasing with increasing pH (from pH 6 to 8). Using an equilibrium partitioning approach incorporating both [SEM(Ni)]-[AVS] and OC content, lethal and non-lethal toxicity test endpoints were calculated (predicted) and compared to observed toxicity test results using the amphipod, Hyalella azteca, exposed to four Ni-spiked natural sediments varying in OC and AVS content. Generally, lethal and non-lethal toxicity test endpoints were reasonably predictable in low AVS sediments. Due to the apparent lack of equilibrium between dissolved pore-water Ni and the pure Ni sulfide (likely the result of additional dissolved metal binding ligands), and the possible competition of liberated Fe2+ with Ni2+ for binding sites on organic matter, toxicity predictions (based on sediment OC and AVS content) overestimated the combined protective effects of AVS and OC in the sediments containing mid to high (27.87-44.05 micromol/g d.w.) AVS concentrations. Overall, it was found that equilibrium partitioning-based sediment quality guidelines can be improved through the incorporation of nickel complexation to sedimentary OM (in addition to AVS), although further research is required to fully describe nickel-OM interaction.

Influence of dissolved organic matter on nickel bioavailability and toxicity to Hyalella azteca in water-only exposures.

Dissolved organic matter (DOM) is known to reduce the bioavailability of metals in aquatic systems. This study evaluated the effects of DOM from various sources (e.g., Little Bear Lake sediment, Suwannee River, peat moss) and various DOM fractions (humic acids, HA; fulvic acids, FA) on the bioavailability of nickel (Ni) to Hyalella azteca, a common freshwater benthic invertebrate. In particular, this study was conducted to evaluate the effect of surficial sediment DOM on Ni bioavailability. Short-term (48 h) acute toxicity tests with H. azteca conducted in synthetic water demonstrated that the aqueous Ni concentrations required for lethality were greater than what could be significantly complexed by environmentally relevant concentrations of dissolved organic carbon (DOC: 0.6-30.4 mg/L). At Ni concentrations sublethal to H. azteca (500 microg/L), the bioavailability of Ni was significantly reduced in the presence of representative surface water DOC concentrations regardless of DOC source or fraction. DOC fraction (i.e., FA and HA) differentially affected Ni speciation, but had little or no effect on Ni accumulation by H. azteca. Tissue Ni was found to be strongly dependent upon the Ni(2+) concentration in the exposure solutions and the Ni:DOC ratio. Overall, the concentration of DOC played a greater role than either DOC source or fraction in determining Ni speciation and hence bioavailability and toxicity to H. azteca.