Is it justifiable to pool Chironomus species in trace element contamination studies?

Larvae of the insect Chironomus (Chironomidae: Diptera) have great potential for estimating the bioavailability of sedimentary trace elements because they are common in fine sediments and tolerate high concentrations of these contaminants. Their use as biomonitors is limited by the fact that they are difficult to identify as to species, and the species can differ in their trace element concentrations. To determine whether pooling species would compromise their use as trace element biomonitors, we identified species of Chironomus larvae collected from 22 lakes and measured their concentrations of 9 trace elements. We found that the concentrations of arsenic, barium, cobalt, copper, manganese, and nickel did not generally differ between sympatric Chironomus species, which indicates that they could be pooled for analyses of these trace elements. In contrast, we found that cadmium (Cd), selenium (Se), and zinc (Zn) concentrations differed between species living at the same site according to their feeding behavior, that is, Chironomus species feeding on oxic sediments tended to have higher Cd and Zn concentrations, whereas those feeding on deeper anoxic sediments had higher Se concentrations. Because Se and Zn concentrations in sympatric Chironomus species usually differed by only a factor of 2, separating species based on their feeding behavior might not be as crucial as for Cd if larval Se and Zn concentrations vary greatly from site to site. Environ Toxicol Chem 2019;38:145-159. © 2018 SETAC.

Organic selenium, selenate, and selenite accumulation by lake plankton and the alga Chlamydomonas reinhardtii at different pH and sulfate concentrations.

Selenium (Se) concentrations measured in lake planktonic food chains (microplankton <64 µm, copepods, and Chaoborus larvae) were strongly correlated with the concentrations of dissolved organic Se. These correlations were strengthened slightly by adding the concentrations of dissolved selenate to those of organic Se. To better understand the role of Se species and the influence of water chemistry on Se uptake, we exposed the green alga Chlamydomonas reinhardtii to selenite, selenate, or selenomethionine at various H+ ion and sulfate concentrations under controlled laboratory conditions. At low sulfate concentrations, inorganic Se species (selenate >> selenite) were more readily accumulated by this alga than was selenomethionine. However, at higher sulfate concentrations the uptake of selenite was higher than that of selenate, whereas the uptake of selenomethionine remained unchanged. Although the pH of the exposure water did not influence the uptake of selenate by this alga, the accumulation of selenomethionine and selenite increased with pH because of their relative pH-related speciation. The Se concentrations that we measured in C. reinhardtii exposed to selenomethionine were 30 times lower than those that we measured in field-collected microplankton exposed in the same laboratory conditions. This difference is explained by the taxa present in the microplankton samples. Using the present laboratory measurements of Se uptake in microplankton and of natural Se concentrations in lake water allowed us to model Se concentrations in a lake pelagic food chain. Environ Toxicol Chem 2018;37:2112-2122. © 2018 SETAC.

Evaluating Benthic Recovery Decades after a Major Oil Spill in the Laurentian Great Lakes.

The long-term effects of oil spills on freshwater organisms have been little studied. In 1950, a large oil spill (10 million L) covered the harbor area of Parry Sound, Ontario, the deepest port in the Laurentian Great Lakes. Ecological impacts were not studied at the time, but 25 years later three-quarters of the Chironomus cucini larvae (Insecta, Diptera, Chironomidae) living in the harbor area were reported to be deformed. We returned six decades after the spill and found that the frequency of deformities had returned to background levels and that the community of burrowing invertebrates has largely recovered. By dating sediment cores and measuring the depth distribution of oils, we conclude that, although the oil persists six decades after the spill, sufficient uncontaminated sediment has covered the oil thereby putting it out of reach of most burrowing animals. Provided that the sediment remains undisturbed, the buried oil is unlikely to exert further negative effects on the biota in spite of the fact that it will likely persist for centuries.

Hepatic oxidative stress and metal subcellular partitioning are affected by selenium exposure in wild yellow perch (Perca flavescens).

Yellow perch (Perca flavescens) collected from 11 lakes in the Canadian mining regions of Sudbury (Ontario) and Rouyn-Noranda (Quebec) display wide ranges in the concentrations of cadmium (Cd), nickel (Ni), selenium (Se), and thallium (Tl) in their livers. To determine if these trace elements, as well as copper (Cu) and zinc (Zn), are causing oxidative stress in these fish, we measured three biochemical indicators (glutathione (GSH), glutathione disulfide (GSSG) and thiobarbituric acid-reactive substances (TBARS)) in their livers. We observed that 44% of the yellow perch that we collected were at risk of cellular oxidative stress and lipid peroxidation. Considering all fish from all lakes, higher liver Se concentrations were coincident with both lower proportions of GSSG compared to GSH and lower concentrations of TBARS, suggesting that the essential trace-element Se acts as an antioxidant. Furthermore, fish suffering oxidative stress had higher proportions of Cd, Cu and Zn in potentially sensitive subcellular fractions (organelles and heat-denatured proteins) than did fish not suffering from stress. This result suggests that reactive oxygen species may oxidize metal-binding proteins and thereby reduce the capacity of fish to safely bind trace metals. High Cd concentrations in metal-sensitive subcellular fractions likely further exacerbate the negative effects of lower Se exposure.

Metal (Ag, Cd, Cu, Ni, Tl, and Zn) Binding to Cytosolic Biomolecules in Field-Collected Larvae of the Insect Chaoborus.

We characterized the biomolecules involved in handling cytosolic metals in larvae of the phantom midge (Chaoborus) collected from five mining-impacted lakes by determining the distribution of Ag, Cd, Cu, Ni, Tl, and Zn among pools of various molecular weights (HMW: high molecular weight, >670-40 kDa; MMW: medium molecular weight, 40-<1.3 kDa; LMW: low molecular weight, <1.3 kDa). Appreciable concentrations of nonessential metals were found in the potentially metal-sensitive HMW (Ag and Ni) and LMW (Tl) pools, whereas the MMW pool, which includes metallothioneins (MTs) and metallothionein-like proteins and peptides (MTLPs), appears to be involved in Ag and Cd detoxification. Higher-resolution fractionation of the heat-stable protein (HSP) fraction revealed further differences in the partitioning of nonessential metals (i.e., Ag = Cd ≠ Ni ≠ Tl). These results provide unprecedented details about the metal-handling strategies employed by a metal-tolerant, freshwater animal in a field situation.

Trace metals partitioning among different sedimentary mineral phases and the deposit-feeding polychaete Armandia brevis.

Trace metals (Cd, Co, Cu, Fe, Mn, Ni, Pb, Zn) were determined in two operationally defined fractions (HCl and pyrite) in sediments from Ensenada and El Sauzal harbors (Mexico). The HCl fraction had significantly higher metal concentrations relative to the pyrite fraction in both harbors, underlining the weak tendency of most trace metals to associate with pyrite. Exceptionally, Cu was highly pyritized, with degrees of trace metal pyritization (DTMP) >80% in both harbors. Dissolved Fe flux measurements combined with solid phase Fe sulfide data indicated that 98 mt of Fe are precipitated as iron sulfides every year in Ensenada Harbor. These Fe sulfides (and associated trace metals) will remain preserved in the sediments, unless they are perturbed by dredging or sediment resuspension. Calculations indicate that dredging activities could export to the open ocean 0.20±0.13 to (0.30±0.56)×10(3) mt of Cd and Cu, respectively, creating a potential threat to marine benthic organisms. Degrees of pyritization (DOP) values in Ensenada and El Sauzal harbors were relatively low (<25%) while degrees of sulfidization (DOS) were high (~50%) because of the contribution of acid volatile sulfide. DOP values correlated with DTMP values (p≤0.001), indicating that metals are gradually incorporated into pyrite as this mineral is formed. Significant correlations were also found between DTMP values and -log(Ksp(MeS)/Ksp(pyr)) for both harbors, indicating that incorporation of trace metals into the pyrite phase is a function of the solubility product of the corresponding metal sulfide. The order in which elements were pyritized in both harbors was Zn≈Mn

Using Sulfur Stable Isotopes to Understand Feeding Behavior and Selenium Concentrations in Yellow Perch (Perca flavescens).

We measured selenium (Se) concentrations in yellow perch (Perca flavescens) muscle and their prey collected from four Se-contaminated lakes located near metal smelters in the eastern Canadian cities of Sudbury and Rouyn-Noranda. Yellow perch Se concentrations were related to their weight in two of the four lakes. Measurements of sulfur stable isotopes (δ(34)S) in yellow perch muscle and stomach contents showed that larger fish tended to feed less on zooplankton and more on benthic invertebrates than did smaller fish. Because Se concentrations are lower and δ(34)S signatures are higher in zooplankton than in sediment-feeding invertebrates, there was an inverse relationship between animal Se concentrations and δ(34)S signatures in all of our study lakes. δ(34)S signatures were highly effective in characterizing these food web relationships. Selenium concentrations in yellow perch were 1.6 times those of its prey, which indicates that Se is biomagnified by this fish in our study lakes. Estimated Se concentrations in yellow perch gonads suggest that in two of our study lakes one-third of fish are at risk of reproductive toxicity.

Subcellular partitioning of non-essential trace metals (Ag, As, Cd, Ni, Pb, and Tl) in livers of American (Anguilla rostrata) and European (Anguilla anguilla) yellow eels.

We determined the intracellular compartmentalization of the trace metals Ag, As, Cd, Ni, Pb, and Tl in the livers of yellow eels collected from the Saint Lawrence River system in Canada (Anguilla rostrata) and in the area of the Gironde estuary in France (Anguilla anguilla). Differential centrifugation, NaOH digestion and thermal shock were used to separate eel livers into putative "sensitive" fractions (heat-denatured proteins, mitochondria and microsomes+lysosomes) and detoxified metal fractions (heat-stable peptides/proteins and granules). The cytosolic heat-stable fraction (HSP) was consistently involved in the detoxification of all trace metals. In addition, granule-like structures played a complementary role in the detoxification of Ni, Pb, and Tl in both eel species. However, these detoxification mechanisms were not completely effective because increasing trace metal concentrations in whole livers were accompanied by significant increases in the concentrations of most trace metals in "sensitive" subcellular fractions, that is, mitochondria, heat-denatured cytosolic proteins and microsomes+lysosomes. Among these "sensitive" fractions, mitochondria were the major binding sites for As, Cd, Pb, and Tl. This accumulation of non-essential metals in "sensitive" fractions likely represents a health risk for eels inhabiting the Saint Lawrence and Gironde environments.

Uptake and subcellular distributions of cadmium and selenium in transplanted aquatic insect larvae.

We transplanted larvae of the phantom midge Chaoborus punctipennis from a lake having lower concentrations of Cd and Se (Lake Dasserat) to a more contaminated lake (Lake Dufault) located near a metal smelter in Rouyn-Noranda, Quebec. Transplanted individuals were held in mesh mesocosms for up to 16 days where they were fed with indigenous contaminated zooplankton. Larval Cd and Se burdens increased over time, and came to equal those measured in indigenous C. punctipennis from contaminated Lake Dufault. Larval Se burdens increased steadily, whereas those of Cd showed an initial lag phase that we explain by a change in the efficiency with which this insect assimilated Cd from its prey. We measured Cd and Se in subcellular fractions and found that larvae sequestered the majority (60%) of the incoming Cd in a detoxified fraction containing metal-binding proteins, whereas a minority of this nonessential metal was in sensitive fractions (20%). In contrast, a much higher proportion of the essential element Se (40%) was apportioned to metabolically active sensitive fractions. Larvae took up equimolar quantities of these elements over the course of the experiment. Likewise, Cd and Se concentrations in wild larvae were equimolar, which suggests that they are exposed to equimolar bioavailable concentrations of these elements in our study lakes.

Relating selenium concentrations in a planktivore to selenium speciation in lakewater.

We measured selenium (Se) speciation in the waters of 16 lakes located near two major metal smelters and compared it to Se concentrations in a potential biomonitor, the planktivorous insect Chaoborus. We used this sentinel because planktonic algae and crustaceans, which are lower in the trophic chain leading to Chaoborus, are more difficult to separate and identify to species, whereas many fish species are not obligate planktivores. Percentages of selenate and organo-Se were generally higher in acidic lakes, whereas those of selenite were usually greater in alkaline waters. Chaoborus Se concentrations varied widely among lakes and, with the exception of a single high-sulfate lake, were significantly and highly correlated with those of dissolved organo-Se plus selenate (Se(VI)). We suggest that Chaoborus larvae would be highly effective for monitoring the Se-exposure of planktonic food webs in lakes.

Using various lines of evidence to identify Chironomus species (Diptera: Chironomidae) in eastern Canadian lakes.

Chironomus Meigen (Diptera, Chironomidae) larvae are usually the largest sediment-burrowing chironomids, and as such often constitute a major part of the freshwater infaunal biomass. However, use of this genus in ecological, environmental and paleoecological studies is hampered by the fact that Chironomus larvae are difficult to identify to species because the larvae of many species are morphologically similar. We used a combination of morphological, cytological and genetic techniques to distinguish Chironomus larvae collected from 31 water bodies located in eastern Canada, producing 17 distinguishable groupings. These groups of larvae were ultimately identified as belonging to 14 known species (C. anthracinus, C. bifurcatus, C. cucini, C. decorus-group sp. 2, C. dilutus, C. entis, C. frommeri, C. harpi, C. maturus, C. nr. atroviridis (sp. 2i), C. ochreatus, C. plumosus, C. staegeri and C. 'tigris') and three other species that remain unidentified (C. sp. NAI-III). No single approach served to delimit and identify larvae of all 17 Chironomus species that we collected. Although we expected that morphological criteria alone would be insufficient, our results suggest that DNA barcoding, using either the mitochondrial cox1 or the nuclear gb2ß gene, was also inadequate for separating some Chironomus species. Thus we suggest that multiple approaches will often be needed to correctly identify Chironomus larvae to species.

Subcellular metal partitioning in larvae of the insect Chaoborus collected along an environmental metal exposure gradient (Cd, Cu, Ni and Zn).

Larvae of the phantom midge Chaoborus are common and widespread in lakes contaminated by metals derived from mining and smelting activities. To explore how this insect is able to cope with potentially toxic metals, we determined total metal concentrations and subcellular metal partitioning in final-instar Chaoborus punctipennis larvae collected from 12 lakes situated along gradients in aqueous Cd, Cu, Ni and Zn concentrations. Concentrations of the non-essential metals Cd and Ni were more responsive to aqueous metal gradients than were larval concentrations of the essential metals Cu and Zn; these latter metals were better regulated and exhibited only 2-3-fold increases between the least and the most contaminated lakes. Metal partitioning was determined by homogenization of larvae followed by differential centrifugation, NaOH digestion and heat denaturation steps so as to separate the metals into operationally defined metal-sensitive fractions (heat-denaturable proteins (HDP), mitochondria, and lysosomes/microsomes) and metal-detoxified fractions (heat stable proteins (HSP) and NaOH-resistant or granule-like fractions). Of these five fractions, the HSP fraction was the dominant metal-binding compartment for Cd, Ni and Cu. The proportions and concentrations of these three metals in this fraction increased along the metal bioaccumulation gradient, which suggests that metallothionein-like proteins play an important role in metal tolerance of Chaoborus living in metal-contaminated environments. Likewise, a substantial proportion of larval Zn was in the HSP fraction, but its contribution did not increase progressively along the metal gradient. Despite the increases in Cd, Ni and Cu in the HSP fraction along the metal bioaccumulation gradient, some accumulation of non-essential metals (Cd and Ni) was observed in putative metal-sensitive fractions (e.g., HDP, mitochondria), suggesting that metal detoxification was incomplete. In the case of Cd, there appears to be a threshold body concentration of about 50 nmol Cd g(-1) dry weight, above which Cd detoxification becomes more effective and below which Chaoborus does not "turn on" its detoxification machinery to the fullest extent. We speculate that acclimation or adaptation of Chaoborus to these highly metal-contaminated environments may have resulted in a capacity to tolerate some metal spillover without comprising essential biological functions such as growth and reproduction.

Subcellular partitioning of cadmium in the freshwater bivalve, Pyganodon grandis, after separate short-term exposures to waterborne or diet-borne metal.

The dynamics of cadmium uptake and subcellular partitioning were studied in laboratory experiments conducted on Pyganodon grandis, a freshwater unionid bivalve that shows promise as a biomonitor for metal pollution. Bivalves were collected from an uncontaminated lake, allowed to acclimate to laboratory conditions (≥25 days), and then either exposed to a low, environmentally relevant, concentration of dissolved Cd (5nM; 6, 12 and 24h), or fed Cd-contaminated algae (∼70nmol Cdg⁻¹ dry weight; 4×4h). In this latter case, the bivalves were allowed to depurate for up to 8 days after the end of the feeding phase. As anticipated, the gills were the main target organ during the aqueous Cd exposure whereas the intestine was the initial site of Cd accumulation during the dietary exposure; during the subsequent depuration period, the dietary Cd accumulated in both the digestive gland and in the gills. For the gills, the distribution of Cd among the subcellular fractions (i.e., granules>heat-denatured proteins (HDP)∼heat-stable proteins (HSP)>mitochondria∼lysosomes+microsomes) was insensitive to the exposure route; both waterborne and diet-borne Cd ended up largely bound to the granule fraction. The subcellular distribution of Cd in the digestive gland differed markedly from that in the gills (HDP>HSP∼granules∼mitochondria>lysosomes+microsomes), but as in the case of the gills, this distribution was relatively insensitive to the exposure route. For both the gills and the digestive gland, the subcellular distributions of Cd differed from those observed in native bivalves that are chronically exposed to Cd in the field - in the short-term experimental exposures of P. grandis, metal detoxification was less effective than in chronically exposed native bivalves.

Nickel dynamics in the lakewater metal biomonitor Chaoborus.

Nickel (Ni) is a widespread contaminant present at toxic concentrations in aquatic systems in the vicinity of some mining and smelting operations. However, its accumulation by aquatic animals has been little studied and there are few biomonitors for this metal. Recently, larvae of the aquatic insect Chaoborus were shown to be effective as biomonitors for Ni concentrations in lakewater. Since animals are more effective as biomonitors when we understand how they take up their contaminants (from water or from food) and the rate at which they exchange contaminants with their surroundings, we set out to measure these parameters for Chaoborus. To achieve these goals, we exposed the components of a laboratory food chain (green alga, cladoceran, Chaoborus) to realistic Ni concentrations. We found that the majority ( approximately 65%) of the Ni taken up by Chaoborus flavicans comes from lakewater, with the remainder coming from its planktonic prey (Daphnia magna). This result is consistent with the low mean efficiency (14%) with which C. flavicans assimilated Ni from its prey. To explain the low efficiency of Ni uptake from food we measured the subcellular distribution of Ni in prey, which predicted that the majority of the Ni in prey ( approximately 55%) was available for assimilation by the predator. This potential Ni uptake efficiency was only reached in animals that ingested few prey, likely because their gut passage time was longer than those ingesting many prey. We also measured Ni uptake and loss by C. flavicans exposed to Ni in water then used these data to parameterize a mechanistic bioaccumulation model that allowed us to describe Ni exchange between this insect and water. Lastly, we used these model constants, along with field measurements of Ni in 10 Canadian lakes, to predict Ni concentrations in field populations of Chaoborus. Model predictions overestimated Ni concentrations in field populations by a factor of 4. We suggest that uncertainties in the rate constant for Ni uptake from water and a lack of measured Ni concentrations in the prey eaten by Chaoborus larvae in the field could explain this difference.

Influence of prey type on nickel and thallium assimilation, subcellular distribution and effects in juvenile fathead minnows (Pimephales promelas).

Because fish take up metals from prey, it is important to measure factors controlling metal transfer between these trophic levels so as to explain metal bioaccumulation and effects in fish. To achieve this, we exposed two types of invertebrates, an oligochaete (Tubifex tubifex) and a crustacean (Daphnia magna), to environmentally relevant concentrations of two important contaminants, nickel (Ni) and thallium (Tl), and fed these prey to juvenile fathead minnows (Pimephales promelas). We then measured the assimilation efficiency (AE), subcellular distribution and effects of these metals in fish. Fish assimilated dietary Tl more efficiently from D. magna than from T. tubifex, and more efficiently than Ni, regardless of prey type. However, the proportion of metal bound to prey subcellular fractions that are likely to be trophically available (TAM) had no significant influence on the efficiency with which fish assimilated Ni or Tl. In fish, the majority of their Ni and Tl was bound to subcellular fractions that are purportedly detoxified, and prey type had a significant influence on the proportion of detoxified Ni and Tl in fish. We measured higher activities of cytochrome C oxidase and glutathione S-transferase in fish fed D. magna compared to fish fed T. tubifex, regardless of the presence or absence of Ni or Tl in prey. However, we measured decreased activities of glutathione S-transferase and nucleoside diphosphate kinase in fish fed Tl-contaminated D. magna compared to fish from the three other treatment levels.

Assessment of nickel contamination in lakes using the phantom midge Chaoborus as a biomonitor.

Nickel (Ni) can be present in concentrations of concern in waters near mining and industrial sites. We tested species of the phantom midge Chaoborus as a biomonitor for this trace metal by collecting water and Chaoborus larvae from 15 lakes located along a Ni gradient mainly in the vicinity of smelters located in Sudbury, ON, Canada. We measured pH, trace metals, major ions, as well as inorganic and organic carbon concentrations in lakewater for use in calculating ambient metal speciation using the Windermere Humic Aqueous Model (WHAM). Nickel concentrations in Chaoborus species varied widely among our study lakes and could be related to concentrations of the free Ni2+ ion in lakewater if competitive interactions with hydrogen ions (H+) were taken into account We verified this inhibitory effect in the laboratory by exposing Chaoborus punctipennis to constantfree Ni2+ ion concentrations at various H+ ion concentrations. As expected, larvae exposed to high concentrations of H+ ions accumulated less Ni. Overall, our results suggest that Chaoborus larvae would be an excellent biomonitor for Ni in lakewater and as such would be a useful component of risk assessment strategies designed to evaluate Ni exposure to aquatic organisms in lakes.

Subcellular distribution of cadmium in two aquatic invertebrates: change over time and relationship to Cd assimilation and loss by a predatory insect.

We set out to determine if the efficiency of cadmium (Cd) assimilation and loss by a freshwater predator (the alderfly Sialis velata) differs when it is exposed, for various lengths of time, to Cd in either an insect (Chironomus riparius) or a worm (Tubifex tubifex). Prey were exposed to Cd in sediments for up to 28 days and then fractionated to measure Cd distributions in their cells. Cadmium subcellular distributions varied little over time for a given preytype but differed substantially between the two prey species; for example, the cytosol comprised a larger proportion of Cd in the insect (76%) than in the worm (34%). The predator assimilated proportionally more Cd from the insect (72%) than from the worm (46%) and these assimilation efficiencies were similar to the proportion of prey Cd that would theoretically be available to it (cytosolic Cd + organelle Cd). However, measurements of Cd in the predator's feces confirmed that to obtain an exact 1:1 relationship between predator assimilation efficiency and prey subcellular distribution we had to assume that approximately 50% of the Cd associated with the organelle fraction of T. tubifex was unavailable for digestion by the predator. Losses of Cd from the predator also varied depending on the type of prey that were the source of its Cd.

Selenium assimilation and loss by an insect predator and its relationship to Se subcellular partitioning in two prey types.

Subcellular selenium (Se) distributions in the oligochaete Tubifex tubifex and in the insect Chironomus riparius did not vary with Se exposure duration, which was consistent with the observations that the duration of prey Se exposure had little influence on either Se assimilation or loss by a predatory insect (the alderfly Sialis velata). However, these two prey types differed in how Se was distributed in their cells. Overall, the predator assimilated a mean of 66% of the Se present in its prey, which was similar to the mean percentage of Se in prey cells (62%) that was theoretically available for uptake (that is, Se in the protein and organelle fractions). Likewise, data for cadmium, nickel and thallium suggest that predictions of trace element transfer between prey and predator are facilitated by considering the subcellular partitioning of these contaminants in prey cells.

The internal distribution of nickel and thallium in two freshwater invertebrates and its relevance to trophic transfer.

Although nickel and thallium are present at potentially harmful concentrations in some lakes, there is little information on their bioaccumulation and transfer up aquatic food webs. To measure the propensity of animals for accumulating and transferring these contaminants along food chains, we exposed two common types of invertebrates, an insect (Chironomus riparius) and a worm (Tubifex tubifex), to these metals spiked into sediment. We then measured the subcellular distribution of Ni and Tl in these invertebrates to estimate the likelihood that these metals will have toxic effects on these prey or be transferred to higher trophic levels. In both species, at least half of their Ni and TI was present in fractions that are purportedly detoxified (granules and metal-binding proteins). Furthermore, based on information in the literature concerning prey subcellular fractions that are likely to be trophically available (TAM), we estimate that much of the Ni and TI in these animals (43-84%) is available for transfer to a predator. To test this prediction, we fed these invertebrates to the alderfly Sialis velata, and measured the efficiency with which this predator assimilated Ni and Tl from each prey type. The majority of both trace metals (58-83%) was assimilated by the predator, which suggests that these contaminants would be easily transferred along aquatic food chains and that models describing Ni and Tl accumulation by aquatic animals should consider food as a source of these metals. The proportion of metal that could potentially be taken up by a consumer (% TAM) and the actual percentage assimilated by S. velata fell on or reasonably close to a 1:1 line for the 4 prey-metal combinations.