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.

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.

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.

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.

Sub-cellular partitioning of cadmium, copper, nickel and zinc in indigenous yellow perch (Perca flavescens) sampled along a polymetallic gradient.

Sub-cellular metal distributions were studied in indigenous yellow perch (Perca flavescens) collected from eight lakes located along a cadmium (Cd), copper (Cu), nickel (Ni) and zinc (Zn) concentration gradient. Ambient dissolved metal concentrations were measured to evaluate exposure and total hepatic metal concentrations were determined as a measure of metal bioaccumulation. Metal partitioning among potentially metal-sensitive fractions (cytosolic enzymes, organelles) and detoxified metal fractions (metallothionein) was determined after differential centrifugation of fish liver homogenates. Major proportions of hepatic Cd and Cu were found in the heat-stable cytosolic peptides and proteins fraction (HSP), a fraction including metallothioneins, whereas the potentially metal-sensitive heat-denaturable proteins fraction (HDP) was the largest contributor to the total Ni and Zn burdens. The concentrations of Cd, Cu and Ni (but not Zn) in each sub-cellular fraction increased along the metal contamination gradient, but the relative contributions of each fraction to the total burden of each of these metals remained generally constant. For these chronically exposed fish there was no threshold exposure concentration below which binding of Cd or Ni to the heat-denaturable protein fraction did not occur. The presence of Cd and Ni in the HDP fraction, even for low chronic exposure concentrations, suggests that metal detoxification was imperfect, i.e. that P. flavescens was subject to some metal-related stress even under these conditions.

Modeling cadmium exchange by an aquatic moss (Fontinalis dalecarlica).

Although aquatic mosses are widely used as metal biomonitors in rivers, there are few effective models to describe metal uptake and loss by these plants. To fill this gap, we exposed the aquatic moss Fontinalis dalecarlica for 28 d to three Cd concentrations (approximately 5-50 nM) in a flow-through laboratory system. Cadmium accumulation by F. dalecarlica was rapid during the first few days of exposure and slowed thereafter but did not reach a steady state within our 1-month long experiment. This lack of a plateau in moss concentrations suggests that, for biomonitoring purposes, the duration of moss exposure should be considered either through a model of the type that we tested or by standardizing the exposure time of mosses transplanted in the field. During the subsequent 22-d elimination phase of our experiment, Cd concentrations in mosses did not return to their initial levels. This result suggests that a two-compartment model is likely to be more effective at describing Cd losses than would a one-compartment alternative. Indeed, predictions of a two-compartment model closely fitted our experimental data, which augurs well for the wider use of this model for other moss species and metals.

Cadmium-handling strategies in two chronically exposed indigenous freshwater organisms--the yellow perch (Perca flavescens) and the floater mollusc (Pyganodon grandis).

Laboratory experiments on a variety of aquatic organisms suggest that metallothionein-like proteins (MT) play an important role in the regulation of essential metals, and in the sequestration and detoxification of non-essential metals (e.g., Cd). However, the importance of metallothionein production relative to alternative strategies of metal detoxification, and its effectiveness in metal detoxification, remain largely unexplored in field situations. In the present study we explored metal-handling strategies in an adult benthic bivalve (Pyganodon grandis) and in juvenile yellow perch (Perca flavescens), exposed to Cd in their natural habitat. The two biomonitor species were collected from lakes located along a Cd concentration gradient. Ambient dissolved Cd concentrations were determined by in situ dialysis as a measure of metal exposure. Sub-cellular Cd partitioning was determined in target tissues (bivalve gills and digestive gland; perch liver) by differential centrifugation, and metallothionein was measured independently by a mercury-saturation assay in the bivalve tissues. Malondialdehyde concentrations were measured as a potential indicator of oxidative stress. Ambient dissolved Cd concentrations ranged from 0.06 to 0.57 nM in the nine lakes from which bivalves were collected, and from < 0.3 to 6.7 nM in the eight lakes from which yellow perch were sampled. Bioaccumulated Cd also varied from lake to lake, more so for the bivalve than for the yellow perch; the [Cd]max/[Cd]min ratios for the various tissues decreased in the order: bivalve gill Cd 28 > bivalve digestive gland Cd 18 > perch hepatic Cd 14. In the two lakes that were common to both the bivalve and perch studies, i.e. lakes Opasatica and Vaudray, accumulated Cd concentrations were consistently higher in the bivalve than in the perch. Cadmium-handling strategies were similar in the bivalve digestive gland and perch liver, in that Cd was mainly associated with the heat-stable protein (HSP) fraction. Furthermore, in these organs the contributions from the "mitochondria" and "lysosomes + microsomes" fractions were consistently higher than in the gill tissue. In the bivalve gill, the HSP fraction could only account for a small proportion (10+/-3%) of the total Cd burden, and the metal was instead largely sequestered in calcium concretions (58+/-13%). Along the Cd-exposure gradient, Cd detoxification appeared to be reasonably effective in the bivalve gill and digestive gland, as judged from the protection of the heat-denaturable protein (HDP) fraction. However, in both organs Cd concentrations did increase in potentially metal-sensitive organelles (mitochondria), and malondialdehyde concentrations increased along the exposure gradient in the gills (but not in the digestive gland). Cadmium detoxification seemed less effective in juvenile yellow perch. As total hepatic Cd increased, Cd concentrations increased in all sub-cellular fractions, including the HDP fraction that was well protected in the bivalve. The relative proportions of Cd in the various fractions did not vary appreciably along the exposure gradient and there was no evidence of a threshold exposure concentration below which sensitive metal pools were protected.

Effect of sedimentary cadmium on the behavior of a burrowing mayfly (Ephemeroptera, Hexagenia limbata).

Behavioral responses of aquatic invertebrates can serve as precocious indicators of adverse effects caused by metals. We conducted experiments to determine if the behavior of nymphs of the burrowing mayfly Hexagenia limbara (Ephemeroptera) was modified by the presence of cadmium (Cd) in sediments. We first exposed H. limbata to concentrations of sedimentary Cd found in lakes subject to smelter emissions (20 microg/g dry wt). We then compared the burrowing activity of control and Cd-contaminated nymphs by means of radiographs. We also studied the time allotted by nymphs to various activities (irrigation, walking, burrowing/feeding, cleaning, and turning) as determined from films taken using an infrared video camera. The results obtained suggest no significant difference in the burrowing activity of control versus Cd-contaminated individuals. Furthermore, the time spent in various behavioral activities was similar between control and Cd-contaminated nymphs. Given these results, we conclude that Cd has no effect on the behavior of H. limbata for the range of Cd concentrations that we measured in nymphs.

Increases in food web cadmium following reductions in atmospheric inputs to some lakes.

We measured the toxic metal cadmium (Cd) over a 13-year interval in lakes located near two metal smelters to determine whether reductions in atmospheric Cd emissions have led to reductions in the Cd present in aquatic food webs. Although Cd in the lake water consistently declined over time, Cd in animals increased in some lakes. This apparent contradiction was explained when we considered the simultaneous reductions that have occurred in lake water acidity; under these conditions, animal Cd can increase if there are insufficient hydrogen ions to out-compete Cd ions at biological uptake sites. We conclude that the risk to organisms from metals has increased in some lakes recovering from smelter emissions.