Temporal and spatial variation in Hg accumulation in zebra mussels (Dreissena polymorpha): possible influences of DOC and diet.

Zebra mussels (Dreissena polymorpha) are filter feeders located near the base of the foodweb and these animals are able to utilize a variety of carbon sources that may also vary seasonally. We conducted both a spatial and a temporal study in order to test the hypotheses: (1) dissolved organic carbon (DOC) concentrations influence Hg accumulation in zebra mussels sampled from a series of lakes and (2) seasonal variations in diet influence Hg accumulation. In the spatial study, we found a significant negative relationship between Hg concentrations and DOC concentrations, suggesting an influence of DOC on Hg bioaccumulation. In the temporal study, we used stable isotope ratios of nitrogen (δ(15)N) and carbon (δ(13)C) as ecological tools to provide a temporally integrated description of the feeding ecology of zebra mussels. Both δ(15)N and δ(13)C varied seasonally in a similar manner: more depleted values occurred in the summer and more enriched values occurred in the fall. Mercury concentrations also varied significantly over the year, with highest concentrations occurring in the summer, followed by a progressive decrease in concentrations into the fall. The C/N ratio of zebra mussels also varied significantly over the year with the lowest values occurring mid-summer and then values increased in the fall and winter, suggesting that there was significant variation in lipid stores. These results indicate that in addition to any effect of seasonal dietary changes, seasonal variation in energy stores also appeared to be related to Hg levels in the zebra mussels. Collectively results from this study suggest that DOC concentrations, seasonal variation in diet and seasonal depletion of energy stores are all important variables to consider when understanding Hg accumulation in zebra mussels.

Uranium bioaccumulation in a freshwater ecosystem: impact of feeding ecology.

The objectives of our study were: (1) to determine if there was significant uranium (U) bioaccumulation in a lake that had been historically affected by a U mine and (2) to use a combined approach of gut content examination and stable nitrogen and carbon isotope analysis to determine if U bioaccumulation in fish was linked to foodweb ecology. We collected three species of fish: smallmouth bass (Micropterus dolomieu), yellow perch (Perca flavescens) and bluegill (Lepomis macrochirus), in addition to several invertebrate species including freshwater bivalves (family: Sphaeriidae), dragonfly nymphs (order: Odonata), snails (class: Gastropoda) and zooplankton (family: Daphniidae). Results showed significant U bioaccumulation in the lake impacted by historical mining activities. Uranium accumulation was 2-3 orders of magnitude higher in invertebrates than in the fish species. Within fish, U was measured in operculum (bone), liver and muscle tissue and accumulation followed the order: operculum>liver>muscle. There was a negative relationship between stable nitrogen ratios ((15)N/(14)N) and U bioaccumulation, suggesting U biodilution in the foodweb. Uranium bioaccumulation in all three tissues (bone, liver, muscle) varied among fish species in a consistent manner and followed the order: bluegill>yellow perch>smallmouth bass. Collectively, gut content and stable isotope analysis suggests that invertebrate-consuming fish species (i.e. bluegill) have the highest U levels, while fish species that were mainly piscivores (i.e. smallmouth bass) have the lowest U levels. Our study highlights the importance of understanding the feeding ecology of fish when trying to predict U accumulation.

The impacts of ontogenetic dietary shifts in yellow perch (Perca flavescens) on Zn and Hg accumulation.

Yellow perch (Perca flavescens) undergo several ontogenetic dietary shifts, and consequently these fish feed at different trophic levels and rely on different carbon sources over their lifetime. Stable isotope ratios of nitrogen (δ(15)N) and carbon (δ(13)C) are powerful ecological tools that are used to provide a temporally integrated description of the feeding ecology of aquatic animals such as fish. The main objective of this study was to use stable isotopes of nitrogen and carbon to determine if dietary changes affected mercury (Hg) and zinc (Zn) accumulation in yellow perch ranging in size from approximately 5 cm to 27 cm. Results showed that Hg bioaccumulation generally increased with increasing trophic level in fish feeding at higher trophic levels, however, the relationship between Hg levels and δ(15)N was non-linear showing no relationship in small fish (less than 15 cm). In contrast, there was a negative, linear relationship between δ(15)N and Zn, suggesting that as perch fed at progressively higher trophic levels, less of Zn bioaccumulated. No relationship was observed between δ(13)C and metal levels in perch. Collectively, these results demonstrate a contrast in the behavior of Zn and Hg bioaccumulation in yellow perch as a function of trophic status.

Seasonal variations in hepatic Cd and Cu concentrations and in the sub-cellular distribution of these metals in juvenile yellow perch (Perca flavescens).

Temporal fluctuations in metal (Cd and Cu) concentrations were monitored over four months (May to August) in the liver of juvenile yellow perch (Perca flavescens) sampled from four lakes situated along a metal concentration gradient in northwestern Quebec: Lake Opasatica (reference lake, low metal concentrations), Lake Vaudray (moderate metal concentrations) and lakes Osisko and Dufault (high metal levels). The objectives of this study were to determine if hepatic metal concentrations and metal-handling strategies at the sub-cellular level varied seasonally. Our results showed that Cd and Cu concentrations varied most, in both absolute and relative values, in fish with the highest hepatic metal concentrations, whereas fish sampled from the reference lake did not show any significant variation. To examine the sub-cellular partitioning of these two metals, we used a differential centrifugation technique that allowed the separation of cellular debris, metal detoxified fractions (heat-stable proteins such as metallothionein) and metal sensitive fractions (heat-denaturable proteins (HDP) and organelles). Whereas Cd concentrations in organelle and HDP fractions were maintained at low concentrations in perch from Lakes Opasatica and Vaudray, concentrations in these sensitive fractions were higher and more variable in perch from Lakes Dufault and Osisko, suggesting that there may be some liver dysfunction in these two fish populations. Similarly, Cu concentrations in these sensitive fractions were higher and more variable in perch from the two most Cu-contaminated lakes (Dufault and Osisko) than in perch from the other two lakes, suggesting a breakdown of homeostatic control over this metal. These results suggest not only that metal concentrations vary seasonally, but also that concentrations vary most in fish from contaminated sites. Furthermore, at the sub-cellular level, homeostatic control of metal concentrations in metal-sensitive fractions is difficult to maintain in perch with high hepatic metal concentrations.

A field study examining metal elimination kinetics in juvenile yellow perch (Perca flavescens).

Currently little is known about how and at what rate fish eliminate metals in natural environments. To address this knowledge gap we examined metal elimination kinetics in the field using juvenile yellow perch (Perca flavescens) that were caught in a metal-contaminated lake (elevated levels of Cd, Cu and Zn) and transplanted to cages held within a reference lake. Fish were sampled from the cages over 75 d and changes in metal concentrations were measured in the gills, gut, liver and kidney. In transplanted fish, Cd concentrations decreased most rapidly in the gills and gut, i.e. from organs in contact with the ambient water and food; biological half lives (t1/2) were 18 and 37 d, respectively, for each organ. Longer half-lives were observed in the liver (75 d) and kidney (52 d) for this metal. Elimination of excess Cu by the liver and gut occurred much more rapidly, with estimated half-lives of labile Cu being 8 and 4 d, respectively, for these two organs. In contrast to Cd and Cu, there was little Zn elimination. To compare how the liver handles different metals during elimination, we used a differential centrifugation approach to examine changes in metal concentrations (Cd and Cu) at the sub-cellular level. Consistent with the long half-life observed for Cd at the whole organ level, there was no significant loss of Cd from any of the sub-cellular fractions. Copper, on the other hand, was lost from both the organelle and cellular debris fractions. As these fractions likely contain structures such as lysosomes, we suggest that Cu is depurated from the liver by direct elimination of these sub-cellular vesicles. These field results clearly demonstrate how the liver handles essential (Cu, Zn) and non-essential metals (Cd) differently during depuration.

Dynamics of Cd, Cu and Zn accumulation in organs and sub-cellular fractions in field transplanted juvenile yellow perch (Perca flavescens).

Juvenile yellow perch (Perca flavescens) were caught in a reference lake and transplanted to cages held within a lake impacted by mining activities, with elevated levels of aqueous bioavailable copper (Cu(2+)), zinc (Zn(2+)) and cadmium (Cd(2+)). Fish were sampled from the cages over 70 d and changes in metal concentrations were followed over time in the gills, gut, liver and kidney. In addition, the hepatic sub-cellular partitioning of the three metals was determined by differential centrifugation of liver samples, yielding the following fractions: cellular debris; organelles; heat-denaturable proteins (HDP); and heat-stable proteins (HSP) (including metallothionein). In transplanted fish, Cd concentrations increased in all the organs sampled, whereas Cu mainly increased in the gills, gut and liver but not the kidney; some slight accumulation of Zn occurred in the kidneys and gills of the transplanted fish. The sub-cellular partitioning results demonstrated that metal-handling strategies in juvenile yellow perch differed among metals. Cellular sequestration in the HSP fraction was an important strategy used by these fish in response to increased ambient Cd. Accumulation of Zn was not seen in the organs examined, indicating that transplanted perch were able to either reduce influx, or increase efflux rates of this metal. The response of yellow perch to elevated ambient Cu appeared to combine the strategies used for Cd and Zn, as both cellular sequestration and reduced accumulation were observed in transplanted fish.

The potential impact of climate on human exposure to contaminants in the Arctic.

Many northern indigenous populations are exposed to elevated concentrations of contaminants through traditional food and many of these contaminants come from regions exterior to the Arctic. Global contaminant pathways include the atmosphere, ocean currents, and river outflow, all of which are affected by climate. In addition to these pathways, precipitation, animal availability, UV radiation, cryosphere degradation and human industrial activities in the North are also affected by climate change. The processes governing contaminant behaviour in both the physical and biological environment are complex and therefore, in order to understand how climate change will affect the exposure of northern people to contaminants, we must have a better understanding of the processes that influence how contaminants behave in the Arctic environment. Furthermore, to predict changes in contaminant levels, we need to first have a good understanding of current contaminant levels in the Arctic environment, biota and human populations. For this reason, it is critical that both spatial and temporal trends in contaminant levels are monitored in the environment, biota and human populations from all the Arctic regions.