91 (FIS-1117-826214) Uses and abuses of stable isotopes in ecotoxicology.
Start time: 8:00 AM
Fisk, A¹, Kidd, K², ¹ University of Georgia, Athens, GA, USA² University of New Brunswick, Saint John, NB, Canada
Stable isotopes of carbon (¹³C) and nitrogen (¹⁵N) have become a common tool in ecology for quantifying carbon flow and food web structure. The fate and potential effects of anthropogenic contaminants are largely determined by carbon flow and food web structure, and so, not surprisingly, stable isotopes have become a popular metric for explaining observed patterns of contaminants in food webs. However, there has been a recent wave of publications in the ecological literature questioning some of the assumptions made when using stable isotopes and there is a call for more laboratory experiments. For example, there has been little attention paid to the kinetics of stable isotopes (i.e., how quickly they change after a diet switch) and how they vary across tissue types. Kinetics will determine the length of time it will take an organisms to achieve a stable isotope signature that is in equilibrium with its diet; recent feeding studies suggest this may take 100s of days in fish and other poikilotherms although it is highly dependent on tissue type. Such assumptions or caveats need to be considered when they are applied to contaminant data because the time for a contaminant to achieve equilibrium in an can vary from hours to years and is specific to the chemical. Ignoring these assumptions could lead to erroneous conclusions about the dynamics of contaminants in the environment but consideration of these assumptions could lead to greater insights. This talk will summarize some of the assumptions/caveats involved in stable isotope research and how they affect the application and utility of this tool in ecotoxicology.

92 (KID-1117-850549) Advantages and Limitations of Using Stable Isotope Ratios to Understand the Accumulation of Persistent Organic Pollutants in Aquatic Food Webs.
Start time: 8:20 AM
Kidd, K.¹, Fisk, A.², Borgå, K.³, Muir, D. ⁴, ¹ University of New Brunswick, Saint John, NB, Canada² University of Georgia, Athens, GA, USA³ Norwegian Polar Institute, Tromsø, Norway⁴ Environment Canada, Burlington, ON, Canada
An increasing number of studies are using ratios of stable nitrogen isotope ratios (¹⁵N) to characterize the trophic positioning of organisms, and quantify the length of the food web and biomagnification of persistent organic pollutants (POPs) in freshwater and marine systems. These studies have shown that concentrations of DDT, PCBs and other chlorinated organics in biota are significantly related to their trophic position. The slope of regression of the (log) pollutant versus trophic level based on ¹⁵N has been termed a bioaccumulation rate and its antilog the trophic magnification factor (TMF). The TMF can be positive or negative depending on the extent of magnification or metabolism of a contaminant by upper-trophic-level biota. Studies have now been completed in several Great Lakes (both Laurentian and African), in productive and unproductive systems, in marine and freshwaters, and at arctic through tropical latitudes; these results can be contrasted on a broader scale to improve our understanding of food web magnification of POPs. As an example, the TMFs of log p,p'-DDE versus ¹⁵N are consistently positive and vary by a factor of 2 from tropical to arctic latitudes in freshwater food webs. In contrast, negative TMFs have been observed for compounds such as phthalate esters, indicating that food web metabolism of some pollutants can be quantified with this technique. Data from tropical, temperate and arctic lakes and oceans are compiled herein to determine whether patterns in the slopes of the relationships of specific POPs versus trophic level/¹⁵N exist across systems, and whether these differences can be explained by system-specific factors such as productivity or by broader influences such as climate. In addition, we discuss the advantages to and limitations of using this technique in studies of pollutants in aquatic food webs.

93 (CAM-1117-764050) Assessing biomagnification of mercury in food webs using stable isotopes: A global perspective.
Start time: 8:40 AM
Campbell, LM¹, Dixon, DG², Evans, M³, Hecky, RE², Gerwutz, S⁴, Kendall, C⁵, Kidd, K⁶, Lockhart, L⁷, Muir, DG⁸, ¹ School of Environmental Studies, Queen's University, Kingston, ON, Canada² Department of Biology, University of Waterloo, Waterloo, ON, Canada³ National Water Research Institute, Environment Canada, Saskatoon, SK, Canada⁴ Department of Geography, University of Toronto, Toronto, ON, Canada⁵ United States Geological Survey, Menlo Park, CA, USA⁶ Department of Biology, University of New Brunswick, St. John, NB, Canada⁷ Freshwater Institute, Department of Fisheries & Oceans, Winnipeg, MB, Canada⁸ National Water Research Institute, Environment Canada, Burlington, ON, Canada
For the past decade, stable isotopes of nitrogen and carbon have been used as an effective tool to assess food web structure in freshwater and marine ecosystems globally. Stable nitrogen isotope ratios (¹⁵N) have been used as a trophic position indicator, while stable carbon isotope ratios (¹³C) have been used to assess carbon sources to biota. The ability of mercury (Hg), particularly methylmercury (MeHg) to biomagnify through food webs is well known. As such, biomagnification factors (BMFs) can be obtained for an ecosystem using the slope of the regression of Hg concentrations in biota from an ecosystem against their trophic position, as indicated by ¹⁵N values. The importance of BMF to Hg burdens in food webs can be compared with other factors such as food chain length, latitude and lake trophic status. Databases for lakes ranging over a wide gradient of sizes and trophic status in tropical, temperate and arctic latitudes have been assembled over the last 10 years. In addition, a review of other mercury biomagnification studies that incorporated stable isotope analyses is included. This provides an opportunity to compare Hg biomagnification and trophodynamics across latitudes, lake sizes and climate regimes. Regression slopes of log Hg vs ¹⁵N were found to range from 0.16 to 0.29 across systems, with most slope values around 0.20. Food chain length, as estimated from lower trophic species to top trophic predators were calculated, and were found to be a more significant predictor of Hg concentrations in top predators. Despite similar biomagnification factors and longer food chain lengths in many African lakes, Hg concentrations in tropical pisivorous fish are lower than those in North American lakes. This suggests that while Hg BMF's are relatively consistent, the bioavailability of Hg may be different in tropical and temperate lakes.

94 (STE-1117-835479) Trophic enrichment of trace elements in aquatic food webs: A paradigm shift from the organics world.
Start time: 9:00 AM
Stewart, A¹, Croteau, M-N¹, Luoma, S¹, ¹ U.S. Geological Survey, Menlo Park, CA, USA
Our understanding of contaminant enrichment in aquatic food webs has been greatly enhanced through the use of stable isotopes of carbon and nitrogen. Stable isotope techniques are extremely effective in explaining how concentrations of organic contaminants such as organic mercury biomagnify along food webs. Relationships between inorganic pollutants and trophic position have been less transparent, leading to erroneous conclusions that most metals are not enriched in food webs in nature. Here we measured Se, Hg and Cd concentrations in algae, invertebrates and fish from estuarine and freshwater food webs from the San Francisco Bay/Delta to illustrate how traditional application of stable isotopes in studies of organic contaminants or mercury may not be appropriate for trace elements. Analysis of these food webs show how trophic enrichment is a function of feeding guild, consumer physiology, habitat, prey type and trophic position rather than a single relationship with trophic position or percent lipid. By dividing food webs into discrete units based on these ecological and physiological relevant processes we are able to identify important trophic links that lead to enhanced exposure in predators. A shift in the way we perceive food webs is essential if we are to identify those individuals and species most at risk for trace element toxicity.

(58076) COFFEE BREAK.
Start time: 9:20 AM

95 (WAL-1117-810307) Using stable isotopes to track biomagnification of polychlorinated biphenyls (PCBs) through stream food webs.
Start time: 10:00 AM
Walters, D¹, Fritz, K¹, Johnson, B¹, Lazorchak, J¹, ¹ US EPA, Office of Research and Development, Cincinnati, Ohio, USA
Biomagnification studies in streams are rare. This is surprising because PCBs and other persistent organic pollutants are known to biomagnify in aquatic ecosystems, and contaminated sediments are pervasive in stream ecosystems. Most studies of biomagnification are from lentic systems, which are characterized by organic matter and sediment retention. It is unclear of PCBs biomagnify and persist in streams, which are characterized by sediment and organic matter export. We investigated PCB contamination in Twelvemile Creek (Clemson, SC, USA), a stream that received > 400,000 lbs of PCBs from 1955-1978. Our goals were to determine if PCBs biomagnify in streams and to measure the relative importance of heterotrophic and autotrophic pathways for biomagnification. Sites were sampled during spring (n=6) and fall (n=4) 2003-04. Major components of the foodweb were analyzed for total PCBs (n = 525), ¹³C and ¹⁵N (n = 1325). Biomagnification was apparent as mean ¹⁵N strongly predicted PCBs (r² = 0.58), and PCBs consistently increased with trophic level. We found no consistent patterns related to carbon pathways when the entire foodweb was considered. ¹³C and PCBs were uncorrelated, suggesting that carbon source was unrelated to biomagnification. However, analysis of individual trophic groups indicated that carbon source effects PCB concentrations. PCBs in grazing insects (970 ng g^-1) were more than double those in leaf shredding insects (370 ng g^-1) even though PCB concentrations were similar for periphyton and leaves. Periphyton ¹³C was significantly related to grazer ¹³C (r² = 0.34) but not to shredders. These results suggest that greater reliance on algal resources contributes to higher PCB concentrations within trophic levels. Our results show that PCBs biomagnify and persist in streams, but the importance of autotrophic and heterotrophic pathways is unclear. Although this work was reviewed by EPA and approved for publication, it may not reflect official Agency policy.

96 (PET-1117-834808) Stable isotopes provide evidence for the biomagnification of radiocesium in a contaminated aquatic environment.
Start time: 10:20 AM
Peters, E¹, Jagoe, C², Unrine, J², Romanek, C², Rosales, Y¹, Batts, R¹, Brant, H², ¹ Chicago State University, Chicago, IL, USA² Savannah River Ecology Lab, Aiken, SC, USA
Anthropogenic radionuclides can be released to the environment during nuclear power generation or through the production, testing or use of nuclear weapons. Evaluation of risks associated with radionuclide contamination requires understanding their movement, accumulation, and retention in the environment. Among fission products, radiocesium (especially the longer-lived ¹³⁷Cs), is important because it is a biochemical analog of potassium and can accumulate in the soft tissues of organisms. Unlike other fission products (and most metals), radiocesium appears to biomagnify. This observation is controversial because, while some studies have found biomagnification of radiocesium, others have not, or have even reported decreased concentrations at higher trophic positions. The available data suggest that the potential for biomagnification varies among environments and may be related to trophic complexity. However, few prior studies of radiocesium have used objective, unambiguous methods to quantify trophic relationships or differences in diet among the organisms sampled. To investigate biomagnification of ¹³⁷Cs, we employed stable isotope ratios of C and N as indicators of trophic position and carbon source in biota from Pond B on the Savannah River Site, a nuclear production facility in South Carolina, USA, that was contaminated with ¹³⁷Cs in the 1960s.We collected plants, aquatic invertebrates and fishes from Pond B over a two-year period, and measured ¹³⁷Cs concentrations, ¹⁵N, and ¹³C. Aquatic plants contained relatively low concentrations of ¹³⁷Cs. Among fishes, ¹³⁷Cs concentrations increased by a factor of 4.5 (95% C.I. 3.5 to 5.9) per trophic level, assuming a shift in ¹⁵N of about 3.5‰ per trophic level. The best regression model of ¹³⁷Cs concentration (R² = 0.46) incorporated both ¹⁵N and ¹³C as predictors. As assimilation efficiencies of ¹³⁷Cs and K are comparable, both the bioaccumulation and biomagnification of ¹³⁷Cs likely result from slower whole-body elimination rates of ¹³⁷Cs than for K.

97 (PAT-1117-815938) An evaluation of stable nitrogen isotopes and PCBs as bioenergetic tracers in aquatic systems.
Start time: 10:40 AM
Paterson, G¹, Drouillard, K¹, Haffner, G¹, ¹ Great Lakes Institute for Environmental Research, Windsor, Ontario, Canada
The stable isotope of nitrogen (¹⁵N) has been used as a predictor of persistent organic pollutant accumulation in aquatic ecosystems. Such applications rely on two central assumptions; a constant trophic enrichment of 3.4‰ for ¹⁵N between predator and prey and that ¹⁵N and pollutants such as PCBs are tracking similar bioenergetic processes. In this study, we investigated ¹⁵N ratios and PCB concentrations in two fish species in relation to age, size and diet. Using a dietary mixing model and a growth and metabolism based accumulation model, ¹⁵N exhibited enrichment with increasing size and approached steady state with respect to dietary ¹⁵N values by the second year of growth. Young of the year fish, however, exhibited lower nitrogen isotope enrichment over the diet indicating that the commonly held trophic enrichment factor of 3.4‰ is more suitable for older, slow growing individuals. A PCB accumulation model predicted pollutant accumulation to transition from water based bioaccumulation (bioconcentration) in small rapidly growing individuals to accumulation dominated by dietary exposure (biomagnification) by a body size approaching 100g. Significant increases in PCB accumulation were also attributed to decreased specific growth rates such that ≤ 60% of body mass was gained on an annual basis. It was concluded that growth related changes in species bioenergetics regulate both contaminant accumulation and ¹⁵N dynamics. For contaminants such as PCBs, these mechanisms have been well defined. However, for stable isotopes, substantial research is required focusing on the kinetics of assimilation, fractionation, and elimination in aquatic species.

98 (VID-1117-248940) Using stable isotopes to monitor the bacterial degradation of naphthenic acids and polycyclic aromatic hydrocarbons.
Start time: 11:00 AM
Videla, P¹, Farwell, A¹, Butler, B¹, Dixon, D¹, ¹ University of Waterloo, Waterloo, Ontario, Canada
Large amounts of process-affected water (tailings) are generated from the extraction of bitumen from the oil sands in Alberta. The tailings consist of sand, clay, unrecovered bitumen, and large quantities of naphthenic acids (NAs) and polycyclic aromatic hydrocarbons (PAHs). NAs are complex mixtures of carboxylic acids that have been analytically characterized by C numbers (C5-C33) and Z families (Z=0, linear; Z=-12, 6 rings). The PAHs associated with oil sands are primarily alkylated PAHs, dominated by substituted dibenzothiophenes and phenanthrenes/anthracenes. NAs are of concern for their acute toxicity effects, PAHs for their carcinogenic and mutagenic potential. A "zero-discharge" policy prevents the release of tailings to receiving waters. As such, prior to reclamation, the tailings are placed into settling basins. To some extent the NAs and PAHs are biodegradable which could potentially reduce their toxicity. Biodegradation of NAs and PAHs in reclaimed aquatic environments is important in determining the structure and function of the mature aquatic ecosystems. To improve our understanding of energy sources and trophic level interactions, stable carbon and nitrogen isotopes have been analyzed for food web components of various oil sands reclaimed systems. Previous studies of benthic invertebrates indicated changes in stable isotope values (¹³C depletion and ¹⁵N enrichment) associated with elevated levels of NAs and PAHs in reclaimed systems. To gain a better understanding of the cycling of carbon and nitrogen sources at the base of the aquatic food web, laboratory studies have been initiated to examine the levels of isotope fractionation associated with the biodegradation of NAs and PAHs. Oil-sands derived bacterial cultures were supplied oil sands-extracted and commercially prepared NAs and PAHs of varying profiles and concentrations. At various time intervals, microbial growth, concentrations of NAs, PAHs, CO₂, DIC, DOC and NA profile, as well as ¹³C and ¹⁵N for the biomass, CO₂, DIC, DOC were characterized.