title: Use of a food web model to evaluate the factors responsible for high PCB fish concentrations in Lake Ellasjøen, a high arctic lake.
abstract: OBJECTIVE
  Lake Ellasjøen, located in the Norwegian high arctic, contains the highest concentrations of polychlorinated biphenyls (PCBs) ever recorded in fish and sediment from high arctic lakes, and concentrations are more than 10 times greater than in nearby Lake Øyangen. These elevated concentrations in Ellasjøen have been previously attributed, in part, to contaminant loadings from seabirds that use Ellasjøen, but not Øyangen, as a resting area. However, other factors, such as food web structure, organism growth rate, weight, lipid content, lake morphology, and nutrient inputs from the seabird guano, also differ between the two systems. The aim of this study is to evaluate the relative influence of these factors as explanatory variables for the higher PCB fish concentrations in Ellasjøen compared with Øyangen, using both a food web model and empirical data.
  
  
  METHODS
  The model is based on previously developed models but parameterized for Lakes Ellasjøen and Øyangen using measured data wherever possible. The model was applied to five representative PCB congeners (PCB 105, 118, 138, 153, and 180) using measured sediment and water concentrations as input data and evaluated with previously collected food web data.
  
  
  RESULTS
  Modeled concentrations are within a factor of two of measured concentrations in 60% and 40% of the cases in Lakes Ellasjøen and Øyangen, respectively, and within a factor of 10 in 100% of the cases in both lakes. In many cases, this is comparable to the variability associated with the data as well as the efficacy of the predictions of other food web model applications.
  
  
  CONCLUSIONS
  We next used the model to quantify the relative importance of five major differences between Ellasjøen and Øyangen by replacing variables representing each of these factors in the Ellasjøen model with those from Øyangen, in separate simulations. The model predicts that the elevated PCB concentrations in Ellasjøen water and sediment account for 49%-58% of differences in modeled fish PCB concentrations between lakes. These elevated sediment and, to a lesser extent, water concentrations in Ellasjøen are due to PCB loadings from seabird guano. However, sediment-water fugacity ratios of PCBs are consistently greater in Ellasjøen compared with Øyangen, which suggests that internal lake processes also contribute to differences in sediment and water concentrations. We hypothesize that the nutrients associated with guano influence sediment-water fugacity ratios of PCBs by increasing the stock of pelagic algae. As both these algae and the guano settle, their organic carbon content is degraded faster than PCBs, which causes an extra magnification step in Ellasjøen before these detrital particles are consumed by benthic organisms, which are in turn consumed by fish. The model predicts that the remaining approximately 50% of the differences in PCB concentrations observed between the fish of these lakes are due to other subtle differences in their food web structures.
  
  
  CONCLUSIONS
  In conclusion, based on the results of a food web model, we found that the most dominant factors influencing the higher PCB fish concentrations in Lake Ellasjøen compared with Øyangen are the higher sediment and water concentrations in Ellasjøen, caused by seabird guano. Together, sediment and water are predicted to account for 49%-58% of differences in fish concentrations between lakes. Although seabird guano provides a source of nutrients to the lake, in addition to contaminants, empirical data and indirect model results suggest that nutrients are not leading to decreased bioaccumulation, in contrast to what has been observed in temperate, pelagic food webs.
  
  
  CONCLUSIONS
  The results of this study emphasize the importance of considering even small differences in food web structure when comparing bioaccumulation in two lakes; although the food web structures of Ellasjøen and Øyangen differ only slightly, the model predicts that these differences account for most of the remaining approximately 50% of the differences in PCB fish concentrations between the two lakes. This study further demonstrates the utility of food web models as we were able to predict and tease apart the influence of various factors responsible for the elevated concentrations in the fish from Lake Ellasjøen, which would have been difficult using the field data alone.
authors.affiliation: Department of Geography, University of Toronto, 100 St. George Street, Toronto, Ontario, M5S 3G3, Canada. sarah.gewurtz@utoronto.ca
authors.forename: Sarah B
authors.initials: SB
authors.lastname: Gewurtz
authors.forename: Nilima
authors.initials: N
authors.lastname: Gandhi
authors.forename: Guttorm N
authors.initials: GN
authors.lastname: Christensen
authors.forename: Anita
authors.initials: A
authors.lastname: Evenset
authors.forename: Dennis
authors.initials: D
authors.lastname: Gregor
authors.forename: Miriam L
authors.initials: ML
authors.lastname: Diamond
mesh_terms: D000818:Animals; D001110:Arctic Regions; D004785:Environmental Pollutants / Q000378:metabolism; D005399:Fishes / Q000378:metabolism*; D020387:Food Chain; D005618:Fresh Water; D019015:Geologic Sediments; D007448:Invertebrates / Q000378:metabolism; D008954:Models, Biological; D009664:Norway; D011078:Polychlorinated Biphenyls / Q000737:chemistry* / Q000378:metabolism*; D015048:Zooplankton / Q000378:metabolism
chemical_list: D004785:Environmental Pollutants; D011078:Polychlorinated Biphenyls