2003a, Dehn et al. 2006a). Conclusions on Hg biomagnification are mixed. Dehn et al. (2006b) found little DNA Damage inhibitor evidence for Hg biomagnification, whereas data from Atwell et al. (1998) suggest a significant, positive log relationship between [Hg] and δ15N values in arctic food webs. Theory and empirical studies show

that [Pb] is lowest in top trophic level consumers (Michaels and Flegal 1990), because Pb is biodepleted relative to its biogeochemical analogue calcium. The combination of [Pb] and stable Pb isotopes (207Pb/206Pb) have been especially useful in documenting historical shifts in the source(s) and sometimes concentrations of Pb between preindustrial and modern times (Smith

et al. 1990, Outridge et al. 1997, Caurant et al. 2006). Most of the studies on organochemical contaminants evaluate exposure at the community or ecosystem level and present data from multiple trophic levels RG7204 solubility dmso that often include one or more marine mammal species. Significant, positive correlations among PCB, DDT, and FOC concentrations and trophic level, as derived from δ15N values, are strong evidence for bioaccumulation of these compounds in marine food webs (Jarman et al. 1996, Fisk et al. 2001, Hobson et al. 2002, Tomy et al. 2004). Coupled contaminant and δ13C analysis also suggest differences in FOC contaminant loads among marine mammal species that occupy nearshore versus offshore habitats (Van de Vijver et al. 2003). The blending of contaminant and SIA also yields information on population structure and niche variation at the individual, population, or species level. At first, contaminant concentrations alone were used in this capacity (see review by Aguilar 1987). More recently, however, researchers have begun to integrate toxicological and isotopic proxies. In essence, geographic

variability in natural elements (i.e., food web isotope values) or anthropogenic compounds (i.e., contaminants) provides independent but complimentary chemical tracers that can have signatures unique to the region(s) in which an organism forages. This strategy has been applied to small Interleukin-3 receptor cetacean populations in the southwestern Mediterranean Sea, the northeast Atlantic Ocean, and Black Sea (Das et al. 2000, 2004b; Borrell and Aguilar 2005; Borrell et al. 2006); ringed seal populations in the Canadian Arctic (Fisk et al. 2002a); minke whales in the North Atlantic (Born et al. 2003); and killer whale ecotypes in the North Pacific Ocean (Herman et al. 2005; Krahn et al. 2007, 2008). As with most ecological applications of stable isotope analysis, diet and habitat preferences are the primary pieces of information acquired through study of population structure and niche separation.