Fatty acid carbon isotopes as tracers of trophic structure and contaminant biomagnification in Arctic marine food webs.

Mercury (Hg) and persistent organic pollutant (POP) accumulation among species and biomagnification through food webs is typically assessed using stable isotopes of nitrogen (δN) and carbon (δC) in bulk (whole) tissues. Yet, bulk isotopic approaches have limitations, notably from the potential overlap of isotope values from different dietary sources and from spatial variation in source (baseline) signals. Here, we explore the potential of fatty acid carbon isotopes (FA δC) to (1) evaluate the trophic structure of a marine food web, (2) distinguish feeding patterns among four marine mammal consumers, (3) trace contaminant biomagnification through a food web, and (4) explain interspecific variation in contaminants among high-trophic position predators. In the Cumberland Sound (CS) food web of Nunavut, Canada, ranging from zooplankton to Greenland shark (Somniosus microcephalus), FA δC values for the monounsaturated FAs, 20:1 and 22:1 isomers, did not vary across the food web, while the long-chain polyunsaturated FA, 22:6n3 showed δC values that were enriched by ~1.5 ‰ with each trophic position. Values of δC for shorter-chain and saturated FAs varied widely across this food web. In East Greenland (EG) marine mammals, FA δC values were significantly higher in migratory sub-Arctic species relative to Arctic residents. Linear models using FA δC as explanatory variables for contaminant concentrations demonstrated that baseline-corrected δC values of certain dietary FAs explained more variation in POP concentrations than did bulk stable isotopes in EG marine mammals. However, bulk δN better explained Hg variation in the CS food web. This study details the FA δC instrumental methods, such that other researchers can test this novel approach on other species, locations, and food webs to further evaluate whether the δC values of certain diet-derived FAs consistently show limited or predictable trophic fractionation and may therefore be useful for assessing the accumulation and biomagnification of lipophilic contaminants.