Ecomorphological divergence drives differential mercury bioaccumulation in polymorphic European whitefish (Coregonus lavaretus) populations of subarctic lakes.

Resource polymorphism, whereby ancestral trophic generalists undergo divergence into multiple specialist morphs, is common in salmonid fish populations inhabiting subarctic lakes. However, the extent to which such resource specialization into the three principal lake habitats (littoral, profundal, and pelagic) affects patterns of contaminant bioaccumulation remains largely unexplored. We assessed total mercury concentrations (THg) of European whitefish (Coregonus lavaretus (L.)) and their invertebrate prey in relation to potential explanatory variables across 6 subarctic lakes, of which three are inhabited by polymorphic (comprised of four morphs) and three by monomorphic populations. Among invertebrate prey, the highest THg concentrations were observed in profundal benthic macroinvertebrates, followed by pelagic zooplankton, with concentrations lowest in littoral benthic macroinvertebrates in both lake types. Broadly similar patterns were apparent in whitefish in polymorphic systems, where average age-corrected THg concentrations and bioaccumulation rates were the highest in pelagic morphs, intermediate in the profundal morph, and the lowest in the littoral morph. In monomorphic systems, age-corrected THg concentrations were generally lower, and showed pronounced lake-specific variation. In the polymorphic systems, we found significant relationships between whitefish muscle tissue THg concentration and gill raker count, resource use, lipid content and maximum length, whilst no such relationships were apparent in the monomorphic systems. Across all polymorphic lakes, the major variables explaining THg in whitefish were gill raker count and age, whereas in monomorphic systems, the factors were lake-specific. Whitefish resource polymorphism across the three main lake habitats therefore appears to have profound impacts on THg concentration and bioaccumulation rate. This highlights the importance of recognizing such intraspecific diversity in both future scientific studies and mercury monitoring programs.