Does trophic structure dictate mercury concentrations in top predators? A comparative analysis of pelagic food webs in the Pacific Ocean

Large pelagic predators in the marine environment accumulate relative high levels of mercury (Hg), yet it is difficult to identify the ecological factors that control the fate and flow of contaminants in marine systems, in part due to the difficulty in experimenting with large, dynamic ecosystems. By comparing the eastern tropical Pacific (ETP) and central north Pacific (CNP) pelagic food webs, we determined how differences in trophic pathways, versus differences in baseline Hg conditions, might translate into varying levels of Hg contamination in upper trophic levels. We developed coupled contaminant-trophic mass balance models to estimate Hg concentrations or Hg-elimination rates for species groups in each region finding generally higher Hg concentrations in the ETP model. Specifically we found Hg concentrations of yellowfin, skipjack, bigeye, and albacore tunas were two- to four-fold greater in the ETP than the CNP. These estimated differences reflected previous empirical observations of Hg concentrations in tunas across these regions. Regional differences in tuna Hg concentrations can be solely explained by differences in diets of tuna and their prey, although changes in Hg input at the base of the food web can strongly influence Hg concentrations in top predators as well. The coupled models allowed us to test hypotheses on the sources of Hg variation in top predators at a food web scale, incorporating the indirect effects of different food web structures into our regional Hg estimates. Our ability to draw firm conclusions regarding causal effects underlying known differences in Hg levels across ocean regions is restricted by data limitations that led to different assumptions made to construct the food web models.