Trophic magnification rates of eighteen trace elements in freshwater food webs

Trace elements play diverse roles in animal physiology ranging from essential micronutrients to potent toxicants. Despite animals accumulating many trace elements through their diets, relationships between trophic positions and biological concentrations of most trace elements remain poorly described. We report trophic transfer rates of Al, As, Ba, Cd, Co, Cu, Fe, Hg, Mn, Ni, Pb, Se, Sr, Ti, Tl, U, V, and Zn from 31 freshwaters located in distinct biogeographic regions. Elemental concentrations and stable nitrogen isotope ratios (proxies for trophic position) were determined in zooplankton, molluscs, insects, and fishes from all sites. Trophic magnification factors (TMFs) were calculated as the mean fraction of each element that transferred from prey to predators at each site. TMFs >1 indicate biomagnification and TMFs <1 indicate biodilution. Mercury was the only biomagnifying element (median TMF = 3.77), and selenium neither biomagnified nor biodiluted (median TMF = 1.01). All 16 remaining elements biodiluted, with median TMFs ranging from 0.07 (uranium) to 0.60 (thallium). We used a model selection procedure to determine whether intrinsic physical and chemical elemental properties explained differences in TMFs among elements. Elements with high covalent bonding indices (Q) had marginally greater TMFs than elements with low Q values. Based on their high Q values, we recommend investigation into the trophic transfer rates of ten additional trace elements, some of which may biomagnify through some aquatic food webs. The high variability in TMFs within elements suggests that ecological factors are likely more important than intrinsic elemental properties at determining elemental TMFs.