Variability of total mercury concentrations in Steller sea lion bone locations and bone elements

Abstract

Mercury (Hg) is a contaminant of global concern with impacts on the health and resilience of marine wildlife. Some forms of ingested Hg are bioaccumulated and biomagnified with greatest concentrations observed in long-lived top predators. High concentrations of Hg in mammals impair immune function, neurochemistry, and reproduction. Steller sea lions (Eumetopias jubatus, SSL) are a long-lived top predator, and some populations have total mercury concentrations ([THg]) in fur that are above the threshold values of concern for toxic effects. While this information is available for present-day populations of SSLs, [THg] in historic and ancient populations of SSLs are unknown. Bone is a biologically resilient material that is well preserved over millennial time scales and can be used to study Hg concentrations in these historic and ancient populations; however, little is known about the variability and distribution of Hg in bone. This study examined [THg] within individual bones and across bone elements in skeletons of SSLs to understand the distribution and variability of Hg in bone. Bones were acquired from necropsies or museum collections from fetuses (stillborn), pups (newborn to 3 months), juveniles (4 months to 5 years), and adults (> 5 years). Bones were analyzed for [THg] with a Nippon MA-3000 (Nippon Instruments Corporation, Tokyo) direct mercury analyzer. To determine differences and variability of [THg] within bones, [THg] were quantified in compact and spongy bone of pups (n = 5) and non-pups (n = 5) at seven locations from the proximal to distal end of long bones. Spongy bone [THg] near the epiphyseal plates were greater compared with mid-diaphysis locations in pups (p = 0.01). Spongy and compact bone [THg] near the epiphyseal plates were greater and more variable compared with mid-diaphysis locations in non-pups (p < 0.01). To determine differences and variability of [THg] among bone elements, [THg] were quantified in compact and spongy bone of three bone elements for pups (rib, long bone, phalange) and non-pups (occipital, mandible, nasal turbinate). Among pup bone elements, there were no differences in [THg] of compact bone (p = 0.17); however, spongy bone in ribs had greater [THg] than long bones (p = 0.03) and phalanges (p = 0.01). Among non-pup bone elements, there were no significant differences in [THg] in spongy bone; however, compact bone in nasal turbinates had greater [THg] compared with mandibles and occipitals (p < 0.01). Differences in bone composition, growth, and turnover rate likely affect Hg distribution and concentration within long bones and among bone elements. This study provides insight into a highly variable tissue that may allow for retrospective analysis of contaminants, such as Hg.