Mercury Storage, Transformation, and Transfer in the Colorado Rocky Mountains

Mercury (Hg)a geologically sourced elementcycles in the atmosphere at levels three-fold higher than the pre-Industrial era. Although Hg emissions have declined within the U.S., two-thirds of atmospheric Hg is from reemitted legacy pools on the Earths surface making it important to constrain how Hg cycles within and among ecosystems. High alpine ecosystems can be a sink for up to 80% of atmospheric Hg deposition that reaches them. The ecosystem fates of Hg, however, are largely unknown, including the degree to which it transforms into methylmercury (MeHg), a neurotoxin that bioaccumulates and biomagnifies in food webs. This study investigated patterns of Hg storage and mobilization within food webs across the Niwot Ridge Long-term Ecological Research site in the Colorado Rocky Mountains. Field collection included bulk atmospheric deposition, surface soils across moisture and organic matter (OM) gradients; vegetation from shrub, graminoid, and herbaceous functional groups; and organisms from terrestrial (grasshoppers and pika), aquatic (phytoplankton and zooplankton), and mixed terrestrial-aquatic (caddisflies, ants, birds, weasels, and fish) food webs. All samples were analyzed for total Hg (THg) and MeHg at the Wisconsin State Hygiene Lab. Major findings include that herbaceous plants have significantly higher THg concentrations (9.35 ± 3.65 ng g-1) than graminoids (5.37 ± 2.02 ng g-1) most likely due to greater leaf surface area. Total Hg concentrations increased from fresh vegetation to partially decomposed American Pika food stashes to decomposed litter, probably driven by atmospheric Hg sorbing to particle surfaces over time. Soil THg concentrations were highest in wetter areas of the landscape with greater OM and sulfur concentrations. The highest MeHg values (1.58 to 6.50 ng g-1) were observed in riparian and wetland areas where sulfate reducing bacteria likely stimulate MeHg production. Mercury concentrations in pika hairs were below detection, whereas concentrations in weasel hairs were elevated (625 ± 267 ng g-1). Together, these results indicate that 1) organisms feeding from aquatic parts of alpine landscapes and at higher trophic levels accumulate more Hg in their tissues, and 2) alpine ecosystems play an important role in transforming and storing Hg.