Resolving a paradox—high mercury deposition, but low bioaccumulation in northeastern Puerto Rico

  • James B. ShanleyEmail author
  • Mark Marvin-DiPasquale
  • Oksana Lane
  • Wayne Arendt
  • Steven Hall
  • William H. McDowell


At a “clean air” trade winds site in northeastern Puerto Rico, we found an apparent paradox: atmospheric total mercury (THg) deposition was highest of any site in the USA Mercury Deposition Network, but assimilation into the local food web was quite low. Avian blood THg concentrations (n = 31, from eight species in five foraging guilds) ranged widely from 0.2 to 32 ng g−1 (median of 4.3 ng g−1). Within this population, THg was significantly greater at a low-elevation site near a wetland compared to an upland montane site, even when the comparison was limited to a single species. Overall, however, THg concentrations were approximately an order of magnitude lower than comparable populations in the continental U.S. In surface soil and sediment, potential rates of demethylation were 3 to 9-fold greater than those for Hg(II)-methylation (based on six radiotracer amendment incubations), but rates of change of ambient MeHg pools showed a slight net positive Hg(II)-methylation. Thus, the resolution of the paradox is that MeHg degradation approximately keeps pace with MeHg production in this landscape. Further, any net production of MeHg is subject to frequent flushing by high rainfall on chronically wet soils. The interplay of these microbial processes and hydrology appears to shield the local food web from adverse effects of high atmospheric mercury loading. This scenario may play out in other humid tropical ecosystems as well, but it is difficult to evaluate because coordinated studies of Hg deposition, methylation, and trophic uptake have not been conducted at other tropical sites.


Methylmercury Trophic uptake Demethylation Puerto Rico Sulfate reduction Iron reduction 



We thank Angel Torres and Manuel Rosario for help with sampling, and USGS staff in Menlo Park (CA) for laboratory analyses (Jennifer Agee, Le Kieu, Evangelos Kakouros, and Michelle Beyer). We acknowledge Michael Bank for facilitating analysis of biota and vegetation samples, which motivated this study. This research was supported by the USGS Water, Energy, and Biogeochemical Budgets (WEBB) program, which was funded by USGS Climate Research and Development (now Land Resources Science Program). Supplemental support for MM-D was provided by the USGS Toxic Substances Hydrology Program. Support was also provided by NSF grant DEB-1457805 and the Luquillo Critical Zone Observatory NSF grant EAR-1331841. Part of this research was carried out in cooperation with the University of Puerto Rico. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflict of interest.

Ethical approval

All applicable international, national, and/or institutional guidelines for the care and use of animals were followed.

Supplementary material

10646_2019_2108_MOESM1_ESM.docx (295 kb)
Supplementary Materials


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Authors and Affiliations

  1. 1.U.S. Geological SurveyMontpelierUSA
  2. 2.U.S. Geological SurveyMenlo ParkUSA
  3. 3.Biodiversity Research InstitutePortlandUSA
  4. 4.USFS, International Institute of Tropical ForestryLuquilloUSA
  5. 5.Department of Ecology and Evolutionary BiologyIowa State UniversityAmesUSA
  6. 6.Department of Natural Resources and the EnvironmentUniversity of New HampshireDurhamUSA

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