Abstract

The amount of mercury cycled in the atmosphere and upper ocean waters has increased two to three-fold since industrial times.¹ Mercury can enter the environment through various anthropogenic and natural sources such as fossil fuel combustion, mine tailings, and hydrothermal vents.⁷ Once introduced to water and sediments, inorganic mercury can be converted to its organic form, monomethylmercury (CH₃Hg⁺ or MMHg), which is a potent neurotoxin.² Because this environmental contaminant is able to cross the blood-brain and placental barriers, it can have severe toxicological effects on marine carnivores.²

We are investigating the prevalence of total mercury and monomethylmercury in nearshore seawater at Año Nuevo, a 4,000-acre reserve located along the Central California coastline.⁵ This reserve hosts dense populations of pinniped and marine bird species, including the largest mainland breeding rookery of the Northern elephant seal, Mirounga angustirostris.^4,6 Initial colonization by the Northern elephant seal population began in 1961 and the rookery has since grown to over 4,000 individuals.⁶ Although the reserve is located in an undeveloped area isolated from direct anthropogenic and geologic mercury inputs, a 1977–1978 study reported markedly elevated concentrations of mercury in the tissue of mussels from Año Nuevo relative to the 42 other coastal sites that were evaluated.⁴ Because MMHg is a bioaccumulative contaminant, its concentration is magnified at each successive link in the food chain.¹ As a result, MMHg concentrations in aquatic predators can be orders of magnitude higher than the water in which they live.^3,7 Therefore, seawater impacted by elephant seal fecal matter, molted integument, and decaying carcasses may contain elevated mercury concentrations relative to other coastal systems.

In April 2012 we collected unfiltered total mercury and MMHg samples from nearshore seawater at six locations along the Año Nuevo shoreline. Our sampling event coincided with the female and juvenile molting seasons, during which hundreds of female and juvenile elephant seals assemble on the reserve's mainland beach to conserve energy while shedding their entire top layer of skin and corresponding fur in a matter of four to six weeks.⁸ Total mercury concentrations in Año Nuevo nearshore seawater was as high as 41 pM - an order of magnitude higher than typical concentrations found at other local coastal sampling sites (< 5 pM). Our preliminary results suggest that top marine predators, such as the Northern elephant seal, may interrupt the conventional bioaccumulation of mercury up the trophic food web by contaminating lower levels in semi-enclosed ecosystems, essentially recycling mercury in the nearshore marine environment.

Acknowledgements

This project would not be possible without the financial support of grants from Stevenson College of University of California, Santa Cruz, The Friends of Long Marine Lab, Institute of Marine Sciences, University of California, Santa Cruz, and the Dr. Earl H. Myers and Ethel M. Myers Oceanographic and Marine Biology Trust of Pebble Beach. We are grateful to Jeremy Merckling, Kristina Hayes, Emily Edwards, and Sina Sadeghi for field support and members of the Flegal Biogeochemical Cycling of Trace Metals Laboratory at University of California, Santa Cruz for technical assistance.

* Presenting author

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