Methylmercury in Coastal Marine Fog

Methylmercury in Coastal Marine Fog

 

 

Our research group has recently observed large (pM) concentrations of toxic monomethyl mercury (MMHg) in marine advective fog relative to rainwater from the same region.  MMHg has the ability to bioaccumulate within the food web, potentially posing serious health risks for upper trophic groups, including humans. The processes behind this finding, as well as the impacts to coastal watersheds and trophic interactions, has become the subject of a collaborative multidisciplinary program investigating the cycling of mercury from oceans to fog and deposition to land.  The coastal zone is a complex and dynamic mosaic of water masses and biological processes forced by interactions with the atmosphere, continent and shelf. As such, the biogeochemical cycling of Hg in the California coastal zone is incompletely understood. Nonetheless, some general features of the distribution and cycling of Hg in this region are emerging. The focus of our work is to identify the marine features and mechanisms associated with this transfer.

 

Forms/species of Mercury (Hg)

Elemental Mercury (most abundant, stable form) - Hg°

Inorganic (highly reactive, most unstable form) - Hg(II) / Hg²+

“Organic” Mercury (most toxic form)

–Mono-methylmercury - MMHg   (CH3-Hg+)

–Di-methylmercury  - DMHg (CH3-Hg-CH3 )

A series of cruises sought to break up the coastal zone into compartments (mixed layer, bottom sediments, microlayer, fog, etc) and measure the various mercury species within each compartment. Using some traditional tools of oceanography like the CTD (Conductivity, Temperature, Depth) rosette and plankton tows, mixed with some specialized equipment (glass plate microlayer sampler, active strand fog collector), the Chemical Oceanography and Marine Pollution Studies Labs characterized marine mercury concentrations along the western US seaboard. The cruises led to some interesting discoveries that morphed into graduate student theses. One of these included the enrichment of MMHg in the sea-surface microlayer, which could be a source for MMHg in coastal fog as surface water aerosolizes with wave action. CTD and water collection data exhibited a MMHg and DMHg maxima at a depth that couldn't be explained by low oxygen levels similar to other studies.