RESUMEN
Atmospheric deposition is the main source of mercury to the open ocean while gas evasion and particle sinking are the major pathways for removal of mercury from surface waters. Evasion rates for mercury at the sea surface in the equatorial Pacific Ocean were estimated from measurements of mixed layer dissolved gaseous mercury obtained during a cruise in January through February 1990 aboard lhe NOAA Malcom Baldrige. The average flux of 500 ± 340 pmol m-2 day-1 day was similar to previous estimates for this region. Preliminary fux calculations for lhe mixed layer of lhe equatorial Pacific Ocean suggested that high evasion rates of Hg° could only be supported by lhe supply of HgR via upwelling, and subsequent fortnation of Hg° within the mixed layer. A numerical model was developed to test these observations. The steady-state, one-dimensional advectiondiffusion-reaction model took into account vertical eddy diffusion, particulate uptake and release of dissolved mercury, particle sinking, and upwelling. Model simulations confimed that lhe system is severely substrate limited, wilh maximal sustainable evasion rates of less than 200 pmol m-2 day-1 in the absence of upwelling. Additionally, lhe model results suggested that upwelling rates of 80 to 160 m year would be sufficient to support an evasion rate of 500 pmol m-2 day-1 with physical conditions and Hg concentrations similar to those found during the cruise. This work has provided futher evidence lhat lhe supply of labile inorganic mercury Iimits Hg° formation (and methylation) in lhe aqualic environment.