Mercury sources, distribution, and bioavailability in the North Pacific Ocean: Insights from data and models

Abstract

Fish harvested from the Pacific Ocean are a major contributor to human methylmercury (MeHg) exposure. Limited oceanic mercury (Hg) data, particularly MeHg, has confounded our understanding of linkages between sources, methylation sites, and concentrations in marine food webs. Here we present methylated (MeHg and dimethylmercury (Me2Hg)) and total Hg concentrations from 16 hydrographic stations in the eastern North Pacific Ocean. We use these data in combination with information from previous cruises and coupled atmospheric‐oceanic modeling results to better understand controls on Hg concentrations, distribution, and bioavailability. Total Hg concentrations (average 1.14 ± 0.38 pM) are elevated relative to previous cruises. Modeling results agree with observed increases and suggest that at present atmospheric Hg deposition rates, basin‐wide Hg concentrations will double relative to circa 1995 by 2050. Methylated Hg accounts for up to 29% of the total Hg in subsurface waters (average 260 ± 114 fM). We observed lower ambient methylated Hg concentrations in the euphotic zone and older, deeper water masses, which likely result from decay of MeHg and Me2Hg when net production is not occurring. We found a significant, positive linear relationship between methylated Hg concentrations and rates of organic carbon remineralization (r2 = 0.66, p < 0.001). These results provide evidence for the importance of particulate organic carbon (POC) transport and remineralization on the production and distribution of methylated Hg species in marine waters. Specifically, settling POC provides a source of inorganic Hg(II) to microbially active subsurface waters and can also provide a substrate for microbial activity facilitating water column methylation.

Rapidly increasing methyl mercury in endangered ivory gull ( Pagophila eburnea ) feathers over a 130 year record

“…This corresponds with a 45Â increase from our earliest (1880; 0.09 mg g 21 ) and most recent adult sample (2004; 4.11 mg g 21 ; figure 3). Based on these projections, we anticipate that ivory gulls will show MeHg concentrations of 20 mg g 21 , the concentration believed to cause deleterious effects in piscivores [14], by 2105, though oceanic Hg is expected to increase much more rapidly, increasing fourfold between 2005 and 2050 [58,59], meaning we could expect deleterious concentrations in approximately 50 years. The recently signed Minimata Convention, and changes in global climatic patterns, however, will affect Hg distribution in the Arctic, and consequently its uptake by ivory gulls, though to an unknown extent [60,61].…”

Blood dynamics of mercury and selenium in northern elephant seals during the lactation period

“…The most neurotoxic form of Hg that can be found in animals is the organic form, methylmercury e MeHg (Khan and Wang, 2009). Consuming fish elevated in MeHg causes adverse health effects ranging from neurodevelopmental deficits in offspring to increased risk of myocardial infarction in adults (Khan and Wang, 2009;National Research Council, 2000;Sunderland et al, 2009).…”

Methylmercury production below the mixed layer in the North Pacific Ocean

“…3). Published depth profiles of MMHg from the central Pacific 3,4 show increases with depth from ∼20 to 50 fM near the surface to ∼40 to 400 fM at 600 m depth. Our results indicate that between 46 and 80% of MMHg produced in the photic zone is photodegraded there and that 60-80% of MMHg bioaccumulated at 590 m is produced below the SML.…”

“…But it has been suggested that Hg concentrations in surface and shallow subsurface waters of the Pacific Ocean will change significantly over the next few decades 3,10 . For example, it has been predicted 3 that the concentration of THg mercury in North Pacific intermediate waters (200-1,000 m depth) will double by the year 2050, relative to 1995, assuming contemporary atmospheric Hg deposition rates.…”

“…MMHg production in the oxygen minimum zone (OMZ) of the oceanic water column has been hypothesized 7,13 and recent field studies have offered new evidence supporting in situ methylation not only in OMZs (refs 3,6) but also in oxic surface waters 4,5,[15][16][17] . Vertical differences in ocean biogeochemistry and the relative contributions of different Hg sources to the overall MMHg burdens in fish have wide-ranging implications for understanding how MMHg levels in marine biota will respond to future changes in anthropogenic Hg emissions 3,18 .…”