More Mercury Deeper

Figure 2 from Drazen et al. 2009. Log-transformed mean THg concentrations (μg/kg) at the mean log(mass) of 4.24 or approximately 17.4 kg plotted as a function of median depth of occurrence for 9 species of pelagic fishes.

Figure 2 from Choy et al. 2009. Log-transformed mean THg concentrations (μg/kg) at the mean log(mass) of 4.24 or approximately 17.4 kg plotted as a function of median depth of occurrence for 9 species of pelagic fishes.

ResearchBlogging.org

Mercury is distributed across the earth whether it is in the atmosphere, biosphere, or geosphere.  In the marine realm, the methylated form of mercury (CH3Hg+) is the form of mercury most easily bioaccumulated. But let’s take a step back and ask how the oceans got mercury in the first place.  One source may be from the atmosphere.  Fifty percent of atmospheric concentrations originate from volcanic activity and recently, the other fifty percent from anthropogenic sources such as mining activities and manufacturing.  Some input may also occur from terrestrial sources such as rivers and groundwater carrying inorganic mercury into coastal regions. Recent work suggests that biomediated methylation of mercury occurs in the oceans by sulfate-reducing bacteria living in low oxygen regions. The study suggests that these bacteria sink to mid-depths, where they decompose and release methylmercury. This biomethylated mercury accounts for as much as 29% of all mercury in subsurface ocean waters of the Pacific.  Methylated mercury concentrations are highest below the thermocline and nearly undetectable in surface waters. Findings for another groups largely corroborate this model for the Mediterranean, i.e. not all methylated mercury comes directly from coastal or river sources .  But the relative contribution of all these sources, whether the methylated form that enters into the ocean or formed in situ, and many other specifics of the process are either unknown or only recently understood.

A recent study by Choy et. al in PNAS begins to look at these depth related patterns in mercury concentration for bioaccumlation in 9 predatory pelagic fish and 56 species of their prey (cephalopods, fishes, and crustaceans) in the Pacific.  After accounting for age and size, both positively correlated with methylmercury concentrations, the researchers can explain 76% of mercury concentrations with depth where both prey and predator species spends most of their time.  This deeper depth, 600-1000 meters, corresponds to the low oxygen depths in the Pacific.

These results lend support for both the in situ biomethylation of mercury in the open oceans and that it is enhanced in oxygen poor depths.  Choy et. al’s data support recent conclusions that the main source of methylmercury in the open ocean is from the deep water column and not export from coastal regions or the euphotic zone.”

Of course, this also suggests that eating deep-sea fish may also put you at a much greater risk of mercury poisoning.  Just one of the many reasons not to eat our deep-sea brethren.

Source Wikipedia Commons: This chart shows the level of atmospheric mercury deposition detected in ice cores from the Upper Fremont Glacier in Wyoming. Heightened deposition rates correspond to volcanic and anthropogenic events over the past 270 years. Preindustrial deposition rates can be conservatively extrapolated to present time (4 ng/L; in green) to illustrate the increase during the past 100 years (in red) and significant decreases in the past 15-20 years.

Source Wikipedia Commons: This chart shows the level of atmospheric mercury deposition detected in ice cores from the Upper Fremont Glacier in Wyoming. Heightened deposition rates correspond to volcanic and anthropogenic events over the past 270 years. Pre-industrial deposition rates can be conservatively extrapolated to present time (4 ng/L; in green) to illustrate the increase during the past 100 years (in red) and significant decreases in the past 15-20 years.

Sunderland, E., Krabbenhoft, D., Moreau, J., Strode, S., & Landing, W. (2009). Mercury sources, distribution, and bioavailability in the North Pacific Ocean: Insights from data and models Global Biogeochemical Cycles, 23 (2) DOI: 10.1029/2008GB003425 (see also Cossa et. al 2009)

Choy, C., Popp, B., Kaneko, J., & Drazen, J. (2009). The influence of depth on mercury levels in pelagic fishes and their prey Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0900711106

Dr. M (1634 Posts)

Craig McClain is the Assistant Director of Science for the National Evolutionary Synthesis Center, created to facilitate research to address fundamental questions in evolutionary science. He has conducted deep-sea research for 11 years and published over 40 papers in the area. He has participated in dozens of expeditions taking him to the Antarctic and the most remote regions of the Pacific and Atlantic. Craig’s research focuses mainly on marine systems and particularly the biology of body size, biodiversity, and energy flow. He focuses often on deep-sea systems as a natural test of the consequences of energy limitation on biological systems. He is the author and chief editor of Deep-Sea News, a popular deep-sea themed blog, rated the number one ocean blog on the web and winner of numerous awards. Craig’s popular writing has been featured in Cosmos, Science Illustrated, American Scientist, Wired, Mental Floss, and the Open Lab: The Best Science Writing on the Web.





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