Wednesday, November 26, 2008

Keeping an eye on mercury

Mercury kills, and its presence in the environment is an indicator of potential trouble on the horizon for all species, including humans. Anthony DePalma reports in the NYT on a study recently released by the BioDiversity Research Institute on increased levels of mercury in Bald Eagles in the Catskill Mountains region of New York.

We've known for quite a few years that mercury was a problem in the northeastern U.S. and southeastern Canada. Charlie Driscoll and his colleagues, working under the umbrella of the Hubbard Brook Research Foundation, reviewed the existing data on mercury deposition in the Northern Appalachian ecoregion and showed that mercury levels here routinely exceed human and wildlife health thresholds. Chris Rimmer, at the Vermont Center for Ecostudies, and his colleagues reported in the journal Ecotoxicology in 2005 that even an upland species like Bicknell's Thrush, which inhabits high elevation forests in the Northern Appalachians during the breeding season, has levels of mercury in its blood in excess of accepted health thresholds.

The presence of mercury in a species like Bicknell's Thrush is particularly disturbing. While still not acceptible, mercury in Bald Eagles is at least understandable. Ionic forms of mercury deposited in aquatic ecosystems are converted into methylmercury (a form that causes severe neurological damage and death) by bacteria; methylmercury is then bioaccumulated up through the aquatic food chain and eventually gets incorporated into the tissues of Bald Eagles, which eat a lot of fish.

But Bicknell's Thrush? Either this upland songbird eats far more aquatic insect larvae than previously understood (and methylmercury is bioaccumulating in these short-lived insects to a much greater extent than previously believed) or there are terrestrial pathways from ionic mercury to methylmercury that we don't fully understand, which means that more species are susceptible to mercury poisoning than we thought.

Either way, it's not good news. And neither is the fact that mercury levels are on the rise.


Kent McFarland said...

Steve- Nice post. I have a bit of an updated on the Bicknell's Thrush findings. We are putting together a manuscript right now that examines 4 years of work we did looking at many of the food web compartments. We find Hg from leaves and needles of trees (which accumulate them over the season as they grow), then into the leaf litter and detritus, into invertebrates (and within these, the higher on the food web the more Hg. Spiders are very high), Red-backed salamanders, Bicknell's Thrush, and then raptors (Saw-whet owls are lower than Sharp-shinned hawks). The interesting finding is that Bicknell's actually begin to lose mercury in their system. We think this might be due to a diet shift over the summer from items like spiders early in the season to moth larvae later in the season. So we see mercury moving throughout the food web in this terrestrial environment in s similar manner as one finds in aquatic systems. I will keep you posted on our progress on this work as we near publication.

Steve Trombulak said...

Kent --

In what form is the mercury you are finding in the leaves, needles, leaf litter, etc? Ionic or MeHg?

Kent McFarland said...

This is whole Hg analysis. We just didn't have the funding to do MeHg, which is much much more expensive to analyze. Some other published and unpublished studies have recently looked at MeHg in needles and leaves and arthropods. There is evidence that needles are either taking up MeHg or possibly Hg is methylated in the needles.