Coastal birds, innocent vectors of heavy metal pollution

Common eider (Somateria mollissima). Credit, Andreas Trepte.Coastal birds are polluting important habitat areas with toxic heavy metals according to a new study in the Proceedings of the National Academy of the Sciences.

Neal Michelluti and fellow researchers looked at two ponds in the Canadian High Arctic where seabirds congregate in large numbers and impact soils through the mass deposit of their droppings.

"Birds feeding on different diets will funnel different 'cocktails' of metal contaminants from the ocean back to terrestrial ecosystems, which can then affect other living organisms," Michelutti said.

This particular study site gave the researchers a unique opportunity to examine this process. The remote Arctic setting meant that any unusually high level of contaminants in the soil would likely have originated directly from the bird droppings rather than a nearby industrial source.

Furthermore the two ponds, which are located just 1km apart, are each visited en masse by a separate species of seabird - the arctic tern which preys on fish and the common eider which dines at a lower level of the food chain on benthic mollusks.

According to the study, "the segregation of the tern and eider seabird colonies into separate drainage basins
of the two study ponds has created a rare situation that allows for a quasi-experimental approach to paleolimnology."

Looking at sediment core samples, the scientists found that the site frequented by terns had elevated concentrations of mercury and cadmium while the area inhabited by common eiders contained higher levels of lead, aluminum, and manganese.

The heavy metals at each site correspond with those previously recorded in the tissue of the respective resident species which provides further evidence that the distinct diets of the two seabirds are contributing to the contamination patterns of the ponds.

This process of avian-assisted contamination could be polluting biologically productive coastal areas across the world given that large seabird colonies occur on every continent.

"The seabirds are obviously not directly to blame for the elevated metal concentrations in the ponds," study co-author Jules Blais says. "They are simply carrying out their natural behaviors and lifecycles, but have become unwitting vectors of pollutants in an increasingly industrial age."

--by Rob Goldstein

Michelutti, N., Blais, J., Mallory, M., Brash, J., Thienpont, J., Kimpe, L., Douglas, M., & Smol, J. (2010). Trophic position influences the efficacy of seabirds as metal biovectors Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1001333107