Toxic Turnabout

He’s conducted science around the globe, from Venice, Italy, to Taiwan, from San Diego Bay to Antarctica, performing studies and assessments on biological light production, environmental health, and marine ecotoxicology.  But when an opportunity came knocking to conduct research in his native homeland, Dimitri Deheyn jumped at the chance.

“I saw it as a way to give back to where I grew up,” said Deheyn, a Scripps Institution of Oceanography at UC San Diego project scientist and native of the African nation formerly known as the Belgian Congo.

Deheyn and several international colleagues recently completed a study analyzing dredging—the excavation of marine sediments—and water toxicity from two lagoons in Togo, a small West African nation that extends to the Atlantic Ocean’s Gulf of Guinea.

Working with Kissao Gnandi, a Fulbright Foundation scholar who came to Scripps in 2006 on a visiting research assignment, Deheyn and his colleagues analyzed sediment cores extracted from two lagoons in Togo’s southern city of Lomé. The researchers compared the toxicological content of sediment from an eastern Lomé lagoon that has been dredged over the years to counteract waste input with sediment from a western lagoon that also receives significant waste input but is not dredged.

With its perceived cleansing effects from dredging, conventional wisdom would say that the dredged eastern lagoon would be healthier, less contaminated, and likely inhabited by fish and other animals more palatable than the non-dredged lagoon.

Not so, say the researchers. In their study published in a recent issue of AMBIO: A Journal of Human Environment, the researchers found that contamination levels of toxic metals, including methyl mercury, which can accumulate in fish and be deadly for human consumption, were higher in the dredged lagoon, appearing to contradict the purported cleansing effect.

Deheyn says the study demonstrates that conventional beliefs about dredging should be evaluated on a case-by-case basis.

“When you consider the two lagoons it was fair to believe that eating fish or swimming in the dredged lagoon would be safer,” said Deheyn. “But with dredging events you have to consider that metals have been in place in the sediment for many years and you run the risk of releasing contaminants that were trapped in the sediment.”

Furthermore, Deheyn says, dredging also removes dissolved organic material that otherwise traps or filters newly added contaminants.

Deheyn says the effects of dredging have been controversial in scientific circles and this latest study adds another piece to the muddy picture. Customized risk assessments are needed wherever dredging is considered around the world, he says.

The contamination levels evidenced in both lagoons were traced to human activities and waste, a result that Deheyn says goes back more than 50 years and is rooted in European colonization of Lomé and early infrastructure designs that route sewage through lagoons rather than into the open ocean.

The authors also detected surprising levels of manganese and zinc within the sediment, elements that have been associated with recent airplane fuel additives and are likely related to an airport located near the lagoons.

In addition to Gnandi and Deheyn, coauthors of the research include Seunghee Han, M. Hassan Rezaie-Boroon, and Magali Porrachia. AMBIO, a journal published by the Royal Swedish Academy of Sciences, was founded in 1972, the year of the first United Nations Conference on the environment. The journal addresses scientific, social, economic, and cultural aspects related to the human environment.

—Mario C. Aguilera