Formula measures pollutant build up in various animals

May 19, 2006, volume 36 number 5
By Stuart Colcleugh

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It seemed obvious to Adrian deBruyn. The same biological processes that cause toxic chemicals to accumulate in fish must work virtually the same way with all animal species.

The problem was, no one had come up with a mathematical model to clarify the relationship. So deBruyn, an ecologist and toxicologist with SFU's school of resource and environmental management (REM), decided to tackle the task.

The result, which he co-developed with REM chemist and toxicologist Frank Gobas, is an elegantly simple formula that promises to dramatically improve our understanding of how persistent organic pollutants (POPs) build up in different animal species.

Gobas developed one of the most widely used mathematical models of how persistent organochlorines such as dioxins and PCBs accumulate in fish. Others subsequently adapted the model for different animal species. But deBruyn sought a universal model that could be extrapolated throughout the animal kingdom to account for variations between species.

“These pollutants are absorbed in animals' diets, but they are often poorly eliminated. So they build up through a process called biomagnification as they are passed up food chains, increasing in concentration with each step,” deBruyn explains.

“That's why pollutants can hammer predator species at the top of a food chain like eagles and wolves, even though they may live thousands of kilometres from any pollution source. Gobas developed a model for fish and others adapted it for groups such as marine mammals and humans. But each model was tailored to a particular species or group of species and the results were very difficult to generalize.”

So deBruyn looked for connections to another universal process, known as bioenergetics, through which organisms manage their energy resources. “All animals essentially do the same stuff. We just do it to different degrees. Some things are more important than others, depending on whether you breathe water or air. Different animals eat different things and live in different ways. But it ended up all coming down to the way animals budget their energy.”

deBruyn and Gobas adapted the equations used in bioenergetics research to model biomagnification in a broad range of animals, from caterpillars to carnivores. Then deBruyn looked for independent studies that had measured bioenergetic parameters for these animals, such as how efficiently they digested their food, how much energy they spent on activity and keeping warm and how efficiently they grew.

“There was a set of about 35 species that fell out for which I could find both sets of data with a reasonable amount of quality,” says deBruyn. “So we used the bioenergetic data to run the model for those species and we compared that to what the real-world biomagnification data showed. And it was spectacular, it was beautiful.”

The model - dubbed BMFmax - matched documented contamination levels to an extraordinary degree for most species. And of equal importance, it revealed that the pollutant absorption rate from an animal's stomach and its growth rate were the two most important biological processes affecting biomagnification. Animals with low absorption rates and high growth rates, such as caterpillars, had much lower contaminant levels than animals with high absorption rates and low growth rates, such as polar bears.

“The model can also accurately predict which organisms are most likely to suffer from the effects of specific chemicals, whether they be foxes, or spiders, or fish or worms,” says Gobas.

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