How do contaminants such as flame retardants and mercury make their way into the Arctic Ocean food chain? This question is crucial to northern people who depend on marine mammals for part of their diet. Robie Macdonald, a marine geochemist with Fisheries and Oceans Canada and one of the world’s leading experts in the field, says that one of the clues to solving the puzzle is fresh water, and a second is the organic carbon cycle. Both are key to sustaining life. 

To understand how contaminants get into the Arctic Ocean and enter the tissues of animals, says Macdonald, you need to know how the ocean’s environment works. “You have to look at the Arctic Ocean as a whole — ice, currents, production and destruction of organic carbon and, of course, the food web that leads ultimately to iconic animals such as polar bears. All of this forms the connective tissue that links contaminants from deposition to emergence at levels of concern in top predators.”

What’s the main thing that makes the Arctic Ocean different from other oceans? “Most people will say ice,” says Macdonald. “But the ice in the Arctic Ocean is like a magician’s right hand — the one he wants you to watch. The hidden things, which make his trick work, are in his left hand. For the Arctic Ocean, that’s fresh water. Fresh water is the reason the Arctic Ocean freezes.”

Fresh water is constantly pouring into the Arctic Ocean from the many rivers that drain the Arctic landscape. This stratifies the surface water of the Arctic Ocean, producing a low-density surface layer that is isolated from the heat in the deeper ocean, enabling it to freeze.

All that fresh water carries material from the land and deposits it in the ocean. “The Arctic Ocean is heavily influenced by what is happening on land,” says Macdonald. “Much of the change in the Arctic Ocean originates there. When permafrost thaws or vegetation changes, rivers bring different materials into the ocean.” Rivers can also carry contaminants which were transported by wind from distant industrial sites before settling onto the ground, later to be washed into a watercourse. The same stratification that enables the ocean’s surface to freeze also prevents surface water from mixing with deeper water. This limits primary production — the growth of the plants at the bottom of the food chain — which in turn facilitates the entry of contaminants into the ocean’s food webs.

Macdonald and other researchers use stable isotopes to distinguish the sources of fresh water in the Arctic Ocean in order to shed light on the pathways contaminants take before entering the ocean and where they concentrate once they arrive.

Macdonald’s research, as part of Aboriginal Affairs and Northern Development Canada’s Northern Contaminants Program, contributed to Canada’s successful international efforts to curb the release of persistent organic pollutants such as PCBs. For the future, he says, more and better information — especially from long-term environmental monitoring — is crucial to understanding how contaminants behave in a changing Arctic Ocean. He draws inspiration from the citizens of the North. “The people most affected by change in the Arctic are those who live there”, says Macdonald. “Future generations will have to deal with change: we need to leave them the tools to deal with it.”

Robie Macdonald is the 2014 laureate of the Canadian Polar Commission’s Northern Science Award, which recognises a significant contribution to knowledge and understanding of the Canadian North and the transformation of knowledge into action.