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Algae as Missing Link

Biologists recently got one step closer to solving the mystery of how methyl mercury moves up the aquatic food chain. A new study by Dartmouth College biologists suggests a link between the amount of algae in a water body and the amount of mercury passed up the food chain in that same water body and could explain why levels of mercury in water don’t always correspond with those found in fish.

The study found that when lots of algae is present, methyl mercury is dispersed widely throughout the single-celled algae. Since the mercury is dispersed, the daphnia (water fleas) that eat it are not exposed to high levels of mercury. However, when there is less algae present, the mercury becomes more concentrated, and the daphnia take up more of it with each meal.

Algae may also explain why certain California water bodies like Clear Lake that have been impacted by mining and should have high levels of mercury in fish, don’t.

"By all rights Clear Lake should have among the highest fish mercury in the world," says U.C. Davis’ Darell Slotton. "But it doesn’t—not nearly." What Clear Lake does have, says Slotton, is massive densities of algae and other suspended solids in the water column. Nearby Davis Creek Reservoir, says Slotton, has clearer water but three times the methyl mercury in identical fish. So while too much algae is ordinarily considered a water quality problem, in Clear Lake at least, it may be providing a benefit.

"If the algae problem was lessened, my prediction is that the mercury problem would get proportionally that much worse," he says. Slotton hopes that his CALFED study on Cache Creek, which will be completed this year, will clarify the mechanisms by which mercury is transferred. LOV

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