Crystal clear. But less algae means more mercury in fishes' favorite food.

Murky Ponds Minimize Mercury

The chef presents you with two fish on a plate: One, he says, was caught in a clear mountain lake, the other pulled out of a nearby scummy green pond. What should you do? Take the pond fish.

Researches have known that fish from low-nutrient waters (like the clear lake) have more mercury in their bodies than fish from waters teeming with life. But what caused the difference was not obvious. Now, by tracing the path of mercury in experimental tubs, researchers have singled out algae as an important link in the flow of mercury into water fleas, the dietary staple of many freshwater fish. But somewhat counterintuitively, more algae may translate into less mercury in the fleas.

Aquatic ecologists Paul Pickhardt and Carol Folt and colleagues at Dartmouth College in Hanover, New Hampshire, varied the amount of nutrients in 12 large outdoor tubs to control how much algae grew in each tank. After allowing algae to grow for 9 days, the researchers spiked the mini-ecosystems with an isotope of mercury. They took samples of the algae 24 hours later and added water fleas to the mix shortly after that. Three weeks later, the researchers fed dissolved samples of water fleas into a mass spectrometer to measure mercury. The amount of mercury in the algae went down as the total mass of algae went up, the researchers reported online 18 March in the Proceedings of the National Academy of Sciences. And the water fleas in the clearer ponds had 2 to 3 times more mercury than the high-nutrient ponds--which suggests that any fish that made a meal of the fleas would end up with more mercury too.

The work sheds some light on why mercury piles up in animals in low-nutrient waters, says biogeochemist Jani Benoit at Wheaton College in Norton, Massachusetts. But, she says, it still doesn't answer the fundamental puzzle of why smaller populations of algae concentrate more mercury.

Related links
Folt's site
U.S. Geological Survey page on Mercury in the Environment
FDA advisory on Mercury in fish