Zooplankton diet of mercury varies. (Environment).

In lake ecosystems, methylmercury--which can be toxic to people--moves up the food chain, from algae to minute, floating animals called zooplankton to fish. New experimental evidence demonstrates that the amount of methylmercury in zooplankton decreases dramatically after an algal bloom.

On the basis of computer models and field samples, some researchers have suspected that such blooms dilute toxic metals by spreading them out among the much larger number of individual algae cells--and thus offering zooplankton less-contaminated algae to feed upon.

To test this idea, graduate student Paul C. Pickhardt of Dartmouth College in Hanover, N.H. and his colleagues at Dartmouth and the University of Michigan in Ann Arbor set up 12 hot-tub--size water tanks to simulate a simple lake ecosystem. The researchers filled the tanks with water and different amounts of algae and then added various amounts of nitrogen and phosphorous nutrients. As expected, more algae grew in the tanks with more nutrients.

Next, the team added elemental mercury and methylmercury, an organic form of the toxic metal. The scientists used two isotopic versions of the metal that they could later identify using sensitive laboratory equipment. For the elemental mercury, Pickhardt and his colleagues used mercury-201, an isotope containing 201 protons and neutrons. In contrast, the methylmercury added to the tanks contained mercury-200.

Two days after adding the traceable mercury, the researchers introduced a common zooplankton called Daphnia to the tanks. Then, 2 and 3 weeks later, the researchers took samples of the water, algae, and Daphnia.

Measuring mercury-200 and mercury-201 during the experiment, the team found that algal cells in those tanks with larger algae populations contained, on average, less mercury and methylmercury. Moreover, Daphnia in tanks with more algae contained much less methylmercury than did Daphnia in tanks with less algae. The results also showed that--regardless of the amount of algae present--Daphnia always accumulated much more methylmercury than elemental mercury.

This data indicates that fish could be exposed to 3 to 4 times as much methylmercury when a lake isn't experiencing an algal bloom as when it is, says research team member Carol Folt. The work appears in the April 2 Proceedings of the National Academy of Sciences. --J.G.