Their results, to be published in the April 2 issue of the Proceedings of the National Academy of Sciences (available online at http://www.pnas.org today), suggest that there is a link between the amount of algae in the water and the amount of mercury going up the food chain, and their findings may help explain why levels of mercury in the water don't always indicate corresponding levels in fish.

In a controlled experiment, the researchers followed mercury as it moved from the water, was taken up by algae, and eventually found its way into small animals called Daphnia, which eat the algae. Daphnia, a type of zooplankton, is in turn a food source for many species of fish.

While not toxic to the Daphnia or the fish at the levels found normally in nature, methylmercury biomagnification presents a serious health hazard for humans and other animals that eat the fish. Under biomagnification, there is a systematic increase in the concentration of elements found in tissue of organisms, as they move up the food chain.

The study finds that when there is a lot of algae present, methylmercury is dispersed widely throughout the single-celled algae. As a result, Daphnia that eat the algae aren't exposed to high levels of mercury. However, in systems with less algae, the mercury is more concentrated on each plant cell, so the Daphnia eat more mercury with each meal.

"Now we understand more fully the connection between mercury in the water and mercury in fish," said Paul Pickhardt, senior author on the paper and a graduate student at Dartmouth. "We suspected there was an algal link, but few laboratories had the technology to make such precise measurements before. With our trace-metal techniques, we've achieved mercury detection levels that are 50 times more sensitive than any other method."

The technology in question provides the ability to find and measure tiny amounts of stable isotopes of mercury. The team simulated a freshwater ecosystem with twelve large polyethylene tanks, about three feet across, each holding 450 liters of water. The researchers added minute quantities of stable isotopes of methylmercury and inorganic mercury and measured how much of each was taken up, first by algae and then by the Daphnia.

Each isotope has a slightly different weight, so the researchers were able to measure and follow that weight signature from the water, through the algae and into the Daphnia. They used an inductively coupled plasma mass spectrometer, or ICP-MS. Dartmouth owns one of the few ICP-MS in the northeast, and Pickhardt and his team have developed new methods to measure the mercury at lower levels than ever before.

"These results tell us that over the season in a lake, changes that cause the algae to increase or decrease can also quickly produce changes in the amount of mercury that moves through the ecosystem," said Dartmouth Biological Sciences Professor Carol Folt, co-author on the paper and Associate Director of Dartmouth's Center for Environmental Health Sciences. "It means that there may be large differences in the mercury in animals over time. This is important because right now, scientists and government officials are trying to figure out how and when to measure mercury in order to issue more precise advisories about human consumption of fish."

Inorganic mercury, the kind used in old thermometers, occurs naturally in the atmosphere, certain rocks, oceans, lakes and soil. Methylmercury, the rarer and more toxic form, is primarily found in lake sediments and at extremely low concentrations in lake and sea water.

In the past 100 years, mainly through industrialization, there is now more total mercury in the atmosphere. It is transported by wind and rain and often ends up in bodies of water miles from its source.

The team discovered that inorganic mercury, which is less toxic, does not biomagnify as it moves up the food chain. In comparison, most of the mercury found in fish is methylmercury, the more toxic kind. The methyl form of mercury biomagnifies, which explains why it can be found at high levels in fish even if it is barely detectable in the water.

While this experiment took place in a freshwater system, the predictions are the same for marine systems. In fact, because algae, along with other items low on the food chain, are so scarce in the ocean, these findings might help understand why marine systems are so susceptible to methylmercury uptake and biomagnification into fish such as yellowfin tuna, shark and swordfish.

Celia Chen, another co-author and a Research Assistant Professor of Biological Sciences at Dartmouth, expresses herself as delighted to find evidence that indicates algae is the missing mercury link in the food chain.

Her previous work found that in animals in the field, there were indeed fluctuations in metals that change with the amount of algae. She points out that more research needs to be done, however, to understand the intricacies of how metals move through the food chain.

"Just because low algae systems appear related to greater transfer of mercury, it doesn't mean that high algae systems necessarily reduce uptake," she said. "In a natural lake, a lake where there is lots of algae, there are more processes going on chemically which affect mercury. We want to understand why some lakes have high mercury in their fish and why some don't."

The researchers will work next to complete an intensive study of four lakes, which represent high and low algal systems. The lakes were chosen from their earlier 20-lake survey across the northeast examining metals in each part of the food chain.

This research, featured in the Proceedings of the National Academy of Sciences, is a result of the collaborative efforts of ecologists and trace-metal chemists from Dartmouth's Biology and Earth Sciences Departments. Such interdisciplinary endeavors are the backbone of the National Institute of Environmental Health Science's Superfund Basic Research Program, which provided the funding for this research. - By Sue Knapp

[Contact: Sue Knapp]

19-Mar-2002