Recently, the Food and Drug Administration decided to revisit its recommendations regarding fish consumption for pregnant women because of mercury's harmful effects on developing fetuses. It seems that determining advisories is as complicated as understanding the metal's path through the environment. Now scientists have elucidated one element of its distribution. According to a study published online today by the Proceedings of the National Academy of Sciences, the amount of algae present in a lake may affect how much mercury makes its way up the local food chain. The findings could clarify why concentrations of the element in the water don't always indicate corresponding levels in fish.

Paul C. Pickhardt of Dartmouth College and colleagues set-up and monitored a dozen 450-liter tanks containing varying levels of nutrients and algae as well as precise amounts of traceable mercury and methylmecury (the form that bioaccumulates in fish). They found that the algae incorporated mercury within 24 hours. The researchers next added zooplankton that feeds on algae to the tanks and measured their uptake of mercury. Zooplankton in tanks with low levels of algae, they found, had methylmercury levels two to three times those found in zooplankton from tanks containing higher levels of algal biomass. According to the report, in systems with less algae, each individual cell has a higher concentration of the compound and the zooplankton (which fish feed on) eat more mercury with each meal. "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," study co-author Carol Folt says.

The authors caution that their data contain "a substantial amount of unexplained variation." To better understand differences why fish in similar lakes can have differing levels of mercury, the team plans to study four bodies of water that contain both high and low levels of algae. Discerning the mechanism of mercury distribution is important, Folt comments, 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."