Superfund Research Program

Clues to Methylmercury Levels in Freshwater Fish

Release Date: 05/01/2002

Mercury enters the atmosphere from natural sources, such as volcanoes and forest fires; as a by-product of coal combustion to generate energy; from the use and disposal of mercury in products such as electric lights, computers, and blood pressure gauges; and from the production of chlorine and caustic soda. When mercury is deposited in lakes or waterways, bacteria convert some of it to a much more toxic form - methylmercury (MeHg). Methylmercury is recognized as one of the most hazardous environmental pollutants and at high doses MeHg is a potent human neurotoxin that causes demyelination and delayed nerve conduction.

Fish accumulate metals throughout their lifetime and are a primary source of toxic metal exposure for humans. Mercury concentrations in fish have been related to metal burdens in their zooplankton prey, but potentially dangerous mercury concentrations have been found in fish from lakes that appear by other criteria to be "pristine". Work conducted at Dartmouth College in 20 Northeast USA lakes showed that metal levels in the water alone are not very useful for predicting the level of several metals in fish across this gradient of lake types.

diagram of aquatic food web.

Biological Sciences Professor Carol Folt and graduate student Paul Pickhardt at Dartmouth College worked with a team of researchers at the University of Michigan in studies designed to identify mechanisms that enable toxic metals such as MeHg to move through aquatic food webs into fish. They tested the hypothesis that mercury inputs to lake food webs are impacted by the changes in algal biomass that follow nutrient additions to aquatic systems (primarily phosphorus and nitrogen).

Folt's research team simulated freshwater ecosystems in mesocosm stock tanks inoculated with phytoplankton and zooplankton. Six levels of nutrient loading were examined and the team added constant levels of unique stable isotopes of Hg to the experimental mesocosms. The researchers then measured the uptake of MeHg and inorganic mercury by algae and Daphnia, a common zooplankton herbivore known to be a major food for many planktivorous fish.

One achievement of Dr. Folt's team is that, primarily due to the efforts of Dr. Bjoern Klaue, they achieved spike detection limits 50-100 times lower than those of previous techniques. As a result, they are able to study the extremely low aqueous Hg concentrations that are typical of natural systems.

They made three significant discoveries:

  • "Bloom dilution" occurs. The tanks with greater nutrient enrichment had greater algal biomass, so the same amount of mercury was distributed among a greater number of cells. This resulted in lower mercury per gram of algal material.
  • The concentration of MeHg in Daphnia is related to the concentration of MeHg in the algal cells they ingest. Specifically, MeHg concentrations were consistently and significantly lower in Daphnia from the high nutrient, high initial algal biomass tanks compared to Daphnia from the low nutrient, low initial algal biomass tanks.
  • Zooplankton that graze on algae preferentially accumulate MeHg relative to inorganic Hg. Bloom dilution of inorganic Hg concentrations in the algae had no measurable influence on the accumulation of inorganic Hg in Daphnia. To our knowledge, this is the first study to experimentally demonstrate the preferential accumulation of MeHg relative to inorganic Hg in grazing invertebrates feeding on a relatively intact phytoplankton assemblage.

These results suggest that there is a link between the amount of algae in the water and the amount of mercury going up the food chain.

On March 1, 2002, the Food and Drug Administration (FDA) announced that it will soon schedule a meeting of its Foods Advisory Committee to review issues surrounding methylmercury in commercial seafood. This review will include a re-examination of FDA's most recent Consumer Advisory for pregnant women and women of child-bearing age who may become pregnant. The results of the Dartmouth study indicate that over the season in a lake, changes that cause algae to increase or decrease can also quickly produce changes in the amount of mercury that moves through the ecosystem. This finding is particularly important now as scientists and government officials try to determine how and when to measure mercury in order to issue more precise advisories about human consumption of fish.

For More Information Contact:

Carol L Folt
Dartmouth College
Department of Biological Sciences
Provost Dean of the Faculty
Hanover, New Hampshire 03755-3576
Phone: 603-646-3999

To learn more about this research, please refer to the following sources:

To receive monthly mailings of the Research Briefs, send your email address to