Bioaccumulation and Trophic Transfer of Hg in Aquatic Food Webs

Project Leader: Celia Y. Chen Ph.D. Research Associate Professor, Department of Biological Sciences, Dartmouth Medical School

Project Co-Leaders: Carol L. Folt Ph.D. Professor, Department of Biological Sciences Dartmouth Medical School

Robert Mason Ph.D. Professor of Marine Sciences, University of Connecticut

Clean lake, safe fish?

A loon calls on a pristine lake in Maine. The wind whispers in the pines. You paddle out in your canoe, drop a line—zing!—a fish for dinner. Or is it laced with toxic mercury?

Another lake, outside of Boston. If you're lucky, there's a mallard quacking. Cows graze on the far shore, beside the highway. In places this lake looks as green as the grass; algae drips off your paddle. You drop a line anyway—zing! Should you eat this fish instead?

Yes.

“It's counter-intuitive,” admits Dartmouth researcher Celia Chen, “but fish from the 'pristine' lake in Maine may have higher levels of mercury than fish from the lake surrounded by human beings.”

In 1995, Chen and her colleague Carol Folt began studying the contamination of freshwater lakes with toxic metals: arsenic, lead, cadmium, zinc, and mercury. Mercury was of most concern to their sponsor, the National Institutes of Environmental Health Sciences, because the toxic metal accumulates in the muscles of fish — the fillets we eat. The larger the fish, the more mercury it will contain relative to smaller species in the same lake. But from one lake to the next, the Dartmouth researchers learned, it is harder to guess which fish have higher mercury levels. You can't always predict from how “clean” the water seems whether the fish you catch will be safe to eat.

Consider the green, algae-filled water of that lake outside of Boston. When it comes to mercury, an algae bloom—a rapid increase in the population of algae—can have a surprising effect.

Says Chen, “We found that if a lake had a lot of algae, the concentration of mercury in the fish was often lower.”

Why? Fish don't pick up mercury directly from the water, but algae do. Then the algae are eaten by zooplankton which are eaten by fish. “We reasoned that when the amount of algae suddenly rises, the amount of mercury in any cell of algae would be less,” Folt explains. “So fish would take in the same amount of algae but less mercury.” When the algae are growing really fast the fish can't keep up. Some of the algae go back into the sediments instead of ending up in the fish.

To test their ideas, graduate student Paul Pickhardt and the group developed a completely new experimental technique to measure the movement of mercury from water to algae and then to plankton. They ran these experiments in large cattle tanks. Even though they put the exact same amount of mercury into the water in all of the tanks, after just a couple of weeks the animals in the tanks with the greatest amount of algae had the lowest levels of mercury. “This was very exciting and confirmed our hypothesis that bloom dilution could help explain the patterns that we were seeing in pristine versus algae-rich lakes,” says Folt.

The scientists also made another very important discovery, namely that faster growing fish, or those that were eating healthier food, often ended up with lower concentrations of mercury in their tissues. “This was another very important pattern,” says Folt. “It showed that rapidly growing animals could add body tissue faster than mercury, so that over time, the mercury concentrations became 'diluted' in the tissue of fast growing fish.” They called the process 'growth dilution.'

To test this discovery they turned again to the laboratory. Graduate student Roxanne Karimi together with colleague Nick Fisher from Stony Brook University and the Dartmouth team studied this process using radioactive isotopes of mercury in highly controlled experiments. Their studies were the first to show growth dilution — lower mercury concentrations in faster growing fish — under the types of conditions that could be found in nature.

“We now have found two important ecological pathways, bloom dilution and growth dilution, that make it likely that fish in the more pristine systems, with less algae, will end up with higher concentrations of mercury,” says Folt.

“When we present these results to lake managers,” Chen say, “they're shocked. 'What are you saying? That we should stop controlling algae growth?' And that's not it at all. What we're saying is that our most pristine lakes are often the ones that are most vulnerable to mercury contamination.”

How mercury moves through the environment

Mercury enters the food web from the air. An element found in rocks, some atmospheric mercury is released by volcanic eruptions. But most is emitted from coal-fired power plants. Blown by the wind and washed by rain and snow into streams and rivers, mercury is carried into lakes and estuaries, where some of it settles to the bottom. Bacteria that live in the sediments there attach carbon to the mercury atoms, turning it into methylmercury.

This chemical form is both the most toxic and the easiest for animals to store in their tissues. Its effects have been studied in fish, whales, seals, and seabirds. Methylmercury binds to proteins and easily crosses cell membranes, including the blood-brain barrier and the placenta. A neurotoxin, it harms the brain, affecting memory, cognition, and movement. Affected individuals, such as loons, change behaviors that ultimately reduce their survival and reproduction, putting the population at risk.

People are exposed to mercury mainly through eating fish and shellfish — and 95 percent or more of the mercury in fish is the more toxic methylmercury. According to the U. S. Environmental Protection Agency, fish fillets containing more than .3 parts per million of methylmercury should not be eaten (Canada and the states of Maine and Minnesota suggest you avoid fish with .2 ppm). Fish caught in water with very low concentrations of mercury (less than 1 part per trillion) can nonetheless contain toxic levels of methylmercury. In some aquatic ecosystems, the concentration of methylmercury increases 10 million times as it makes its way up through the food web from microscopic algae to shark and tuna.

But the solution is not to avoid eating fish. Fish contain important nutrients, such as omega-3 oils. Studies show that pregnant women whose diets include fish have healthier children. On the other hand, pregnant women exposed to high levels of mercury can have children with learning issues. To get the benefits of fish, people need to know which fish high are in mercury and which are not.

Scientists like the Dartmouth group are trying to get a clearer understanding of how methylmercury is taken up in aquatic ecosystems and how it ends up in the fish that are most commonly consumed by humans. For this reason she has changed her research focus from freshwater lakes to saltwater estuaries.

“Most of the fish we eat come from marine systems,” Chen notes. “Estuaries are the nursery grounds for these fish. They're also where you find the most contaminants — everything is carried from the watersheds into the estuaries.”

A complex web

If the mercury atoms just sat on the sea-bottom like rocks, there wouldn't be a problem. But when the concentrations of organic carbon, oxygen, iron, and sulfate are right, the bacteria in the sediment change the chemical form of mercury in the sediment to methylmercury. Somehow, the methylmercury works its way out of the sediment and into the water column. Burrowing creatures may move it around; tides and currents may roil the sediment and cause it to swirl up; or it might just chemically diffuse.

Methylmercury is taken up by algae and easily absorbed by creatures that eat it. The algae are eaten by larger creatures, such as copepods or krill, which are eaten by yet larger creatures, and so on up through the food web to whales and sea lions, tuna and humans. The methylmercury is also taken up by animals that live in the sediment such as clams and worms, which are eaten by bottom-feeding invertebrates like crabs and fish.

Many of these steps in mercury's travels are educated guesses, still being filled in with data by scientists such as Chen, Folt, and their collaborator Rob Mason at the University of Connecticut. Yet it's already clear that chemical, physical, and ecological processes are all involved. Chen is interested in how these factors affect the bioavailability of methylmercury — that is, how easily methylmercury is taken up by organisms.

For instance, when Chen tested different saltwater creatures, she found that the concentration of methyl mercury in swimmers was higher than that in bottom-dwellers — even though the methylmercury concentration of the water was lower than the mercury concentration of the sediments.

Nor did the amount of mercury in the sediments tell you if the fish were safe to eat. “Between a pristine site and a polluted site, the concentration of mercury in the sediments can differ by 100-fold,” Chen explains. “But there was only a two- to four-fold difference in the concentration of mercury in the invertebrates and fish we caught in these two sites.

“So what does that tell us about the bioavailability of mercury? Is it 100 times more bioavailable in the polluted site, or only four times? The bottom line is that the bulk concentration of a toxic metal in the sediment is not a good indicator of the bioavailability of the metal.”

To better understand these questions of bioavailability and bioaccumulation, Chen and her colleagues are studying two creatures more closely, one a bottom-dweller and one a swimmer.

The bottom-dweller is Leptocheirus, a tiny crustacean that burrows into sediments. In saltwater tanks in the lab, the researchers will examine the animal's uptake of mercury from the sediment and phytoplankton it eats.

“We're really interested in the role of carbon in that system,” Chen says. “Organic carbon comes from plant matter, from detritus that falls to the bottom of the estuary. Organic carbon can bind to contaminants and keep them from being taken up by phytoplankton—it can keep those contaminants in the sediment.” So carbon should diminish the bioavailabilty of mercury. Yet carbon is also vital to the reaction that turns mercury into methylmercury—which is easier for organisms to take up. Chen hopes that studying Leptocheirus in the lab will untangle carbon's role in making mercury more —or less — bioavailable.

The swimmer is the killifish, a native minnow-sized fish common in estuaries up and down the East Coast. Killifish is the model organism Dartmouth Superfund researcher, Bruce Stanton, uses to understand how arsenic affects the endocrine system, the network of glands that secrete hormones. “The killifish makes a good species for studying toxic metal pollution because you can find it in the most pristine sites and the most contaminated ones,” Chen explains. “It is also a good organism for doing laboratory experiments because killifish are easy to grow.”

In freshwater, Chen and her colleagues had identified growth dilution as a factor that affects bioaccumulation — that is, they learned that the fastest growing fish have the least amount of mercury in their tissues. Fish grow fastest in lakes with the best food supply. To test that finding in saltwater, they will feed killifish (again in saltwater aquaria in the lab) fish-food of their own concoction. “We'll collect a lot of invertebrates from the coast and grind them up to make a food pellet,” she says. The pellets will contain different amounts of nitrogen and phosphorus, important fish nutrients. They will expose killifish to different levels of mercury in the water and sediments. They expect that in saltwater, as in freshwater, the killifish that eat the high-quality food — the pellets highest in nitrogen and phosphorus —will grow more rapidly, diluting the methylmercury in their tissues.

Sampling at high tide

The most painstaking step in Chen's research protocol sounds like it should be easy: testing the water. Because the researchers are interested in the effects of extremely low concentrations of metals, it's very hard to get “pure” water samples. “You can contaminate your own samples so easily,” Chen admits, “and then your data are meaningless.”

In 2008, Chen and her colleagues took samples at high tide from ten sites along the East Coast between Maine and New Jersey. “The site in New Jersey, on the Hackensack River, is very well known for mercury contamination,” she explained. “For all the sites, we wanted places where there were coastal marsh systems, not only manmade structures. Without marsh grass, you don't get the same broad assemblage of animals.” Being able to sample all of the creatures in a complete, natural food web is necessary to understand how the system works.

Chen has been developing her water-testing procedure since she and Folt began their metals research in 1995. Using a fiberglass boat, she carefully scrubs it of all dirt and road-dust before putting it into the water. All her supplies are in plastic tubs that are inside double plastic bags — which were cleaned with an acid solution before being packed since, as Chen says, “Trace metals are everywhere.”

Collecting a water sample takes two people. “One person is 'Dirty Hands' and the other person is 'Clean Hands.' Dirty Hands can open the outer plastic bag. Clean Hands then opens the inner plastic bag — wearing gloves.”

Clean Hands draws the water up through a tube that has also been acid washed and will only be used once. She filters the sample immediately, separating out all particles larger than .4 microns across (by comparison, a human hair is about 75 microns across) then puts the filtered water in one acid-washed sample container, the residue that was filtered out into another, and then puts them both into the inner plastic bag. She closes the inner bag, and Dirty Hands then closes the outer plastic bag.

“But even doing all this, we can still get contamination,” Chen says. So to double-check their procedures, they collect “blanks.”

“We bring a bottle of ultra-clean water with us and put it through a tube and a filter and handle it the same way as our field samples. If we've added any metal to our samples, it will show up in the blanks.”

It would be a lot easier to filter the samples in a laboratory clean-room, but that would have to be done very soon after the sample is taken. “If you wait too long, the fine particles you want to filter out — if they're algae — will start to break down,” explains Chen. “Algae are so delicate. It doesn't take much for them to degrade.”

In addition to a water sample and a blank, for each of the 10 sites, Chen and her colleagues took three sediment samples, and five to 10 samples of organisms, such as fish, snails, and crabs (each with five to 10 individuals). Each is then analyzed in the lab at Dartmouth for inorganic mercury and methylmercury.

“Next summer, we're planning to go back to three sites and take more samples to see how much variability there is within the system.” For instance, in any estuary there are areas where the sediments collect and others where the currents wash them downstream. This creates a lot of variation in the amount of organic matter and metals in different areas of a single estuary.

“Nobody has compared such a broad range of sites and examined both the sediment biogeochemistry and the resident food webs. But this is what we've tried to do since the beginning of this project: First do extensive fieldwork across a broad range of sites, then go back and do more intensive sampling to understand the factors that control the fate of contaminants in these food webs.”

Recent Publications

Celia Chen Pubmed

Carol Folt Pubmed

Robert Mason Pubmed