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Lake Superior Holds Onto Her Dead ... and Her Toxaphene

Water testing.

A researcher takes a sample from Lake Superior to test for pollutants.

Folklore and fact merge in the saying “Lake Superior never gives up her dead.” Bodies tend to remain sunken because Lake Superior’s frigid water slows bacterial action. (Bacteria operating in warmer waters generate enough gas to make dead bodies float after a few days.)

“Lake Superior doesn’t give up her dead because of bacteria and cold water, and she doesn’t give up pollutants for the same reasons,” said Matt Simcik, associate professor of environmental health sciences at the University of Minnesota Twin Cities. “However, here’s the twist. Even though they work slowly, bacteria make up such a large portion of the metabolic activity in Lake Superior that they play a great role in moving energy and toxins into the food web. When a toxin is locked up in the food web, it can’t sink into the sediments, evaporate into the air, or flow out with the water.”

With funding from Minnesota Sea Grant, Simcik and his colleagues reviewed the processes driving pollutants in and through Lake Superior. The resulting article, “Synthetic Organic Toxicants in Lake Superior,” published in the peer-reviewed journal Aquatic Ecosystem Health and Management, summarizes 25 years’ worth of investigations. The researchers report that Lake Superior’s microbial community, cold temperature, vast surface area, and long retention time allow airborne pollutants to enter the lake and stay. They document that:

  • Most toxins make a long journey through the atmosphere before encountering Lake Superior.
  • Once in Lake Superior, a toxin’s fate depends on its ability to cling to or become part of larger things, like algae.
  • Toxins accumulate as they move up food webs.
  • Most toxins are recycled into the food web by microbes.

Toxins in the form of gas or tiny particles become parts of raindrops, snowflakes, and fog. This is true for the most notorious organic pollutants, like PCBs and DDT, which become more concentrated as the water of the Great Lakes flows toward the ocean.

Lake Superior has just the right conditions to hold onto toxaphene, however it swirls with twice as much as Lake Michigan and five times more than Lake Erie.

Toxaphene, one of the most heavily used pesticides in U.S. history, is a composite of more than 670 chemicals. Although the compound was banned from use in the U.S. in 1990, toxaphene persists in the environment with other “legacy” pollutants like PCBs and DDT, and more recent contaminants. It is known to cause adverse health effects in humans and is toxic to aquatic life.

Toxaphene’s atypical behavior stems from its predominantly gaseous form in the atmosphere, and Lake Superior’s size and temperature. Gases, like oxygen and toxaphene, are particularly chummy with Lake Superior. They nimbly cross air-water boundaries and Lake Superior provides them with a particularly broad one. Additionally, cold water (like Lake Superior’s) can absorb higher concentrations of gases than warmer waters (like Lake Erie’s).

For toxaphene and for other organic toxicants, entering Lake Superior is easier than exiting. This is true for four reasons:

  • Colder water decreases the chance that gases can diffuse back into the atmosphere,
  • The lake has a slow evaporation rate,
  • A drop of water (and whatever floats around in it) stays in the Lake Superior Basin for nearly 200 years,
  • Not much gets buried in the lake’s sediments, which build slowly because the lake is unproductive and almost all nutritional items are kept in the food web.
Map of Isle Royale's Lake Siskiwit.

Several studies conclude that over 90 percent of the organic toxicants polluting Lake Superior dropped out of thin air. A compelling example involves fish and an island.

The same contaminants and, in some cases similar concentrations, were discovered in lake trout from Isle Royale National Park’s Lake Siskiwit and from Lake Superior. Since no industries operate on the banks of Lake Siskiwit, the contaminants in question must ride there on winds as tiny particles or gases.

Once in the water, organic toxicants are like burrs; they stick to all sorts of organic things, like zooplankton excrement or algae. If the pollutants hook onto sinking matter, they might ride it to the lake floor. Before the matter can settle into the sediments, populations of voracious bacteria liberate the pollutants. Freedom gives the toxicants a chance to stick to the tiniest species in the benthic food web. One study indicated that only 8 to 33 percent of polycyclic aromatic hydrocarbons (PAHs) escaped the recycling efforts of Lake Superior’s bacterial communities and made it into the sediments.

Organic toxicants also enter the food web when they stick to algae that are then eaten by zooplankton. Or unbound toxicants can work their way into the fatty tissue of a fish after crossing its gill membranes.

When death forces an organism to give up its worldly goods, its treasury of toxicants is either passed to the thing that ate it or recycled by bacteria back into the base of Lake Superior’s food chain. As one thing eats another, toxaphene-like compounds add up, becoming more and more abundant in animal tissues. In aquatic food webs, contaminants may be 106107 times more concentrated in top predator fish as compared to the water they swim through. The bioaccumulation of toxins in Lake Superior sport fish is great enough to warrant fish consumption advisories.

“As we’ve witnessed with toxaphene, pollutants can behave differently in Lake Superior,” said Simcik. “This is important to remember, especially as new chemicals of concern come into focus.”

To order a copy of “Synthetic Organic Toxicants in Lake Superior,” view JR 518 offered on the journal reprints page.

By Sharon Moen
February 2007

Return to February 2007 Seiche

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