Study shows link between clear lakes and contaminated fish
 s spring approaches, thousands of anglers eagerly
anticipate the day they can cast their lines into a clear lake and pull
out fish for dinner. But at the same time, departments of natural resources
in approximately 40 states issue advisories that help fishermen avoid
eating a mercury-contaminated catch.
Now, a team of UW-Madison aquatic chemists and limnologists
has discovered a link between the amounts of dissolved organic matter
(DOM) in bodies of fresh water and increased levels of highly toxic
methyl-mercury in fish.
Methylmercury enters the fresh-water food chain by
binding with microscopic organisms like green algae, which other organisms
subsequently eat. But, using a computer model, the researchers discovered
that when the water also contains high concentrations of dissolved organic
matter, there is a greater tendency for methylmercury to chemically
bind with DOM, rather than green algae. Since DOM is a natural chemical
component of aquatic systems that’s not consumed by organisms
or animals, when methylmercury binds with it, the toxin doesn’t
enter the food web.
Gorski coupled laboratory experiments with computer
modeling that helped him quantify the affinity between methylmercury
and green algae—a relationship researchers previously did not
know. Then he applied the results to a natural system. “I came
up with a model and had algae present, methylmercury present and DOM
present, and tried to predict at what concentrations they would outcompete
each other,” he says.
He began at relatively low DOM levels, like those
found in “clear” northern lakes, and increased DOM concentration
until it roughly equaled that of a more DOM-rich, brown body of water.
“As you start ramping up the DOM concentration, it starts outcompeting
the algae for the methyl-mercury, and then more and more methyl-mercury
gets bound to the DOM,” says Gorski. “So the model predicts,
at really high DOM concentrations, that methylmercury will competitively
bind to the DOM instead of the algae.”
The research may help explain why so many mercury
warnings are issued for fish from clear lakes, says Gorski. But he stresses
that it’s an initial step in being able to predict how methylmercury
enters the bottom of the food chain.
The next step, says Armstrong, would be to determine
what characteristics of DOM control methylmercury bioavailability and
whether those characteristics differ across various fresh-water systems.
“If so, we would like to identify relatively simple methods to
measure these differences so that these measures could be used in surveillance
programs to help identify systems most vulnerable to methylmercury bioaccumulation,”
he says.
He calls the association of methylmercury with natural
dissolved organic matter a double-edged sword. “On one hand, binding
to DOM reduces bioavailability,” he says. “On the other
hand, association with DOM also can carry mercury from surrounding uplands
and wetlands into lakes, meaning that higher DOM inputs into lakes is
not necessarily a ‘good thing’ with respect to mercury levels
in lake food webs.”
Researchers need to understand better the resulting
balance between these two effects of mercury association with DOM, says
Armstrong. In a broader context, they also must learn more about how
quickly mercury levels in aquatic food webs would decline if mercury
emissions into the atmosphere—and their subsequent deposition
onto watersheds—were reduced. “The interaction of mercury
with DOM is one part of the puzzle,” he says.
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