杏吧原创

A TOXIC metal is to be deliberately added to a pristine Canadian lake in a
bid to discover how it gets into fish.

People in Canada and most US states have been told to limit their consumption
of freshwater fish because they contain high levels of mercury. But it鈥檚 not
clear whether fish pick up the toxic metal from new deposits from the atmosphere
or from old pollution released from sediments. If sediments are to blame,
expensive measures to control mercury emissions could be ineffective.

To find out, microbiologist Cynthia Gilmour of the Academy of Natural
Sciences in St Leonard, Maryland, and her colleagues plan to add three traceable
mercury isotopes to a pristine lake in north-western Ontario. One isotope will
be added to the lake itself, one to the surrounding wetlands and one to the
surrounding uplands. In total, about half a teaspoon (37 grams) will be added
over five years, giving a mercury deposition rate equivalent to that currently
seen in the north-eastern US.

鈥淔or the first time we will be able to specifically track how changes in
mercury deposition rates affect accumulation in aquatic food webs,鈥� says
Gilmour.

Similar experiments in the past have either fizzled out or ended in disaster.
When Swedish researchers seeded a lake with selenium, which lowers mercury
levels in fish, it poisoned all the fish in the lake. In another study, an
enormous tent erected over an entire watershed to stop the deposition of
atmospheric mercury also upset the natural workings of the lake, ruining the
experiment.

But there are good reasons for the study. The US Environmental Protection
Agency plans to regulate mercury emissions by 2004. The cost to polluting
industries鈥攃oal-fired power stations are the biggest emitters鈥攃ould
run into billions of dollars a year. Two utility groups are attempting to block
the regulations, claiming that the EPA has not quantified the relationship
between mercury emissions from utilities and levels in fish.