How can you tell if a stream has been polluted? You look at the creatures that live in it.

If chemicals or sediments spill into a stream, they are often swept far downstream before the water can be tested. But their effects can be seen and measured in the myriad of tiny creatures that live in and on the stream bottom -- the benthic invertebrates.

Benthic means bottom-dwelling, and invertebrates are creatures without the bony internal skeletons found in humans and other larger animals. The invertebrates that dwell on the bottom of water bodies are mainly the larval stages of insects like blackflies, mosquitoes, mayflies, stoneflies, and dragonflies.

"They're grazers, predators, shredders. They have all sorts of functions in the aquatic system," says Benoit Godin, head of Environment Canada's Environmental Contaminants section in Whitehorse. They are also an important food source for fish, he adds.

And they are plentiful. A square metre of productive stream bottom might contain as many as twenty thousand individual creatures belonging to a few hundred different species.

Not every stream is that productive. The number and kind of benthic invertebrates depend on the quality of the water and the nature of the stream bed and stream flow. Changes in water quality or the nature of the stream will change the benthic invertebrate population.

That is why benthic invertebrates are so important to scientists. They serve as a warning of change long before the change reaches larger creatures.

"If we go to the fish to look for changes, then usually we have a problem already," says Godin. "We're looking at protection of fish under federal fisheries and contaminants legislation, and we want to be proactive and protective."

Analyzing a sample of benthic invertebrates means looking at the range of creatures that have been collected rather than at individual species, Godin explains.

"Each species of insect has different requirements," he says. "Some are pollution sensitive, some are tolerant of it, and there's a gradient between the two extremes."

If the latest sample from a regular sampling site shows a sharp decline in pollution-sensitive species or a drop in overall numbers of benthic invertebrates, scientists can tell that something has changed the stream. Analysis of the benthic invertebrate sample might even indicate what caused the change.

Before you can draw conclusions, however, you have to know what is in the sample. The Yukon division of Environment Canada sends most of its samples to a laboratory in Victoria. There a taxonomist, a scientist who specializes in identifying different species, sorts and counts the tiny creatures under a microscope. It might take two weeks to sort through the hundreds of species and thousands of individuals contained in a single vial of benthic invertebrates.

"There are very few taxonomists," says Godin. "People who have the patience to do that sort of job have a lot of work in front of them."

They will have even more work in the future. Monitoring benthic invertebrates has become the accepted method of tracking water quality throughout much of the industrialized world.

"Benthic invertebrate monitoring is pretty much part of all mining permit requirements," says Godin. "Environment Canada does some of it, but more and more it is becoming the responsibility of the company to provide the information for the regulatory agencies."

For more information about water quality monitoring in the Yukon, see the Yukon State of the Environment Report 1995, available from Environment Canada in Whitehorse, or contact the Environmental Protection Branch, Environment Canada, Whitehorse, at 667-3400. (Visit the World Wide Web version of the Yukon State of the Environment Report.)