If you stare at the bottom of a shallow stream for long enough, things start to move. Small things creep, and wriggle, and scoot, busy with mysterious activities in their underwater world.

Scientists call them benthic invertebrates, which simply means insects and similar small creatures that live in and on the stream bottom.

If you watch them long enough, you might begin to recognize several different types of organisms, all making a living on the same small patch of streambed.

Which organisms you'll find in a stream depends on the nature of the stream, says Benoit Godin, head of Environment Canada's Environmental Contaminants section in Whitehorse.

Recently Godin checked two Whitehorse-area creeks, McIntyre Creek and McCrae Creek, for benthic invertebrates. McIntyre Creek, he found, is populated mainly by Diptera (blackflies) and Ephemeroptera (mayflies). In McCrae Creek, most of the creatures belong to the families Plecoptera (stoneflies) and Trichoptera (caddisflies).

"Nutrients and oxygen make the difference," he says.

There's a large marshy wetland on McIntyre Creek, rich in algae, which means that the creek below the marsh is high in nutrients. That suits blackfly and mayfly larvae just fine.

Godin says he found many of the blackfly larvae stuck to the algae, happily using fans on either side of their heads to filter and collect broken bits of algae and detritus. The larvae, he says, look like tiny zucchini.

Mayfly larvae also thrive on high levels of nutrients, he says. In fact, mayflies have to take their nutrients while they can. When they metamorphose into adults, they have no digestive tract and never eat again. Mayflies emerge as adults, mate, and lay their eggs in the space of about three days. Then they die.

McCrae Creek, in contrast to McIntyre Creek, is clear and smaller mountain creek, with lower levels of nutrients and high levels of oxygen. The creatures Godin found in McCrae Creek prefer that kind of environment.

Stoneflies are particularly sensitive to the oxygen content of the water, says Godin.

"Like most aquatic insects, they breathe through gills," he says. " The gills are little tufts of what looks like hair under their legs. That's the only way they have to breathe. However their gills are smaller than most insects, so they cannot afford lower oxygen levels in the water."

Caddisflies, or Trichoptera, are among the more interesting benthic creatures to look for. Most of them produce cases to protect the soft portions of their bodies.

Some just spin nets, says Godin, but others build elaborate cases out of material from the stream bottom. Some use only fine sand grains to build a straight or horn-shaped case. Others glue together spruce needles into neat squares or into shaggy bundles. Some build curved cases like a snail. Others use small pebbles to build cases that would rival any masonry work, he says.

One kind of caddisfly, says Godin, cuts tiny sheets of bark that have fallen in the creek and turns them into neat, square case.

"It's very beautiful," he says.

The caddisfly larvae cling inside their cases by means of a hook at the end of the abdomen. Their well-protected mouth parts and legs stick out the other end of the case, allowing them to move around and feed.

"Most of them go on big rocks and scrape the algae," says Godin. " They have gills within the casing, and that's how they get the oxygen."

Like blackflies and mayflies, stoneflies and caddisflies, many of the creatures on the streambed are the sub-adult stages of insects we know better in their adult, flying form. Unlike humans, they spend most of their lives as sub-adults.

"Look at cicadas. They spend a month singing, but they spend 13 years in the ground as a grub," says Godin. "They spend most of their lives growing up."

For more information, contact Benoit Godin at Environment Canada (667-3400) in Whitehorse.