ISLE ROYALE, Mich. (AP) _ Each new submarine descent into Lake Superior has shown that the world's largest freshwater lake is mother to more life forms than previously suspected, a scientist says.

The four-week, $500,000 expedition is the first human exploration of the depths of Lake Superior.

As researchers prepared Tuesday to conclude their look at underwater life and turn their attention to long-buried wreckage of sunken ships, a scheduled dive was canceled by 8-foot waves and 40 mph winds that made launching the submarine more hazardous, said expedition organizer William Cooper.

Cooper, a Michigan State University zoologist, is serving as chief scientist for the last third of the expedition on board the Seward Johnson.

The 176-foot research vessel is mother ship to a 22-foot submarine, the Johnson-Sea-Link II, that has been specially altered for freshwater diving.

Scientists on the Seward Johnson, which docked Tuesday on the northeast tip of Michigan's northernmost island, this week are studying bacteria and algae, Cooper said.

''We take (the sub) down to 600 feet of water and slowly work our way up,'' collecting samples all along the way and learning ''just how much is really down there,'' he said.

Cooper said those samples are analyzed aboard the ship, where researchers are trying to measure, among other things, photosynthesis - the amount of light that is captured and used by the plants at each depth.

On Thursday, the Johnson-Sea-Link II is scheduled to dive toward known shipwrecks that have not been fully documented because of the depth to which they sank.

Until now, lake research has been limited to 200 feet, the lowest depth reachable by scuba divers.

The exploration of Lake Superior is supported by a $500,000 grant from the National Oceanic and Atmospheric Administration and run by Michigan State University and the University of Connecticut.

So far, researchers have dived to the lake's lowest charted point, more than 1,300 feet below the surface, where they found thriving colonies of burbort and sculpin (fish) and a strain of hydra, a tiny invertebrate, that had never before been documented.

They also have taken a subs-eye-view of one of the lake's most populous trout spawning grounds - research that may help scientists create more perfect spawning beds elsewhere - and have vacuumed sediments from the lake's floor in hopes of unlocking the secret of Superior's natural cleaning mechanisms.