Last year, researchers sampled 32 Puget Sound locations and found the harbor's concentrations of Alexandrium were about 1,000 times greater than most other sites in the sound, said Cheryl Greengrove, a UW-Tacoma oceanographer.

"This new documentation points a big finger at Quartermaster Harbor," said David Shull, a biological oceanographer at Western Washington University who also studies Alexandrium. "The bottom is littered with these cysts."

Alexandrium organisms are microscopic but they yield powerful neurotoxins. The toxins don't harm shellfish, which thrive on algae. But paralytic shellfish poisoning can be fatal to marine mammals and people who eat tainted mussels, clams and oysters.

In 2000, paralytic shellfish poisoning struck nine people who ate contaminated shellfish from Carr Inlet, near Gig Harbor. Five were hospitalized. Three had to be hooked up to respirators until their bodies purged the toxins.

The state Department of Health's regular testing of shellfish, regulation of harvests and beach shutdowns keeps most people out of the hospital, scientists said.

"You don't see any dead bodies because they're monitoring," said Rita Horner, a UW phytoplankton expert.

Alexandrium's troublesome history in the Pacific Northwest dates back to 1793, when paralytic shellfish poisoning killed a member of Capt. George Vancouver's crew who ate mussels in British Columbia.

In Washington, three deaths are officially blamed on paralytic shellfish poisoning. A man and two children died in 1942 after eating contaminated shellfish from the Strait of Juan de Fuca.

Some scientists suspect a link between water pollution and harmful algal blooms. But most research on paralytic shellfish poisoning has focused on shellfish rather than Alexandrium.

By studying conditions in Quartermaster Harbor, Greengrove and others expect to find out more about the nature of environments where Alexandrium appears to thrive.

Besides sampling sediment, Greengrove's UW-Tacoma students regularly visit the harbor, sample water and measure and record chemical and physical properties.

Alexandrium typically causes trouble for only a couple of weeks every year, when warmer temperatures coax buried cysts out of dormancy and they emerge from the bottom. It's unclear under what circumstances they produce harmful toxins, Greengrove said.

The Health Department has teamed up with researchers at the Woods Hole Oceanographic Institute in Massachusetts to evaluate a genetic test for Alexandrium in the waters of Puget Sound. The experiment started May 15.

Instead of depending on laboratory mice to test the toxicity of mussels collected around the sound, the test involves siphoning salt water samples through special filters to trap plankton. The filters are shipped to Woods Hole where researchers will use molecular detection methods to count Alexandrium.