The declining populations of the energy-dense creature are threatening lake whitefish and many prey fish upon which salmon, trout and walleye rely.

Purdue University Assistant Professor Marison Sepulvdea says the rice grain-sized Diporeia -- in the same biological class as krill and shrimp -- contain up to 40 percent lipids, making them a vital energy and nutrient source for the entire food web, said Sepulvdea.

Collaborating researcher Tom Nalepa of the Great Lakes Environmental Research Laboratory said the Diporeia are already gone from many large areas of lakes Michigan, Huron, Erie and Ontario, with nearly no Diporeia found in Lake Michigan at depths shallower than 90 meters. Just 15 years ago, their density often exceeded 10,000 animals per square meter at such depths.

Sepulveda and University of Michigan researcher Tomas Hook received a four-year, $560,000 grant from the Great Lakes Fishery Trust earlier this year to further investigate possible causes for Diporeia's decline.

The research findings so far are to appear in next month's issue of the journal Aquatic Toxicology.