CLEVELAND (AP) - Researchers think zebra mussels may be causing a low-oxygen "dead zone" in the central basin of Lake Erie.
A team of 17 government and academic scientists was scheduled to leave Cleveland on Thursday for a week aboard a federal research vessel to conduct experiments and gather water and soil samples on the phenomenon.
Researchers are finding that the thumbnail-size zebra mussel clam and the newly arrived quagga mussel are altering the food chain and habitat in the lake.
"What we're seeing is a complete change in the ecosystem," said Gerald Matisoff of Case Western Reserve University, the U.S. research team project leader.
The working cruise continues research started last summer into the cause of high levels of phosphorus and algae and low levels of oxygen.
Canadian researchers have been conducting similar studies.
High levels of phosphorus are believed to be behind a dead zone that occurs in the central basin of Lake Erie between spring and fall. Colder, more dense water stays at the bottom and cannot be replenished with oxygen by moving to the surface, creating a harsh environment.
Scientists have focused on three theories on what's causing the high phosphorus levels: climate, increased phosphorus releases from farms and sewage treatment plants, and internal changes in the lake.
Matisoff said no theory has been ruled out. However, the strongest evidence points to changes caused by mussels.
They filter particles from the water and release smaller ones that sink and decompose in deeper water. Their actions and presence have altered the food chain because they have replaced tiny animal and plant life that once floated in the water.
By removing that small animal and plant life, the mussels have also made the water clearer. Light now penetrates deeper, allowing algae to grow at deeper levels.
Scientists also are finding that the zebra mussel is no longer the dominant mussel. The quagga, which lives in lower temperatures and deeper waters, is now dominant.
The research could lead to changes in policy that govern the Great Lakes but it's too early to say what that could be, said Herb Gray, Canadian chairman of the International Joint Commission that oversees water resources shared by the two nations.