Year after year on Lake Erie, families motor around in pontoon boats and fishermen haul in catch-of-the-day specials, while the lake's ecosystem struggles to survive.

Scientists say part of Lake Erie's ecosystem suffers from a dead zone - an area of the lake that, lacking enough oxygen, cannot sustain wildlife.

Years ago scientists tried to treat the problem, and the dead zone was thought to be under control. But surveys of the lake's ecological health conducted over the last five years show the dead zone isn't under control - it's growing.

University researchers are trying to find out why.

By disrupting the lake's natural ecological balance, the dead zone threatens an important facet of Michigan's economy: commercial and recreational fishing on the Great Lakes, which brings in about $4.5 billion each year.

A group of Midwestern and Canadian scientists, including seven University researchers, are using a $2.5-million grant from the National Oceanic and Atmospheric Administration for a five-year study to examine why the dead zone is returning and find solutions.

Research following the zone's initial discovery in the 1960s pinpointed phosphorous run-off from farms and water treatment plants as the main cause.

Scientists posited that the phosphorous in fertilizer and sewage formed algae blooms in the lake that sunk to the bottom, feeding bacteria that drains the habitat of oxygen, said Donald Scavia, a professor in the School of Natural Resources and Environment and the project's lead investigator.

After the implementation of the Clean Water Act in 1972, which barred the use of certain fertilizers and detergents and imposed stricter regulation of sewage disposal, the zone began to shrink.

The state and the scientific community thought the dead zone had been killed.

"We thought with the reduction and elimination of phosphorous, the dead zone might disappear," Scavia said.

But the dead zone's recent rebound is making scientists think there might also be another cause. In the initial stages of the study, University researchers are creating digital models of Lake Erie, adjusting the virtual conditions to determine what factors most affect the dead zone.

The models are based on decades worth of data, Scavia said.

By adjusting various settings in the models, researchers can analyze the effects of three potential causes: phosphorous pollution, zebra mussel infestation and global climate change. The models also simulate relationships between the factors.

"The dead zone is probably caused by a combination of the three," Scavia said, "but in some years one will be more important than the others."

The zebra mussel hypothesis speculates that zebra mussels, imported en masse by oceanic trade ships, prevent the distribution of algae through the lake's food web by funneling matter straight to the lake's bottom layer.

Global climate change may affect the lake's ecosystem by warming the lake's top layer to unnatural temperatures, making it expand and compress the bottom layer. When compressed, the bottom layer loses oxygen, and bottom-dwelling fish are exiled from their habitat. This, in turn, throws off the ecosystem's balance.

After researchers determine what is feeding the dead zone, the study will change its focus to determining how the threats can be neutralized, Scavia said, and then making recommendations on how new public policies could solve the problem.

- Allison Santacreu contributed to this report