Ohio officials say they'll investigate how toxic algae managed to taint tap water and leave many thousands of residents of Toledo, Ohio and adjacent southeastern Michigan without drinking water.

The latest in a series of algae blooms contaminating Lake Erie led officials to declare a state of emergency for Ohio's fourth-largest metropolitan area and forced residents to do without drinking tap water over the weekend.

The Ohio National Guard helped deliver bottled water to people and operated a purification system producing potable water.

One state lawmaker says he will hold a hearing on the matter of the increasing algae blooms.

"This is not just one community issue, this is the whole lake," says state Rep. Dave Hall.

Hall says he wants to hear from experts about the origins of algae blooms and the effects of weather, wastewater treatment and farm runoff.

The toxic tap water was the result of the presence of microcystis, a kind of blue-green algae.

Such algae, also known as cyanobacteria, is a particular problem in Lake Erie, which is the warmest and shallowest of the Great Lakes.

That makes it biologically productive and full of phosphorus and nitrogen, the main food source of the algae.

People whose skin is exposed to microcystin can develop blisters or hives, and if ingested the toxin can cause fever, headaches, nausea and diarrhea.

In large amounts, microsystin can cause liver damage.

The blue-green cyanobacteria that created the toxin began to be seen in Lake Erie in the 1990s, and in the summer of 2001 a bloom covered almost 20 percent of the lake's surface, the largest bloom on record.

Toledo is especially at risk from algae blooms and toxins because of geography; it sits at the edge of the western basin of the lake, where it is at its most shallow.

While winds can help break up blooms or move them toward the lake's center where the algae does not threaten water supplies, this year the winds have been pushing the algae close to Toledo's water intake sources.

Agricultural runoff of nitrogen and phosphorus has increased with a transition in recent years from small family farms to large industrial-sized operations, and that combined with climate change, bringing stronger storms and elevated temperatures, has resulted in a perfect environment for the algae to bloom, environmentalists say.

"Imagine you're at an all-you-can eat buffet, and you gorge yourself," says Jennifer Caddick of the advocacy group Alliance for the Great Lakes. "That's what's happening with this algae."