GIBRALTER ISLAND, Ohio — A significant harmful algal bloom is forecast for the western basin of Lake Erie this summer.

The forecast was announced by a group of scientists July 17 during a news conference hosted by The Ohio State University’s Stone Lab, a research station near Put-in-Bay on South Bass Island.

On a new numbering system that rates the blooms on a scale of 1 to 10, this year’s bloom is estimated to be a 5 or 6.

"This does not mean all of western Lake Erie will have a problem," said Rick Stumpf of the National Oceanic and Atmospheric Administration, a nationally known HAB expert. "If (the wind is) calm, there will be places with scum. That is to be expected."

He reviewed the last few years of HABs with 2011’s mega-bloom as the baseline others are compared to.

"2012’s forecast was a mild bloom as expected," Stumpf said. "2013 was much worse than we expected."

Algal blooms are caused by cyanobacteria, a toxic blue-green algae.

Stumpf said several factors play a part in how much bacteria grows, and he has created a model that helps to predict the severity.

"Temperatures in June and July seem to have a great effect," he said.

And the wind blowing around the bacteria makes it seem worse in some areas and not in others.

The four scientists who compared notes to make the forecast agreed the severity could increase if the rest of July is wet and causes more nutrient runoff from the Sandusky and Maumee rivers.

Much of the forecast is based on data collected at Heidelberg University’s National Center for Water Quality Research, based in Tiffin, Ohio. Researcher Laura Johnson provided background on the center’s data collection methods during her presentation.

"We had some big loading events early in the spring, two really big rain events," Johnson said.

During heavy rains, dissolved phosphorous is washed into the rivers, and eventually into the lake.

According to the researchers who spoke, dissolved phosphorous is the main cause of HABs in Lake Erie.

But other nutrients also play a part, said Justin Chaffin, a Stone Lab scientist studying HABs.

Chaffin is conducted experiments to discover what part nitrogen plays in worsening harmful algal blooms.

"Phosphorous is key, but nitrogen plays a secondary role," he said. "The forecast is based solely on phosphorous, but it’s the concentration of nitrogen that determines how fast they grow (in August and September)."

Last year, the size of the bloom was based on dissolved phosphorous levels, he explained, but nitrogen levels caused it to grow larger than expected and to produce larger amounts of toxins.