A Millinocket paper mill is being blamed for an algae bloom in the Penobscot River so massive that leaders of the Penobscot Nation are advising tribal members to stay out of the water.

State environmental officials say the bloom of common blue-green algae, although unusually large, has not been determined to be toxic, so there are no state-issued health advisories for the Penobscot River. A sample from the bloom has been sent away for further analysis.

In the meantime, the Maine Department of Environmental Protection has issued a notice of violation to Katahdin Paper Co. for phosphorus discharges from its Millinocket mill. Phosphorus, which is used in pulp production, can help trigger large blooms of the potentially toxic blue-green algae.

But leaders of the Penobscot Nation are not waiting for the test results, even though it’s the tribe – not the state – that is paying for them.

Late last week, Penobscot leaders issued an advisory recommending that members avoid swimming in or drinking from the Penobscot all the way from Dolby Pond near Millinocket to the coast. The advisory also extends to pets.

“We’re really taking the precautionary approach in the event that it is toxic,” said John Banks, director of the Penobscot Nation’s Department of Natural Resources. Staff at the Penobscot DNR detected the bloom and have been monitoring it for several weeks.

Blue-green algae – which, despite the name, are not algae but bacteria known as “cyanobacteria” – occur naturally in lakes, rivers and ponds. They are a critical part of the global ecosystem.

But given the right conditions – namely, warm summer days and a supply of phosphorus and nitrogen – the cyanobacteria will reproduce rapidly and form large blooms.

Most blooms are not harmful. However, some blooms can release toxins that can irritate the eyes and skin or cause gastrointestinal problems in people – especially children – or animals if the bacteria are ingested. More severe health problems can result from exposure to large amounts of the cyanobacteria.

David Courtemanch, director of environmental assessment at the DEP, said blooms happen periodically in Maine but that they are not so intense that the department feels toxicity analysis is needed.

While Courtemanch acknowledged that it is unusual to have a bloom extend over so large a river, the DEP does not believe state-issued health advisories against swimming in the Penobscot are necessary at this point. The DEP will work with the Penobscot Department of Natural Resources to continue monitoring the bloom.

“I know of no cases, even when the blooms were worse, when we’ve had illnesses reported,” he said.

Last Thursday, the DEP sent a letter notifying Katahdin Paper that the company is violating state water quality laws by discharging phosphorus that caused the bloom from Dolby Pond to Hampden. In addition to affecting use of the river, the bloom has contributed to low dissolved oxygen levels in the water in some areas, particularly in the impoundment of Weldon Dam.

The company is required to report to the DEP by the end of the month on the reasons for the excessive phosphorus discharges. The department is also requiring Katahdin Paper to limit phosphorus discharges during the month of September.

A representative for Katahdin Paper said the company was not aware of the bloom until recently but that the mill is working to address the problem.

“We’re monitoring it as we speak and have been monitoring it for some time,” company spokesman Glenn Saucier said. “Right now we’re doing what we can to work with the state DEP.”

Saucier said he does not believe that the mill has increased its usage of phosphorus lately but, in response to the bloom, the staff is researching alternative compounds for brightening pulp.

Penobscot Nation Chief Kirk Francis, meanwhile, said the recent bloom is symptomatic of what he said are larger problems of state oversight of water quality in the Penobscot and with Katahdin Paper’s permits in particular.

Francis said that after a 2004 blue-green algae bloom, Katahdin Paper agreed to limit phosphorus discharges. But discharge levels were never written into the permit. Now, members of his tribe are again having to limit use of their ancestral river for the same reason, he said.

“Because nothing was done in 2004, we have this [bloom] in the river once again,” Francis said. “I have a camp up on the river and to have to question whether to take my family out on the river is very discouraging.”

DEP spokesman Scott Cowger said the department is conducting a water quality analysis of the Penobscot. That model will be used to create phosphorus discharge limits for Katahdin Paper. But even lacking the discharge limits, the agency can still hold the company accountable for harming water quality, he said.

“There are a number of ways that they have to conform with the law,” Cowger said.