State, municipal and tribal officials are working with paper mills to eliminate an algae bloom that has clogged the Penobscot River for most of the past month.

The organism, a blue-green algae of the genus Anabaena, lives only in fresh water and is common in Maine rivers.

Despite its deceptive common name, the organism isn’t a true algae; it’s what scientists know as cyanobacteria, a simple life form that lives by photosynthesizing but is not a plant.

This summer, the bloom has been denser and more long-lasting than in past years, Andy Fisk, head of the Department of Environmental Protection’s Bureau of Land and Water Quality, said Thursday.

In fact, the green-tinged bloom stretches all the way from Dolby Pond, located on the West Branch of the Penobscot River between East Millinocket and Millinocket, to the mouth of the river in Belfast.

Last week, coastal residents near Lincolnville and Northport may have seen the organism washing up on beaches, as a strong offshore wind and calm seas caused the river to “dump” algae into the Atlantic, Fisk said.

Some species of Anabaena are considered toxic because their shape irritates tissues such as skin and fish’s gills.

The DEP has not identified precisely which species is living in the Penobscot, but state biologists do not believe this bloom to be toxic.

The Penobscot Nation, however, is warning tribal members not to swim in the river, just in case.

Large blooms can cause serious environmental problems if they become too dense because they can sap oxygen that fish and other organisms need to survive. Neither state nor tribal researchers believe that the Penobscot bloom has reached this point.

The bloom was discovered by a researcher from the Penobscot Nation on July 26, as part of a cooperative water quality monitoring program.

Anabaena appears in the samples every summer, but never at this level, said John Banks, environmental director for the Penobscots.

A few days later, the state viewed the bloom from the air and verified it chemically by testing the chlorophyll levels in water samples, Fisk said.

Weather may have played a role in the unusual cyanobacteria bloom, but more likely, the culprit was an increase in phosphorus in the river for the organism to feed on, he said.

Because of where the bloom started, the state considered just two possible sources of phosphorus – the Millinocket city wastewater facility and the Katahdin Paper Co.’s mills.

Last week, DEP staff visited both facilities, checked the phosphorus levels in effluent being released into the river, and determined that the municipal wastewater plant was not releasing more phosphorus than usual.

The mills’ effluent, however, has increased in phosphorus since operations started up again this spring, Fisk said.

In the past, high phosphorus levels had been linked to discharges from Great Northern Paper, the previous owner of the two mills, he said.

There is no law restricting the amount of phosphorus that can be discharged into lakes and rivers, although the state will consider creating such a rule in the next few years.

However, any action that makes a river less “fishable and swimmable” violates water quality rules, so both facilities were notified this week that they may be contributing to the problem, Fisk said.

A spokesman for Katahdin Paper Co. could not be reached for comment late Thursday afternoon, but Fisk said that the mill has already reduced its phosphorus levels somewhat and has volunteered to work with DEP to find means of further reducing phosphorus.