Study: Algae blooms kill one of Narragansett Bay's best natural filters

PROVIDENCE, R.I. --Blue mussels are one of nature's best defenses against algae blooms that can kill fish, plants and other sea life in Narragansett Bay.

But a study by researchers at Brown University finds that mussels died in large numbers after a massive algae bloom blamed for depriving the bay of oxygen in 2001, possibly creating a dangerous cycle that could create worse conditions in the future.

Andrew Altieri, an ecologist who led the study, said an estimated 4.5 billion mussels died after the algae bloom in 2001, or about 80 percent of mussels in the reefs studied.

Before the bloom, Altieri said the mussels could filter the entire volume of the bay in about 20 days. Within weeks of the die-off, he estimated that filtering ability had dropped by 75 percent.

Professor Jon Witman said with fewer mussels to filter the bay, the chances of another algae bloom increase.

"It's like a downward spiral, a feedback loop. And this problem is not unique to Narragansett Bay," he said.

Mussels serve as filters, but they're more easily damaged by oxygen deprivation than clams or quahogs.

"You can almost think of them as a canary in a coal mine because they're pretty sensitive to problems," Altieri said.

He and Witman monitored nine massive mussel reefs in Narragansett Bay during the summer and fall of 2001. They focused on the mytilus edulis, or the common blue mussel often used in pasta dishes.

Blue mussels are one of the predominant filters in Narragansett Bay because they eat by straining tiny phytoplankton from the sea water that passes through their bodies, Altieri said.

Witman said it's difficult to predict how many mussels die during the course of a normal summer, but he doubts it would be 80 percent.

"We were just blown away by this big die-off that started in the summer," he said. "The large scale of it really surprised us."

Arthur Ganz, a retired biologist for the Rhode Island Department of Environmental Management, studied the algae bloom in 2001. He hadn't reviewed the Brown University study, but he cautioned that blue mussels tend to be fickle and can disappear for many reasons, including shifting sands.

"Mussels are a very hard animal to predict. They come and go. They live fast and die young," he said.

While biologists have theorized about the filtering effect of mussels, the Brown University study is one of the first to analyze the interruptions caused by algae blooms, Ganz said.

Conditions become ripe for algae blooms in the summer, when sunlight warms seawater near the coast, scientists said. The risk is especially great when tides are weakest because water is circulating less than normal.

The blooms begin when rain washes nitrates and phosphates into the ocean, which phytoplankton feed on and then multiply. When the phytoplankton die, they fall to the seafloor where their remains are consumed by bacteria that suck oxygen from the ocean.

In some cases, Ganz said, the partially digested muck spreads along the seafloor and gives off reeking sulfur gas.