EASTHAM — The town’s dept. of natural resources closed Salt Pond in the Nauset Estuary on March 13 because of “red tide,” an outbreak of single-cell algae that produces a toxin harmful to human health.

This was the second closure this year of Salt Pond, a popular spot for recreational shellfishing on “Salt Pond Sundays.”

Shorter and milder winters allow for longer-lasting algal blooms, according to Don Anderson, a senior scientist at the Woods Hole Oceanographic Institution’s Center for Oceans and Human Health. Anderson told the Independent that his team had been expecting toxicity levels to rise again after an “unprecedented” bloom in late December that kept Salt Pond closed for three weeks.

In a so-called red tide, microscopic organisms of the genus Alexandrium bloom and produce a neurotoxin that can lead to paralytic shellfish poisoning (PSP). Though the health of the shellfish is not affected by the toxin, they store it in their flesh. A person eating affected shellfish can experience PSP symptoms ranging from tingling in the lips or tongue to paralysis and even death, depending on the number of toxic shellfish consumed.

Despite its name, red tide usually does not turn the water red in New England. Rather, its presence is detected by tests on shellfish meat conducted by the Mass. Div. of Marine Fisheries (DMF).

According to Anderson, Salt Pond, a “kettle hole” formed by glaciers, is an unusual site for studying the swimming organisms that produce the PSP toxin because it is both unusually deep and has a narrow, shallow inlet from the ocean. As a result, the Alexandrium can go through a full life cycle without being washed out with the current.

Devon Nunez, Eastham’s shellfish constable, told the Independent that the town’s natural resources dept. makes sure to send out alerts whenever there is a problem with red tide in Salt Pond. Currently, signs posted at the pond say it is closed to shellfishing. Nunez said more signage specific to red tide closures is on the way.

Scientists at Woods Hole keep track of Alexandrium levels with a submersible microscope that is fastened to a barge in the middle of the pond. The microscope takes some 10 images of algae every second.

Researchers then use machine-learning software similar to facial recognition programs to sort the millions of images taken by the microscope to identify and count the Alexandrium present. The data are used to both monitor harmful algal blooms and track how they spread from year to year. Woods Hole scientists share the data with the DMF and with local shellfish constables.

The DMF on March 19 closed three shellfishing areas in Orleans — Namequoit River, Frostfish Cove, and Meetinghouse Pond — as a precaution.

Two Colonies, Different Trajectories

There are two main colonies of the red tide algae that can affect Cape Cod. In recent years, they have been on diverging trajectories of toxicity.

The first is a yearly bloom of red tide that starts to the north, in the Bay of Fundy, then moves down the Eastern seaboard, sometimes reaching as far south as the New Hampshire and Massachusetts coasts. This coastal red tide has trended less toxic in recent years, according to Anderson.

Conversely, the second colony, the Alexandrium present in Salt Pond, has become more toxic — and scientists have noticed the algae spreading along the back side of Outer Cape Cod.

When the algae that creates red tide is not in bloom, it makes what scientists call a “cyst” that acts much like a seed of a plant. Alexandrium cysts can sit for years at the bottom of a body of water until the conditions are right for them to germinate.

Michael Brosnahan, an assistant scientist at Woods Hole who has been studying red tide for the last 10 years, told the Independent that his team found approximately 30 times more cysts in Salt Pond than usual after a particularly toxic bloom last spring.

Alexandrium cysts are also being found in greater numbers than in the past in Pleasant Bay and particularly in Meetinghouse Pond in Orleans. In samples taken from the bottom of Meetinghouse Pond last week, there were over 900,000 Alexandrium cells per liter of water, a “very high” number, according to Brosnahan.

Pleasant Bay had not seen high toxicity levels for decades until last year. Brosnahan’s team is currently working to determine whether the new blooms there are related to the resident Alexandrium population in Salt Pond or if decades-old cysts left over from blooms in the late 1970s are now able to germinate again because of favorable growth conditions. Anderson said that both possibilities are “equally likely,” but the consequence is clear: there are now cyst deposits in Pleasant Bay that may be a problem for years to come.

Commercial Shellfishing Not Affected

Red tide is much less of a concern for shellfishing in Cape Cod Bay — the heart of the Outer Cape’s commercial shellfishing industry. Unlike in Nauset Marsh and Pleasant Bay, any red tide that could affect the bay side would come from the blooms of the colony traveling down from Maine.

The last time a major outbreak of red tide halted commercial shellfishing in Cape Cod Bay was in 2005.

Still, those who depend on shellfish are aware of the danger red tide poses.

Nancy Kunik, who has been harvesting wild oysters in Wellfleet Harbor off Indian Neck Beach since 2002, said that she was surprised to hear that red tide had closed shellfishing in Salt Pond already.

“It’s a little early in the year to have red tide,” said Kunik. “But we’ve had a mild winter.”

Wellfleet Deputy Shellfish Constable Christopher Manulla said that he has seen red tide affect shellfishing in Wellfleet only once in the nearly 25 years he’s been on the job.

“We have really good communication with the state, and we do water tests frequently to make sure it’s healthy,” said Manulla. “If there was any problem with shellfishing in this town, we would alert as many people as possible in many forms of media to keep the people and the industry safe.”