As clam flats start to reopen along the Maine coast, scientists are at sea in hopes of better understanding the red tide that wreaked havoc on the New England coast this spring.

David Townsend of the University of Maine spent this week sampling populations of the organism that causes the toxic blooms, a plankton named Alexandrium tamarenses, in a current of cold, nutrient-rich water where it thrives.

Known as the Eastern Maine Coastal Current, the underwater river is about 5 degrees colder than the surrounding water, which means that it can hold more oxygen and provide a better environment for the world of microscopic organisms known collectively as plankton.

Miles out to sea, Alexandrium populations are high, but that’s the natural state of things, Townsend explained by satellite phone from the deck of the Cape Hatteras working near Grand Manan on Thursday.

Alexandrium is a type of dinoflagellate – a microscopic organism that defies classification as either plant or animal. It can move, if not exactly “swim,” and rides the currents, taken in by filter feeders such as clams, mussels and oysters. Scattered populations that live throughout the Gulf of Maine are completely harmless.

But when Alexandrium goes into reproductive overdrive, it develops “blooms” that, in extreme cases, can make the water a rusty color and cause shellfish to become poisonous. When people or wildlife eat clams, oysters and especially mussels tainted with high concentrations of Alexandrium, they can develop paralytic shellfish poisoning, a condition that at its most minor causes symptoms similar to a severe case of food poisoning and at its most serious is fatal. Other shellfish, such as lobster and crabs, don’t filter feed and aren’t affected by the red tide.

Red tides seemed to be on the rise in recent decades. Until the 1990s, many people believed the problem was their fault – the symptom of an ocean in shock, an inevitable consequence of pollution.

But about five years ago, a group of scientists, including Townsend, Neil Pettigrew and Andrew Thomas from the Orono campus, decided that old fishermen’s tales of red tide after a rain deserved a closer look. By analyzing thousands of water samples over several years, they proved that the Eastern Maine Coastal Current provides a home for the plankton, which come to shore only when little curled eddies formed by wind push the cold water toward land, causing random red tide blooms that might strike one cove, leaving untouched another, just a few miles away.

“It’s not the fresh water, it’s not the rain, it’s not the runoff – it’s the wind,” Townsend said.

Every year some places, such as Monhegan and Grand Manan islands, suffer severe red tides because they are located right in the plankton’s path. Some parts of Washington County experience the tides almost annually when autumn storms move through, shifting the current slightly as it hugs the shore. Some communities located in “the sandwich” area between Jonesport and Eastern Penobscot Bay rarely experience red tides because the current shifts farther out to sea for a while as it moves south.

This spring, the powerful winds that carried northeasters through the region in May also pushed the current carrying the normal Alexandrium populations toward shore in unusual places. The organisms responded to increased sunlight and nutrients with a population explosion that led to broad shellfishing closures. Biologists called it the worst red tide bloom since 1972.

At one point, most of the Maine coast was closed to at least one type of shellfishing from the New Hampshire border to the Washington County line. This week, however, the Maine Department of Marine resources reopened an area from Small Point to Owls Head for clamming – though a ban on harvesting mussels, snails and some species of oysters remains.

Biologists estimate that it takes three weeks for water in the Eastern Maine Coastal Current to make its complete journey though the Gulf of Maine, which stretches from Nova Scotia to Cape Cod. Now the current is returning to its normal position, and red tides are dissipating.

“The time it took for that water to travel through the gulf allowed the cells to keep growing,” Townsend said.

After two to three weeks, the shellfish in red tide blooms will flush the toxins out of their systems, leaving the clams, mussels and oysters unharmed and safe to eat. This week, several stretches have already reopened to the harvesting of soft-shell clams, the species most commonly used for steamers and fried clams. But the past few weeks have already put such a strain on the shellfishing industry that political leaders are calling for federal emergency funds.

Knowing when and where Alexandrium will likely appear could help fishermen deal with these economic consequences, so scientists are patrolling the ocean, measuring water temperature, sunlight, oxygen levels and other variables that could affect the blooms. Understanding the currents is crucial to understanding red tide, Thomas said.

“The key to any of this probability stuff is putting the instrumentation systems into the ocean,” Thomas said. “What’s needed is a very good observation system so we can understand what’s triggering these things.”

The National Science Foundation has dedicated $200 million over the next five years for this sort of ocean monitoring, and Maine’s congressional delegation is advocating for more. Constant monitoring, with automated systems like the Gulf of Maine Ocean Observing System buoys, could provide the tremendously detailed picture of the gulf that scientists will need to stay one step ahead of red tide, Thomas said.

“If it were easy, it would be rocket science,” he said. “But it’s not. It’s ecology.”

For general information about red tide, visit www.whoi.edu/

redtide. For the full list of shellfish closures from DRM, visit www.state.me.us/dmr/rm/

publichealth/closures/

pspclosures.htm.