A “perfect storm” of weather and ocean conditions probably led to the severe red tide in New England in 2005, according to scientists at an April 18 symposium at the Massachusetts Institute of Technology. While no one predicted a repeat in 2006, no one ruled it out either.
During the spring and summer of 2005, harvesting shellfish was prohibited along a record area of New England shoreline due to contamination by red tide. The closure took heavy tolls on shellfishing, aquaculture and tourism in Maine, New Hampshire, Massachusetts and Rhode Island.
“Red tide” is really a dense concentration of a single-celled, plant-like microorganism called Alexandrium. The microbe produces toxins that attack the nerve cells of animals, including humans.
The problem is that people eat clams, and clams eat Alexandrium. Clams concentrate the toxins as they filter large volumes of water during feeding. When people eat clams, mussels and oysters that have concentrated toxins they can get sick and even die.
To prevent death and illness, scientists have been trying to understand the causes of red tides in the Gulf of Maine. As a result of a research program called ECOHAB-Gulf of Maine, involving technology ranging from satellites to lab experiments, they now know a great deal about how the red tides develop. Unfortunately, they still can’t predict red tides far in advance.
Several storms that blew through the Gulf of Maine in May and June of 2005 were very important to the development of paralytic shellfish poisoning (PSP) in 2005, according to Neal Pettigrew, a physical oceanographer at the University of Maine.
The storms blew from the northeast, intensifying the Eastern Maine Coastal Current that flows south and west along the Maine coast. The intensified current carried a far greater than normal seed population of Alexandrium from the Bay of Fundy. Alexandrium has persisted in the Bay of Fundy for decades.
In typical years, this ocean current turns offshore near Penobscot Bay and causes what scientists call the “PSP sandwich,” an area west of Penobscot Bay to the Kennebec River that rarely experiences PSP toxins. But in 2005, as a result of the storms, the current did not veer south. Instead, it reached all the way to western Maine.
Researchers believe this rare event was important in making the red tide of 2005 the worst in western Maine since 1989.
Another factor seems to have played a role. The winter of 2004-2005 was very snowy and wet, resulting in extremely high runoff from Maine’s rivers.
The added freshwater stimulated the growth of Alexandrium in western Maine waters especially, by fertilizing the water with nutrients such as nitrogen, and by reducing the mixing in the water, according to Don Anderson of the Woods Hole Oceanographic Institution.
The photosynthetic microbe grows and reproduces best where it can obtain a lot of light. Reducing vertical mixing keeps the cells near the surface, enhancing their growth.
Finally, for unknown reasons, there was a large supply — perhaps 10 times more than normal — of Alexandrium cysts in the ocean sediment in the western Gulf of Maine in the fall of 2004, according to Anderson. The cysts are a dormant life stage of Alexandrium that can remain in the sediment for years before germinating and growing like the seeds of an annual plant.
Anderson’s laboratory happened to be studying Alexandrium in Massachusetts Bay and the Gulf of Maine in 2004, and so obtained a lot of information about the cysts. They think the germination of these cysts contributed to the Alexandrium populations that grew and then blew ashore from western Maine to Massachusetts in 2005.
They do not, however, know why there were so many cysts present in the fall of 2004. After all, there had been no comparable PSP event for years, but they could have built up slowly, year after year.
Anderson says it is possible that cysts from the 2005 event will lead to more severe PSP outbreaks in the future, especially in southern Maine and Casco Bay. He also fears we may have entered what he calls a “new regime” where severe PSP events will be more frequent.
But Alexandrium is a normal part of the plankton community in the Gulf of Maine, Dr. David Townsend of the University of Maine reminded the audience at the MIT symposium.
He has studied Alexandrium in the Gulf of Maine since 1998 and finds that Alexandrium often reaches high cell concentrations offshore, south of Penobscot Bay.
Most of the time, as in 1998, no severe PSP event endangers the coast, where aquaculture and clam fisheries are concentrated. In certain years, however, it appears that storms, river runoff, and cyst beds combine to produce severe red tide, as they did in 2005.
As bad as the 2005 red tide was, the toxic algae caused no human deaths or illnesses.
While public health is a primary concern for many investigating red tides, some scientists study the effects of severe blooms of Alexandrium on marine animals and food webs.
Last year, scientists from Maine and Nova Scotia published a scientific paper in which they described how some soft-shelled clams actually resist the effects of the PSP toxins.
The toxins work in a very specific way in animals. They bind to a pore on the surface of nerve and muscle cells, blocking the electrical impulses that travel from nerves to muscles. This leads to paralysis.
After they eat the toxic algae, most clams slow down and cannot burrow into the sand. They get partly paralyzed. But Monica Bricelj of the Institute for Marine Biosciences in Halifax, Nova Scotia, and Laurie Connell of the University of Maine noticed that some clams were unaffected by the toxin.
When they examined the unaffected clams they found a mutated gene. The mutation prevents the toxins from binding at the pore and makes the clams resistant. The mutation also allows some clams to continue feeding, and accumulating toxin when others have been poisoned.
The mutation may make these clams even more dangerous for people because they will contain more toxin than non-mutants, the researchers argue in their report.
While scientists have a good understanding of how the toxins work and how the 2005 red tide began and developed, they do not know why it ended. After June, the red tide gradually receded perhaps because Alexandrium had exhausted its supply of nutrients, or because some other organisms out-competed or preyed upon the Alexandrium.
Although scientists cannot yet predict a red tide, state and local fisheries and public health managers are preparing for one in 2006. Last spring and summer, Maine’s Department of Marine Resources tested more than 3,000 shellfish for PSP toxins. Darcie Couture, director of biotoxin monitoring for Maine, said the DMR has increased manpower and money to do that and more this year, if necessary.
A former journalist, Dr. David E. Avery works on red tide at the Department of Marine Sciences at the University of Connecticut in Groton, Connecticut.