Maine Perspective Front Page
Oceanographers Propose Gulf of Maine Red Tide Theory
A species of marine algae that causes paralytic shellfish poisoning in New England and the Canadian Maritimes appears to be more abundant off shore in the Gulf of Maine than scientists had initially believed, according to preliminary information from the first year of a five-year research project.
The species, known as Alexandrium tamarensis, may take advantage of a natural pattern of nutrients which develops in the Gulf in mid- and late-summer, according to David Townsend of the School of Marine Sciences.
A new theory to explain the phenomenon has emerged from work by Townsend, Neal Pettigrew, Andrew Thomas and their students, all of UMaine, who are collaborating with scientists from other research institutions, including Woods Hole Oceanographic Institution, the University of New Hampshire and Bigelow Laboratory for Ocean Sciences. They have earned more than $5 million in grant support from the National Science Foundation and the National Oceanic and Atmospheric Administration.
"We think we're on the verge of understanding what causes red tide outbreaks in the Gulf," says Townsend. "I presented these ideas to a meeting in Boothbay Harbor last summer."
If confirmed by detailed analysis of 1,200 water samples, the new theory could pave the way for predicting red tide outbreaks and reducing their impacts on human health and the region's shellfish industry. More than 2,000 samples were collected during cruises in the Gulf last summer.
Alexandrium-contaminated shellfish have caused deaths among consumers, most recently during a 1988 outbreak in Prince Edward Island. As a precaution, shellfish beds can be closed for weeks at a time when red tide is detected.
The Theory
According to the theory, Alexandrium grows in the nutrient-rich water that stays just below the surface in the eastern Gulf of Maine. Strong tides in the Bay of Fundy mobilize nutrients from deeper water layers, and currents carry this plume southwest along the coast of New Brunswick and Maine. At a point near Jonesport, part of this current turns south toward deep water.
Alexandrium is found in abundance in and on the edges of this current. In the microscopic world of marine algae, it is a relatively large organism and has the ability to swim. It turns out that swimming may give the species a competitive advantage over other algae, says Townsend, especially in mid- and late-summer.
As summer wears on, a layer of warm water tends to grow at the surface of all northern hemisphere water bodies. If left undisturbed by strong tides or currents, the warm water is like a wet blanket on a smoldering fire. Lacking in nutrients, it tends to dampen the growth of algae. Moreover, it pushes colder, nutrient-rich water down to depths where there is not enough light for algae to grow.
"It turns out from other studies done in the lab that Alexandrium needs high levels of light and nutrients. These two things are usually mutually exclusive in the ocean. In the summertime, other phytoplankton have already used up the nutrients in the surface waters. It's pretty well depleted. The light is highest in the surface waters, and it's dark down deep where there are lots of nutrients. You have to bring the two together, and this tidal mixing in the eastern Gulf of Maine brings them together," Townsend explains.
With nutrient-rich water only a few meters below the surface, the theory goes, Alexandrium is able to swim down to absorb the necessary nutrients at night and return to the surface during the day to bask and grow in sunlight. In the western Gulf of Maine, nutrient-rich water is too deep, beyond the reach of the algae.
During cruises in June, July and August, the researchers sampled water at 215 different points along the coast up to 70 miles offshore.
"By the time we did the August survey, we could look at the printout of how water temperatures changed with depth and say whether we were or were not going to find Alexandrium there. It worked quite well," says Townsend.
Rivers and the Sandwich
The theory also explains other observations that scientists have made about red tide in past years. For example, local pollution sources have been thought to promote red tides. Since freshwater tends to stay on the surface of the Gulf, high levels of nutrients coming out of rivers discharging to the Gulf can indeed give Alexandrium a boost.
Monhegan Island also has been a red tide "hot spot," says Townsend, who wrote a scientific paper in 1983 proposing an explanation. "Turns out I was dead wrong. There's always been a lot of toxicity in the mussels out there. It's always the first one to become toxic in the summer. It's the only one we sample on a regular basis that's off shore. Everything else we sample is along the coast. This is the only outer island that's sampled. If you're going to look for it to occur first, you'd naturally look for it out where it lives."
Another mystery involves a stage in the Alexandrium life cycle known as a cyst. The algae form cysts when they are under stress. When conditions improve, the cysts respond by developing into active cells.
"The cysts are in the water all the time," says Townsend. "It's like saying where do the diatoms come from in the spring in the middle of the Atlantic Ocean? The answer is, there are always some around."
Scientists have also known for many years that red tide algae are rarely seen along a section of the Maine coast from eastern Penobscot Bay to Jonesport. "Either side of that, we see paralytic shellfish poisoning in the mussels that are monitored by the Department of Marine Resources. John Hurst down there refers to this as the sandwich. Nothing in the middle and bread on both sides. Nobody really understood it," says Townsend.
Based on the observation that the nutrient-rich current moves away from the coast at Jonesport, it makes sense that Alexandrium would be much less abundant there.
"The theory provides for the first time a foundation that's purely oceanographic in nature, rather than that there's a source of pollution here, and something else there. It clearly shows that it's the oceanography of the region that controls this. Unless we have oceanographers applying themselves to these problems, these things will remain elusive, and that's been the case until now," Townsend says.
The five-year project is part of an international effort to understand the causes of toxic algae outbreaks which have plagued the world's coastal waters in recent years. Further information about the Gulf of Maine project is available on the Internet, crusty.er.usgs.gov/ecohab/