Vol. 3, No. 2 Contents
Headline Back Issues
Spring 1999
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New theory: Tidal mixing creates red tide incubatorOrono, Maine ---- Last fall, when scientists proposed a new theory about the cause of red tide outbreaks in the Gulf of Maine, they took a cautious course. Their ideas were based on shipboard observations and preliminary data. Since then, laboratory analyses of water samples have provided support for the theory that Alexandrium tamarensis, a species of marine algae that causes paralytic shellfish poisoning (PSP) in New England and the Canadian Maritimes, thrives in the eastern Gulf under conditions associated with cold, well-mixed, nutrient-rich waters and seasonal circulation.
The researchers are nearing the end of the second year of a five-year project and 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. "This is the first time anybody has gone out and really looked from ships out in the Gulf. Up to this point, most of our information came from near-shore measurements and extrapolation." The term "red tide algae" is a misnomer because harmful algae rarely reach densities high enough to turn water red, and they are not caused by the tides. Nevertheless, some species do have red pigments and when they appear in near shore waters in large concentrations, or "bloom," they can contaminate shellfish with toxins. They include diatoms and dinoflagellates such as Alexandrium. The diatom pseudo-nitzschia produces the toxin demoic acid, which causes amnesic shellfish poisoning and was responsible for deaths among shellfish consumers in Prince Edward Island in 1988. Alexandrium causes PSP, which, according to Donald Anderson of WHOI, is a life-threatening syndrome. Symptoms include tingling, numbness, burning sensations, loss of muscle coordination, giddiness, drowsiness, fever, and rash. The most severe cases result in respiratory arrest within 24 hours of consumption of the toxic shellfish. The theory
Alexandrium is found in abundance in and on the edges of this current. In the microscopic world of marine algae, Alexandrium is a relatively large organism with the ability to swim. It turns out that this ability to swim ---- especially vertically ---- may give the species an advantage, says Townsend, particularly during the mid and late summer months. That's because, as the summer wears on, a layer of warm water tends to develop 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 last June, July, and August, the researchers collected more than 2,000 water samples at 215 different points along the coast up to 70 miles (113 kilometers) off the Maine shore. Their transects ranged from Cape Cod to just east of Grand Manan Island (see maps on the Web at crusty.er.usgs.gov/ecohab/). "By the time we did the August survey, we could look at the print out 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 sandwichThe 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 fresh water 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 has also been known as 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. "Well, 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 PSP in the mussels that are monitored by the Maine Department of Marine Resources (DMR). John Hurst at DMR 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 us 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 that have plagued the world's coastal waters in recent years. For more information visit crusty.er.usgs.gov/ecohab/ on the Internet. |
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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. It has emerged from work by David Townsend, Neal Pettigrew, Andrew Thomas, and their students, all of the University of Maine, who are collaborating with scientists from other research institutions including Jennifer Martin of the Canadian Department of Fisheries and Oceans at St. Andrews, New Brunswick; Donald Anderson at the Woods Hole Oceanographic Institution (WHOI); Ted Loder at the University of New Hampshire; and Maureen Keller at the Bigelow Laboratory for Ocean Sciences. 
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 mix deep waters and nutrients and bring them into closer contact with surface waters. 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.