The potential for an outbreak of the phenomenon commonly called "red tide" is expected to be "moderately large" this spring and summer, according to researchers with the Woods Hole Oceanographic Institution (WHOI) and North Carolina State University (NCSU).
This advisory is based in part on a regional seafloor survey of quantities of Alexandrium fundyense — the algae notorious for producing a toxin that accumulates in clams, mussels, and other shellfish and can cause paralytic shellfish poisoning (PSP) in humans who consume them. The survey maps are used with computer models that simulate different scenarios of weather and oceanographic conditions to indicate where and in what abundance the toxic cells might be expected in 2009.
The researchers found concentrations of Alexandrium cysts — the dormant seed-like stage of the algae's life cycle — in the Gulf of Maine to be 40 percent lower than the historically high levels observed prior to last year's bloom, but still higher than the level preceding a major regional bloom in spring 2006 that closed shellfish beds from Canada to Massachusetts Bay.
The Alexandrium survey has been conducted each fall since 2004 as part of several research and event response projects funded by the National Oceanic and Atmospheric Administration (NOAA) Center for Sponsored Coastal Ocean Research (CSCOR). Fall concentrations of Alexandrium cysts are one of the indicators of the magnitude of a potential bloom in spring.
In October 2008, a survey team led by Don Anderson, a senior scientist in the WHOI Biology Department and the lead investigator of the Gulf of Maine Toxicity (GOMTOX) study, spent 10 days collecting seafloor sediment samples between Massachusetts Bay and the Bay of Fundy looking for Alexandrium cysts. After the samples were processed and analyzed, the team determined the cyst abundance to be about 40 percent lower than the high levels observed in fall 2007 (see figure).
This is one line of evidence that leads Anderson and his colleagues to believe the bloom this spring won't be as extensive as last year's, which closed shellfish beds from the Canadian border to the Cape Cod Canal, and on the outer portions of Cape Cod as well.
"Last year at this time, we issued an advisory for a very large regional bloom that did in fact occur. In hindsight, that advisory was 'easy' because the cyst concentrations were higher than we had ever seen – 30 percent higher than in 2004 just before the massive 2005 red tide that many people probably remember," said Anderson. "It's more difficult to make a prediction this year because the numbers of cysts we found are not extreme."
The cysts for this year's bloom have been dormant in the seabed since late last summer, when they were formed at the end of the Alexandrium bloom. With the onset of spring and its warm temperatures and increased light, the cysts are already beginning to germinate, liberating cells that swim to the surface waters. Under the right conditions, a single cell can then divide into several hundred cells within a few weeks. But where and when the resulting bloom will make landfall depends on weather events that cannot be predicted months in advance.
That's where computer modeling can help. While cyst abundance gives a general indication of the magnitude of the bloom, oceanographers Dennis McGillicuddy, a senior scientist in the WHOI Applied Ocean Physics and Engineering department, and his colleague Ruoying He of North Carolina State University (NCSU) have been working with Anderson to simulate the resulting growth and transport of the toxic cells using computer models. Working together over the last decade, the team has developed a computer simulation that incorporates weather conditions, river runoff, and various wind and current patterns to predict the intensity and location of blooms of the toxic algae in the Gulf of Maine.
With data from last year's cyst counts, the WHOI and NCSU team ran their computer model through a range of scenarios, using the predominant regional wind patterns and ocean conditions from each year since 2004.
According to He and McGillicuddy, the simulations suggest a moderately large regional bloom of Alexandrium, falling in the mid range of those that have occurred over the last five years. Red tides during those years have ranged from very large ones that stretched from the Canadian border to southern Massachusetts, including Cape Cod and the Islands, to small outbreaks that have only caused limited harvesting closures in Maine, New Hampshire, and northern Massachusetts.
The researchers have found that toxicity in near-shore shellfish is the highest for years in which northeasterly storms occur, which tend to drive Alexandrium cells toward the southern New England coast. When southwesterly winds dominate, the algae tend to remain offshore. So even when there are a lot of cells in the Gulf of Maine, toxicity can be confined to offshore waters if the winds and currents cooperate.
The Alexandrium cells can be expected to appear in surface waters in early April. By the end of that month, cells usually begin to appear in large numbers in the waters off coastal Maine and New Hampshire, which is when and where the first reports of toxicity typically appear. As the coastal bloom is transported to the south, the Massachusetts shoreline can be impacted as well. The severity of this year's outbreak along the coast will depend in large part on the wind patterns in May, June, and July.
The GOMTOX researchers regularly share their results with coastal resource and fisheries managers in six states and at the National Oceanic and Atmospheric Administration, the Environmental Protection Agency, and the Food and Drug Administration (which oversees the safety of offshore shellfish beyond state limits).
"We can only issue an advisory, as opposed to a forecast," says Anderson. "We can estimate how many cells will be in the water from the germination of cysts and potential growth of that population, but not whether those cells and that water will be blown to shore."
These types of red tide or harmful algal bloom (HAB) advisories can help the economy. Advance warning of a potentially troublesome year for algae can give shellfish farmers and fishermen time to make contingency plans for the timing of their harvest or expansion of aquaculture beds; state agencies can make informed staffing decisions to accommodate additional monitoring that might be required to protect public health and the shellfish industry.
The WHOI and NCSU researchers underscore that even when these major red tides occur, shellfish and other seafood products are safe to eat because they are carefully monitored. Shellfish stocks are regularly tested by state and federal agencies, ensuring that contaminated growing areas are closed to harvesting, and that dangerous product does not reach the market.
This research was supported by NOAA's Center for Sponsored Coastal Ocean Research, and the National Institutes of Health and the National Science Foundation (through the Woods Hole Center for Oceans and Human Health). Additional work examining other species of toxic algae in the Gulf and on Georges Bank is supported by the NOAA Oceans and Human Health Initiative (OHHI).
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