New Maps Provide Clues to the Historic 2005 Red Tide Outbreak in New England And Hints for 2006

Scientists at the Woods Hole Oceanographic Institution (WHOI) have completed two extensive survey and mapping efforts to better understand why the 2005 New England red tide was so severe and to suggest what might lie ahead. WHOI Senior Scientist Don Anderson and colleagues mapped the distribution of Alexandrium fundyense cysts in seafloor sediments immediately before and after the historic harmful algal bloom of 2005.

The first of these analyses shows unusually large numbers of cysts of Alexandrium fundyense in bottom sediments of the Gulf of Maine in late 2004, compared to previous survey results from 1997. The hardy, seed-like cysts lie dormant in ocean sediments until growing conditions are favorable for a bloom of this toxic alga. A subsequent survey conducted last fall, after the 2005 outbreak, shows slightly fewer cysts in the sediments than there were in 2004, with the abundance and distribution of those cysts giving a hint of what might be in store for 2006.

“This is partially good news since the 2005 cyst distribution shows that the species did not create a new southern ’seedbed’ within Massachusetts Bay and southern waters, as we had feared. The cyst abundance we found there may be too low to initiate blooms in the bay proper, although we don’t know if there are cysts in harbors or embayments that might lead to localized toxicity,” said Anderson, director of the National Office for Marine Biotoxins and Harmful Algal Blooms, based at WHOI. “On the other hand, there are still five times as many cysts off western Maine as there were in 1997 and only slightly less than in 2004, so there is certainly the potential for another regional event.”

The presence of large numbers of cysts does not guarantee that 2006 will bring another widespread bloom in southern New England. The large seedbed that has just been mapped supports the likelihood of a significant bloom for the region as a whole. Cells germinated off the coast of Maine tend to be transported south and west by regional-scale ocean currents. Whether or not those cells make landfall in Massachusetts and Cape Cod Bays depends on the wind. “If we have persistent winds from the northeast this spring and early summer, there is a higher probability for a bloom and shellfish closures within the bay, whereas if we get winds from the southwest, which is more common for those months, that scenario is less likely,” Anderson said.

These inferences are based on knowledge gained from a decade or more of research on Alexandrium and its blooms in the Gulf of Maine. The distribution and transport of Alexandrium cells has been documented through numerous blooms, and sophisticated computer models have been developed that allow the WHOI researchers to evaluate the relative importance of the cyst seedbeds, weather patterns, and other factors. Coupled with real-time oceanographic observations, such as those provided by the Gulf of Maine Ocean Observing System (GoMOOS), the prospect for bloom forecasts is brightening.

Looking at the longer term, Anderson and colleagues are wondering if southern New England may be entering a period of more frequent and extensive toxicity. Another massive Alexandrium fundyense bloom in 1972 was followed by two decades of heightened toxicity in Massachusetts Bay that did not slacken until the early 1990s. In western Maine and northern Massachusetts, the toxicity that began in 1972 has not relented.

The outbreak of Alexandrium fundyense in 2005 was the most widespread and intense in New England since 1972, with concentrations of toxic algae growing to 40 – 100 times the normal number and spreading to regions usually unaffected by the species. Toxins from this toxic algae can cause paralytic shellfish poisoning in humans who eat shellfish that have ingested the toxic algae as they feed.

From May to December 2005, a group from Anderson’s laboratory made a dozen trips to track the bloom and then to map out its cyst “footprint” on the WHOI-operated research vessels Oceanus and Tioga, as well as the Environmental Protection Agency’s (EPA) ocean survey vessel Bold. The effort was made possible by “event response” funds from the National Oceanic and Atmospheric Administration’s National Ocean Service, Center for Sponsored Coastal Ocean Research (NOS/CSCOR), and a major research grant from the National Science Foundation and the National Institute of Environmental Health Sciences to the Woods Hole Center for Oceans and Human Health. Ship time on the Bold was provided by the EPA.

With prior support from NOAA’s National Centers for Coastal Ocean Science, Anderson’s group had previously conducted an extensive cyst survey in the fall of 2004, unaware that a massive bloom was coming a few months later in 2005. That survey revealed a substantial seed population waiting for the favorable growing conditions that developed in the spring of 2005. The cyst population in New England waters at the end of 2004 was 10 times larger than when it was last mapped in 1997, with the bulk of this increase occurring off western Maine.

“We believe the very high abundance of cysts off western Maine was a critical factor in the 2005 bloom,” said Anderson. “We don’t know when they were laid down between 1997 and 2004, but we do know that they were in place immediately prior to the historic bloom.” An unusual late-season bloom of Alexandrium in that region in the fall of 2004 may have deposited the cysts.

As they now work to analyze the cyst map produced after the 2005 bloom, Anderson’s group is puzzled by the low abundance of cysts in Massachusetts Bay and the waters to the south of Cape Cod. They expected to find more cysts because the bloom had very high cell abundances in those bays, and spread as far south as Martha’s Vineyard and Nantucket. “We thought we would have much higher numbers down there,” Anderson said. “Frankly, we are scratching our heads trying to figure out why we don’t have more.”

One theory is that marine animals such as microscopic zooplankton may have eaten the Alexandrium cells, terminating the bloom before the cells were able to go through the process to make cysts and settle into the sediments. Another line of thinking suggests that the cells were dispersed naturally by winds and currents before they could aggregate into the concentrated populations needed for cyst formation. Lastly, it may be that the sandy, coarse-grained sediments characteristic of many areas offshore of southern New England may not be favorable to retention of cysts deposited on the bottom.

In parallel with the mapping efforts, researchers in Anderson’s lab are also exploring the genetics of the 2005 bloom. They have isolated and cultured the cells from that bloom, analyzing the genetic and physiological makeup of Alexandrium communities from various locations.

The study is focused on the idea that, as with humans, a single population of algae is actually composed of different “races” or genotypes. For Alexandrium, each genotype varies in the types and amounts of toxins it produces, so knowing the distribution of genotypes and their preferred environmental conditions could help researchers someday predict the extent and toxicity of blooms.

WHOI Research Associate Bruce Keafer, who led many of the 2005 surveys for Anderson’s team, has scheduled at least eight surveys of New England waters this spring to observe the development of the toxic algae. Cruises began this week, again with support from NOAA’s CSCOR program.