This article is made possible, in part, by funds from Maine Sea Grant and the Oak Foundation.

Summer is just around the corner, and that has clammers, shellfish growers and seafood-shack proprietors worried about red tide.

Meanwhile, offshore in the Gulf of Maine, teams of scientists are working around the clock, collecting water samples from the sea surface to the seafloor. They measure water currents, temperature, salinity, oxygen, nutrients, phytoplankton and zooplankton, all in an effort to understand the tiny, single-celled algae that can turn our shellfish toxic.

The culprit in the case of red tide is Alexandrium fundyense, a dinoflagellate phytoplankton about 35 microns across, smaller than the sharpest point of a pen, and barely visible with the naked eye. This particular dinoflagellete is a “background” species amongst the Gulf’s phytoplankton, according to Dave Townsend, a biological oceanographer at the University of Maine. But because it naturally produces a toxin that is 1,000 times more potent than cyanide, A. fundyense attracts a lot of attention.

Like most phytoplankton, A. fundyense grow by dividing and, under favorable conditions, can “bloom” in large numbers. At high enough concentrations, the natural pigment in the cells can make water appear red, hence the name, red tide.

Even at much lower concentrations, relatively modest blooms can have a devastating impact if the cells are carried into near-shore waters where shellfish are grown or harvested for human consumption. Shellfish take in the toxin from the algae as they filter feed and, while most shellfish are immune to the toxin, humans, mammals, and even fish can be sickened or killed from eating the shellfish.

Paralytic Shellfish Poisoning (PSP) events only occur along the coast when two events coincide: favorable conditions for A. funyense to grow in larger numbers than usual, and currents aligning to bring it near shore. åÊ

The Maine Department of Marine Resources runs a much-emulated shellfish monitoring program that tracks toxicity along the whole coast (see “Successful red tide monitoring program could be cut back,” May issue, The Working Waterfront). When tests of shellfish indicate high levels of toxins, sections of the coast are closed to shellfish harvesting. Clam flats can also be closed because of bacterial pollution, especially after heavy rains when runoff can pollute near-shore areas; sometimes these closures overlap with red tide closures, but they are caused by different factors.

Don Anderson, a biologist at Woods Hole Oceanographic Institution and an expert on regional red tides, points out that the idea of offshore PSP events (caused by A. fundyense ) is confusing to many because people generally believe that water farther out to sea is cleaner. In fact, the opposite is true. While less polluted, offshore waters often host higher numbers of A. fundyense, and shellfish collected offshore (including on floating objects or on offshore banks, like Georges Bank) can have higher toxicity than those near shore. For this reason, and the difficulty of monitoring toxicity offshore, Georges Bank has been closed to quahog and clam fishing since 1989, notes Anderson.

PSP made the news in 2007, when four members of a fishing family in Washington County were hospitalized after consuming mussels harvested from a barrel floating offshore. It was the first documented case of human red tide poisoning in at least 30 years. In 2008, three people were admitted to a Machias hospital with PSP, also from mussels. In both cases, the mussels were harvested away from shore in areas that are not tested for shellfish toxicity, but where A. fundyense is often present. (Maine’s shellfish monitoring program is top-rate and shellfish bought from reputable dealers or dug in open areas are safe to eat).

The algae follows a seasonal cycle, typically multiplying to reach highest numbers during spring and summer, and then transforming into hard, pill-capsule shaped “cysts” that sink to the ocean floor for the winter. The cysts germinate in the early spring (much like a seed for a land plant) and therefore serve as the source for blooms in subsequent years.

Don Anderson has studied these cysts in his lab. “The cysts are remarkable” he says. “They actually have an internal clock that controls their germination. In the lab we have exposed them to heat, light everything you would think would make them germinate during fall or winter and they won’t do it, they are hard-wired to wait for spring.”

In recent years, researchers have begun conducting annual surveys to count the A. fundyense cysts in the Gulf of Maine, in efforts to build predictive capacity for red tide. From the deck of a research vessel, scientists deploy a sediment coring device that collects samples from the seafloor, which they bring back to the lab in order to count cysts. In 2009 researchers found more cysts, covering a larger area, than they had ever seen before. Based on this finding, Anderson, Townsend and their collaborators predicted a bad red tide year for 2010, which seems to have proven true with the earliest closure on record.

But high numbers of A. fundyense offshore do not necessarily lead to shellfish toxicity and closures. First, the algae has to make its way inshore. For years, researchers have suspected that certain winds and current conditions might be linked to PSP events along the coast. After analyzing twenty-one years of shellfish toxicity data and comparing to a range of environmental variables, Andy Thomas, a biological oceanographer at the University of Maine, thinks he’s confirmed that suspicion. The only environmental variable that was strongly correlated with shellfish toxicity events was wind.

Researchers are now working to pull together the years of data from ship-board surveys and short-term and long-term weather forecasts into computer models that will provide red tide predictions. Don Anderson notes that this ability has been elusive in the past, but now, with new sensors that will measure A. fundyense concentrations, they are much closer to this goal. “We are building models that can be adjusted in real time and we can see these predictions being possible, both seasonally and on a week to week basis. We should be able to give people a much better idea of when and where toxicity will occur.”

Heather Deese holds a doctorate in oceanography and is the Island Institute’s director of marine programs. Catherine Schmitt is communications coordinator for Maine Sea Grant.