Red Tide

Alexandrium image courtesy of WHOI

Alexandrium image courtesy of WHOI

Last spring, coastal New England, including the waters of Cape Cod Bay, experienced the worst red tide outbreak in decades. Red tide is the common name for several individual species of phytoplankton, a broad term used to classify hundreds of microscopic plant species that primarily drift in the water (the term comes from the Greek “planktos” meaning wandering or drifting). Red tide phytoplankton are contained within a subgroup called dinoflagellates; these plants have two flagella, or threads, that extend from the body and enable them to move through the water column in a corkscrewed manner, although their overall movement is still dictated primarily by wind, tide and current.

The term red tide most often refers to a highly toxic dinoflagellate species, Alexandrium fundyense, which in large enough concentrations colors the water red and can limit or even completely prohibit the harvest of popular New England shellfish such as soft-shell clams (“steamers”), hard-shell clams (“quahogs,” “cherrystones” or “littlenecks”) and oysters. The restriction on harvesting is due to the fact that when these shellfish ingest the Alexandrium from the water column they may concentrate the toxin in their tissues, causing the shellfish to become unsafe for human consumption. If the toxicity of the shellfish tissues reaches high enough levels, a condition known as paralytic shellfish poisoning (or PSP) may result from human consumption.

As the anniversary of last year’s bloom approaches, the Center’s habitat studies team, headed by Dr. Stormy Mayo, has been talking to some of the nation’s top experts on red tide and other harmful algal blooms (HABs) so that the Center can eventually offer assistance to Cape Cod municipalities and the Commonwealth’s Division of Marine Fisheries with early detection of Alexandrium cells in Cape Cod Bay.

To that end, Mayo’s assistants, Associate Scientist Dave Osterberg and Research Assistant Meri Ratzel, are collecting, counting and identifying the single-celled, phytoplankton, including Alexandrium, in order to understand the occurrence of these organisms and their impact on the ecology of Cape Cod Bay. Ratzel recently completed red tide training under Bruce Keifer, who works with Dr. Donald M. Anderson, head of the national office for Marine Biotoxins and Harmful Algal Blooms at the Woods Hole Oceanographic Institution (WHOI); while Osterberg is experienced in the oceanographic sampling practices needed to undertake this new initiative in phytoplankton sampling.

Ratzel has also been working with Dr. Robert Edgar, of Harvard University’s Farlow Herbarium of Cryptogamic Botany to develop the methodology of diatom identification. Diatoms are another group of phytoplankton; unlike dinoflagellates, they have no thread-like tails and therefore no ability to propel themselves through the water column. Their only way of getting from one place to another is through the whims of wind, tide and current. Yet diatoms form the basis of the entire food web within our oceans. As Mayo recently noted, “Life on earth, both on the land and in the oceans, ultimately springs from plants and their photosynthetic processes.”

Because of the fundamental role played by diatoms in the dynamics of marine ecosystems, the Center’s phytoplankton team hopes to begin regular, year-round sampling of Cape Cod Bay in order to determine its phytoplanktonic composition—including diatoms and dinoflagellates as well as other subgroups of phytoplankton. The first few years of this sampling will comprise what is commonly known as a baseline monitoring program; in essence, a snapshot of the bay’s phytoplankton make-up to be compared against the “pictures” taken through similar and consistent sampling in years to come.

The growth, or “blooms,” of all the different phytoplankton species depend upon a combination of complex processes, and each individual type thrives based on its own ideal combination of various environmental factors, much as do houseplants, perennials, shrubs and trees—indeed, any living organism. Hence as the seasons change and the environmental conditions vary year by year, the phytoplankton composition varies also.

Diatoms flourish in early spring because the water column is separated by a thermocline (boundary between warm top layer and colder bottom layer), trapping the diatoms closer to the light-filled and nutrient-rich surface, in the photic (or light-filled) zone. Spring, then, is the season of the diatoms in Cape Cod Bay.

Red tide dinoflagellates including Alexandrium come along a bit later, blooming after the nutrient-rich bottom water has mixed with the warmer top layer. As sunlight penetrates this nutrient-loaded water, red tide begins to reproduce exponentially and a given population of cells can double in size within 24 hours. Once the nutrients in the water have been consumed, usually by midsummer, the blooms of phytoplankton begin to die off. In the case of Alexandrium it is during the die–off that seeds, called “cysts,” are produced. The cysts sink to the floor of the bay, where they remain dormant until the next year.

As of early July 2006, WHOI’s experts do not believe a large cyst field exists in Cape Cod Bay. However, continuous monitoring of red tide seeding may become an important component of the Center’s phytoplankton baseline monitoring program, which is currently seeking start-up funding.

Links to other Red Tide Research:
•Massachusetts DMF Shellfish Management
•WHOI Red Tide Project
•Red Tide Blooms

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