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CORVALLIS, Ore. - Harmful algal blooms that have closed shellfish harvests in the Pacific Northwest and caused "red tides" elsewhere appear to be increasing, scientists say, and the likely suspects are global climate change and increased human impact in coastal zones.

One recent bloom has significantly elevated levels of a toxin called domoic acid in Oregon razor clams, prompting a statewide harvest closure that has been in effect all summer. This follows several other regional closures over the past 2-3 years. Humans who consume shellfish with high levels of domoic acid may suffer vomiting, diarrhea, disorientation and memory loss; in severe cases, domoic acid can result in comas and even death.

Two Oregon scientists - one from Oregon State University and the other from the University of Oregon - are trying to identify these toxic blooms as they occur by combining satellite imagery and physical data in a project funded by the National Oceanic and Atmospheric Administration (NOAA).

They believe that certain areas, including the Heceta Bank off the central Oregon coast, may act as "incubators" for generating the blooms of Pseudonitzschia, a phytoplankton species that can turn toxic, creating domoic acid. When consumed by shellfish, it accumulates in their tissues.

"Historically, the first warning sign we get for these toxic blooms is when domoic acid shows up during routine testing of razor clams and other shellfish - and by then it's a done deal," said Peter Strutton, an assistant professor in OSU's College of Oceanic and Atmospheric Sciences, and co-principal investigator of the study. "Unlike the phytoplankton species that causes red tides off Florida, the blooms off Oregon don't have characteristic pigment that makes them easily visible.

"But we think that we can combine satellite data on chlorophyll levels and ocean conditions to eventually detect these blooms as they develop and provide an early warning system for coastal managers, health officials, and commercial and recreational fishers," Strutton added.

A number of different phytoplankton species bloom regularly off the Pacific Northwest coast and, in fact, are important in feeding the marine food chain. But a breakdown in the metabolic process of Pseudonitzschia - possibly triggered through stress - creates domoic acid, according to Michelle Wood, a professor of biology at the University of Oregon and co-PI on the study.

Wood said it is possible that a symbiotic interaction between the diatoms and certain species of marine bacteria enable, or enhance, the production of toxins. It may also influence the level of toxicity in a bloom, she added.

"Clams and other shellfish take in the toxins when they filter water and the toxin level can vary for a number of reasons," Wood said. "The diatoms that produce domoic acid seem to produce less when they are growing rapidly than when they have used up the nutrients in the water and become stressed physiologically. So as each phytoplankton bloom progresses, there is potential for water masses to break off and carry populations of diatoms at different stages of growth onto shore."

The researchers have combed through data over the last 10 years from the Oregon shellfish monitoring program conducted by the Oregon Department of Agriculture. They are comparing recorded levels of toxicity in razor clams, mussels and other shellfish with archival satellite data showing sea surface temperatures and "ocean color" - chlorophyll levels and rates of fluorescence - in the same regions that the shellfish testing took place.

Strutton says they hope to find an optical signature for potential blooms, and during the next two years visit those areas at peak times to sample the water and drag nets through the surface ocean to measure phytoplankton abundance and toxicity levels.

One area of interest is the Heceta Bank, which the researchers believe is similar to the Juan de Fuca eddy off northern Washington - a known "hot spot" for harmful algal blooms. The Heceta Bank bulges out off the Oregon coast, and the shallow water there creates an eddy effect that appears to send a steady flux of nutrients to the surface, triggering the algal blooms.

"Harmful algal blooms are the negative side of coastal upwelling," Strutton said. "There is growing evidence that these blooms have been increasing over the last 20 years and not only are becoming more frequent, but more intense and with longer duration. We also are starting to record toxic events in places that haven't had them, so there is a concern that they may be spreading.

"The spreading could be caused by the transport of phytoplankton in the ballast water of ships," he added.

Strutton said global climate change leading to warmer ocean waters is one theory behind the increasing incidents of harmful algal blooms. Human activity, including the release of nutrients into the oceans from agriculture fertilizers that leech into river systems, may also be a cause.

"Every spring there is an algal bloom in the Pacific from San Diego, Calif., to Vancouver, B.C., that is a result of warming spring temperatures, upwelling and the general ocean-atmosphere interaction," Strutton said. "Often one species of phytoplankton will dominate, and we need to identify when it is Pseudonitzschia so we can create an early warning system."

Wood said the lag time from onset of an algal bloom to significant toxicity in shellfish may be a few days depending on how "hot" - or full of toxin-producing cells - the water is. The effects can linger.

"Mussels lose their toxicity very quickly, in a matter of days," Wood said, "but razor clams incorporate the toxin in their tissue and remain toxic for weeks - even when they are no longer consuming toxin-producing food."