Forecasting harmful algal blooms in Monterey Bay

Peter Wallerstein of MAR testing the responsiveness of a California sea lion. Photo: Jonathan Alcorn

Peter Wallerstein of MAR testing the responsiveness of a California sea lion. Photo: Jonathan Alcorn

For reasons that mystify scientists, certain types of algal blooms in Monterey Bay have become vastly more toxic in the last 15 years.

“We have seen a  30- to 100-fold increase in domoic acid (an algal toxin) in water samples in the last decade or so,” said Clarissa Anderson, a biological oceanographer at UC Santa Cruz who recently won a California Sea Grant Focus Award to study the blooms.

Nobody knows why the algae produce the toxin – it could be a chemical defense against predators or a means of outcompeting other algae for limited nutrients such as iron – but, the toxin’s presence is a real concern.

Sea lions have been the most obvious victims of domoic acid poisoning as they feed heavily on small fishes that may accumulate the potent algal toxin.

Watch video of California sea lions suffering from seizures.

In the coming months, Anderson will be looking to figure out why algae that are natural to the region (the cells have been identified in ancient seafloor sediment cores) are now making more of the toxin, and whether this is linked to fertilizers and other micronutrients in freshwater flows to the bay.

“We think that the toxicity of these blooms is related to agricultural runoff,” Anderson said. “We are especially interested in “first-flush” storms in the beginning of the rainy season in fall.”

Most algal blooms in California occur in spring and summer during the upwelling season, when winds bring cold, nutrient-rich water to the sunlit waters where the algae live.

Public health officials issue an annual statewide quarantine on all sport-harvested mussels from May 1 through Oct. 31, typically, to protect people from ingesting toxins that have accumulated in the bivalves during the upwelling season.

Recently though, highly toxic blooms have also been observed in the fall after the quarantine has been lifted. Among the main goals of the Sea Grant project is to be able to forecast these blooms to prevent the quarantine from being lifted prematurely.

The focus is on forecasting domoic acid production because of its significance to wildlife and public health.

Since the first documented domic acid outbreak in California in 1998, marine mammal strandings have become a yearly event, according to the Marine Mammal Center in Sausalito, which rescues and treats sick animals. Medical researchers have also noted that Native Americans in the Pacific Northwest who consume a lot of razor clams have higher rates of Alzheimer’s dementia. There is also evidence of delayed mental development among Native American children who have been exposed to domoic acid.

The symptoms of domoic acid poisoning look a lot like dementia or memory loss, Anderson explained. This is because domoic acid is able to mimic an amino acid (glutamic acid) that controls electrical impulses along the central nervous system. When domoic acid binds to the brain’s glutamate receptors, the signaling system is basically jammed in the open or “on” position and the brain’s internal wiring gets fried.

“Even if you don’t get sick, we don’t know what long-term exposure to these toxins does to your brain,” Anderson said. “If you are eating a lot of shellfish from areas where these blooms occur, you probably are being exposed.”

Below is a technical summary of her 1-year Focus Award project:

A diatom (genus Pseudo-Nitzschia) that produces domoic acid.

A close-up of a diatom that produces domoic acid. Steve Oravitz

R/CONT-221 02/01/13–01/31/14
Forecasting river runoff effects on domoic acid production in coastal California
Clarissa Anderson
Raphael Kudela
Christopher Edwards

In Monterey Bay, most of the toxic blooms of microalgae of the genus Pseudo-nitzschia occur during spring and summer upwelling, when macronutrients from depth are brought to the sun-lit surface waters. Though less frequent, highly harmful blooms may also form after “first flush” storms – the first heavy rains of the season, which occur in fall. Freshwater discharges from the Pajaro and Salinas rivers may trigger these blooms by lowering the amount of silicon and decreasing the amount of nitrogen in the bay. In this Focus Award project, scientists will use field data collected before and after first flush events, as well as other environmental and monitoring data, to parameterize a simple numerical model for estimating the rate of domoic acid production. The model assumes that the rate of domoic acid production is proportional to the biomass of domoic-acid-producing algae, multiplied by their growth rate. The proportionality constant is a “domoic acid production factor” that assumes a primary nutrient limiting process (i.e., silicon limitation or nitrate limitation) and is a function of three “tunable” parameters. The main goal of this project is to back-solve (or optimize) these three parameters using existing data and to then check the “fit” using other existing (or new) data. When this is done, scientists will have a “zero-dimensional” model for domoic acid production that they can continue to develop and ultimately “couple” to 3D circulation models. Domoic acid is a potent neurotoxin that has caused marine mammal and seabird die-offs in Central California and can accumulate in seafood, causing “amnesic shellfish poisoning.” New research suggests that chronic low-level exposure to the toxin may pose a health threat to vertebrates. Results from this project will further efforts to understand what causes these harmful algal blooms and forecast their occurrence.

Written by Christina S. Johnson,


NOAA’s California Sea Grant College Program is a statewide, multi-university program of marine research, extension services, and education activities administered by Scripps Institution of Oceanography at the University of California, San Diego. It is one of 33 Sea Grant programs and is part of the National Oceanic and Atmospheric Administration (NOAA), U.S. Department of Commerce. Visit our website ( to sign up for email news or follow us on Facebook or Twitter.


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