This summer’s West Coast algal bloom was unusual. What would usual look like?

Previously, I described a record-breaking algal bloom that is ongoing off the West Coast of North America, stretching from California to the waters of the Gulf of Alaska. High levels of the algal toxin domoic acid led to the closures of recreational razor clam harvests in Oregon and Washington and large portions of Washington’s Dungeness crab fishery. Health advisories were issued to sardine and anchovy fishers along part of the California coastline. NOAA scientists are investigating the toxic bloom as one of many possible causes of death of over 30 whales.

This event is being taken seriously by NOAA scientists who are continuing to monitor the bloom and investigate its causes and impacts, including the possible connection to the unusually warm water in the North Pacific. However, unlike an episode of the TV show CSI, the actual science done to investigate the environmental factors at play during this year’s extreme algal bloom cannot be wrapped up in an hour. While scientists at sea and in research laboratories continue to learn more about this summer’s unusual bloom, I figured it might be a good time to talk about how West Coast algal blooms usually behave.

Micrograph of phytoplankton. Image by NOAA MESA Project, via the NOAA Photo Library.

Algal blooms occur when phytoplankton or microscopic algae rapidly reproduce. Like plants on land, phytoplankton growth is controlled by environmental factors such as light, nutrients, and temperature. The highest concentrations of phytoplankton are usually found in nearshore regions where nutrients are plentiful due to both river runoff and coastal upwelling—periods when strong winds push warm surface waters offshore and draw up deep, nutrient-rich water to replace it.

Also like plants on land, only some species of phytoplankton are toxic. Along the US west coast, the genus of phytoplankton known as Pseudo-nitzschia—which makes domoic acid— is a frequent suspect when blooms turn toxic. One of the most important questions that scientists are trying to answer is why some algae succeed under stressful environmental conditions and others do not. Some types of algae, including Pseudo-nitzschia, are more able to outcompete other phytoplankton during periods when environmental conditions are more “stressful.”

For example, blooms of Pseudo-nitzschia usually occur toward the end of the spring or during late summer-early fall upwelling seasons. During late spring and early summer, water temperatures are warming yet nutrient levels are starting to be depleted from the surface of the ocean. This is a great combination for Pseudo-nitzschia, a common and highly resilient phytoplankton along the west coast.

Satellite image of a massive bloom of phytoplankton off the coast of Oregon and Washington on July 26, 2014. Image courtesy NASA Earth Observatory.

Pseudo-nitzschia (the genus that is responsible for this year’s toxic bloom) can be found across the globe from the Mediterranean, Russia, Japan and Vietnam to South America, Namibia in Africa and New Zealand (Trainer et al. 2012). Some of the most damaging blooms have been observed off the west coast of North America, eastern Canada, Scotland, Ireland, Portugal, Chile, and in New Zealand.

The first recorded instance of human death caused by domoic acid poisoning, though, was in eastern Canada in 1987. Over 100 people ate contaminated mussels from Prince Edward Island; several suffered permanent short-term memory loss and 4 people died. The term “amnesic shellfish poisoning” was coined to describe the human intoxication syndrome caused by domoic acid, reflecting the symptom of short-term memory loss experienced by those exposed to high toxin levels. However, the majority of people who became ill suffered milder symptoms that included vomiting, nausea and diarrhea.

(Here, let me pause for a safety message. There have been no confirmed cases of human illness due to domoic acid since the event in eastern Canada. In the US, rigorous monitoring for harmful algal bloom toxins insures that shellfish purchased from commercial sources are safe for human consumption. Recreational shellfish harvesters should check with state, local, or tribal public health agencies to determine if shellfish from a given location are safe for harvesting.)

Along the U.S. West Coast, there are known “hot spots” for Pseudo-nitzschia, where these toxic blooms tend to occur more often: Point Conception and Monterey Bay, California; Heceta Bank off Newport, Oregon; and the Juan de Fuca eddy at the border of US and Canada. These are oceanic “crock pots” where water can swirl into eddies, allowing toxic blooms to develop and intensify in waters that stay in one place rather than being flushed out. In a particularly impressive event in central California in 1998, the neurotoxin domoic acid produced by a bloom of Pseudo-nitzschia led to the deaths of more than 400 California sea lions.

Monthly average chlorophyll concentrations (milligrams per cubic meter of water) in the North Pacific from July 2002 through July 2015, based on observations from the NASA/NOAA Suomi-NPP satellite.

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