Harmful algal blooms in Puget Sound

An algal bloom is a rapid increase or accumulation in the population of algae in a water system. While most are innocuous, there are a small number of algae species that produce harmful toxins to humans and animals.

Algal bloom. Photo: Eutrophication&Hypoxia (CC BY 2.0) https://www.flickr.com/photos/48722974@N07/5120831456
Algal bloom. Photo: Eutrophication&Hypoxia (CC BY 2.0) https://www.flickr.com/photos/48722974@N07/5120831456

Out of the thousands of species of microscopic marine algae in the world, a handful of species occur in Puget Sound that can produce toxins that are harmful to humans and wildlife. These toxin effects are the most pronounced during periodic “blooms” when these naturally-occurring species proliferate due to a combination of warm temperatures, sunlight, and nutrient-rich waters. The algae are ingested by shellfish, such as clams, oysters, mussels, and geoduck, which concentrate the toxins. Three types of HABs in Puget Sound are closely monitored by state agencies and tribes for issuing public health warnings, and long-term trends are being evaluated in a number of studies. 

Algal bloom

A visible but not harmful algal bloom of Noctiluca Photo: Leo Shaw, The Seattle Aquarium

Paralytic shellfish poisoning (PSP) is caused by toxins produced by the marine alga Alexandrium. Although the toxin does not harm shellfish, it can induce serious neurological disorders or even death when ingested by humans or marine mammals. The earliest documented case on the West Coast was in 1793, when five members of Vancouver’s expedition became ill and one died after eating mussels from the coast of British Columbia. In Washington State, illnesses and deaths in the 1940s launched long-term monitoring programs that have recently been assessed for geographic and temporal changes in PSP incidences. A geospatial map of the first shellfish closures or PSP event in each Puget Sound basin suggests that over time, toxigenic Alexandrium cells have been transported from northern to southern Puget Sound, with the initial “seed” population of cells in Washington State likely originating from the inland or coastal waters of Canada (Trainer et al. 2003). Most recently, closures due to PSP concerns were wide spread in Puget Sound in 2006.

Alexandrium catnella

Alexandrium catnella, an organism responsible for PSP (magnified 200x)

Domoic acid intrusion into Puget Sound: Some species of the marine alga Pseudo-nitzschia produce a toxin called domoic acid that was first documented in razor clams (at levels above U.S Food and Drug Administration action levels) on the outer coast of Washington in 1991. The toxin causes amnesic shellfish poisoning and interferes with nerve signal transmission; in severe cases it can cause shortterm memory loss, repiratory distress, and even death. Following emergency closures, a domoic acid monitoring program was established, and from 1991 to 2003 domoic acid remained an outer-coast problem. However, in September 2003, a bloom occurred near Marrowstone Island in Jefferson County. Domoic acid was detected at low levels over a wide area: as far west as Port Angeles, as far east as east Whidbey Island, and as far south as Port Ludlow (Bill et al. 2006). In September and October 2005, levels of domoic acid exceeding regulatory action limits were measured in commercial mussels from Penn Cove and in clams from Holmes Harbor. Numerous other shellfish species were also affected in other areas including Saratoga Passage and Sequim Bay. If domoic acid closures follow the same southward-migrating trend as PSP closures have in the past several decades, much of Puget Sound will be impacted by this toxin in the near future.

Pseudo-nitzschia australis

Pseudo-nitzschia australis, one of the species responsible for domoic acid poisoning (magnified 100x)

Fish kills: Heterosigma akashiwo is usually rare in plankton, but is capable of forming dense blooms that are often associated with low-salinity surface waters. It is not known to be toxic to humans but can cause extensive fish kills, especially of cultivated salmonids,and wild fish may also be affected. It has been present in Pacific Northwest waters at least since the 1960s and has been associated with fish kills since 1976 (Taylor and Horner 1994). Kills of finfish reared in net pens have also been caused by several species of diatoms, including Chaetoceros convolutus and C. concavicornis, since the early 1960s.

Heterosigma akashiwo

Heterosigma akashiwo (magnified 500x)

Nontoxic algal species: Several other species of algae that are found in Puget Sound waterways can cause damage to fisheries or result in nuisance water discolorations. A summary of these species and their effects is found in Horner et al. (1997).


Bill, B.D., F.H. Cox, R.A. Horner, J.A. Borchert, V.L. Trainer. 2006. The first closure of shellfish harvesting due to domoic acid in Puget Sound, Washington, USA. S. Afr. J. Mar. Sci.

Nontoxic algal species: Several other species of algae that are found in Puget Sound waterways can cause damage to fisheries or result in nuisance water discolorations. A summary of these species and their effects is found in Horner et al. (1997).

Taylor F.J.R. and R.A. Horner. 1994. Red tides and other problems with harmful algal blooms in Pacific Northwest coastal waters. In: Review of the marine environment and biota of Strait of Georgia, Puget Sound and Juan de Fuca Strait. Can. Fish. Aquat. Sci. Tech. Rep. 1948:175-186

Trainer, V.L., B.T.-L. Eberhart, J.C. Wekell, N.A. Adams, L. Hanson, F. Cox, and J. Dowell. 2003. Paralytic shellfish toxins in Puget Sound, Washington State. J. Shellfish Res. 22(1): 213-224

About the Author: 
Sound Science: Synthesizing ecological and socioeconomic information about the Puget Sound ecosystem. 2007. Mary H. Ruckelshaus and Michelle M. McClure, coordinators; prepared in cooperation with the Sound Science collaborative team. U.S. Dept. of Commerce, National Oceanic & Atmospheric Administration (NMFS), Northwest Fisheries Science Center. Seattle, Washington. 93 p.