Marine algae as a group are immensely important to the functioning of our planet, are the base of the marine food web and an important supplier of the world's oxygen. Unfortunately, there are a few dozen species of algae that produce powerful toxins that can be transferred through the food web where they can injure or even kill higher forms of life such as fish, birds, marine mammals and humans that feed on them either directly or indirectly. These harmful algae become noticeable during periodic events known as "harmful algal blooms" (HABs).

HAB events can lead to seabird die-offs, marine mammal injuries and risk to human health. HAB events also cause economic harm as well. Recreational and commercial fisheries shut down during HAB events to protect human health, resulting in lost revenue from fisheries and tourism.

The Pacific Northwest (PNW) region of the U.S. has a long history with HABs that can contaminate shellfish with toxins. The first documented illnesses associated with eating shellfish occurred in 1793, but earlier evidence of HAB-related illnesses can be found in indigenous American oral history. In the Northwest, the HAB species of concern include:

• Diatoms of genus Pseudo-nitzschia, which produce the toxin domoic acid. When ingested, domoic acid can cause the disease Amnesic Shellfish Poisoning (ASP). Pseudo-nitzschia is more commonly found in coastal areas, making the razor clam fishery particularly vulnerable to HAB events and ensuing closures. Dungeness crab, because they feed on razor clams and other shellfish, can also become toxic. In recent years domoic acid has been detected in mussels, clams, and oysters in Puget Sound.
• The dinoflagellate Alexandrium catanella can produce the toxin saxitoxin, which causes the disease Paralytic Shellfish Poisoning (PSP). PSP is commonly found throughout Puget Sound and coastal areas. All molluscan shellfish including clams, mussels, oysters, geoduck, and scallops are capable of accumulating PSP. So can moon snails and other snails.
• The dinoflagellate Dinophysis can produce the toxins okadaic acid and dinophysis toxins which cause the diease Diarrhetic Shellfish Poisoning (DSP). Dinophysis has been detected in PNW's marine waters for some time, but has not been known to produce toxin. However, in June 2011, three people became sick with DSP after eating recreationally harvested mussels from Washington waters that were confirmed to have the toxin.
• The flagellate Heterosigma akashiwo, though not harmful to humans, can kill aquacultured fish in Northwest waters.

State and local agencies along the West Coast continually monitor water quality and shellfish for toxins and will close areas to shellfish harvest if toxins reach a certain minimum level. In the Pacific Northwest, HAB events and the subsequent closing of beaches to shellfish harvest can have a major economic impact on communities which depend on shellfish aquaculture and tourism associated with recreational shellfish harvesting. When fisheries are closed to harvesting due to HABs, it can take several days to several months or longer for the shellfish to flush the toxin from their systems and be safe to eat again.