Overview: Harmful Algal Blooms in Alaska

Satellite showing bloomSatellite image of Bering Sea algal bloom. Provided by the SeaWiFS Project, NASA/Goddard Space Flight Center, and ORBIMAGE

Aquatic algae range in size from microscopic, single-celled phytoplankton to large seaweeds, such as kelp. They form the base of aquatic food webs and are extremely important in both marine and freshwater ecosystems. Most algae species are harmless, but a few produce toxins that can cause illness and death in fish, birds, marine mammals, and humans.

Microscopic phytoplankton occur naturally in aquatic ecosystems. Although they are small, when the temperature, nutrient, and light conditions are right they can reproduce rapidly to high concentrations. When this occurs it is referred to as an algal bloom. Algal blooms are an important source of nutrients for aquatic organisms. However, the concentration of algae can become so high during a bloom that they can harm the health of other organisms by clogging gills or making the water greatly deficient in oxygen, and dense blooms can change the color of the water. The color change is the source of the popular term “red tide.”

Although some species of phytoplankton produce toxins and red tides are often associated with toxic algae, the term "red tide" is misleading. Red tides are not always red (they can be brown, purple, pink, amber, etc.) and they can be composed of algae species that discolor the water but are completely harmless. Toxic algae can be harmful to other organisms, without discoloring the water. The term “harmful algal bloom” (HAB) for a bloom of toxic algae is more accurate than “red tide.”

HABs can occur in marine, estuarine, and fresh waters. When filter feeders such as zooplankton, clams, oysters, mussels, and small fish eat the algae, they accumulate algal toxins in their tissues. The toxins are passed up the food chain to higher level predators such as fish, birds, marine mammals, and humans as they consume the contaminated organisms, causing illness and death.

HABs can harm humans by negatively affecting commercial, recreational, and subsistence fisheries, tourism, and human health. This harm significantly impacts local economies and the livelihoods of coastal residents:

HABs in Alaska are caused by Alexandrium species (paralytic shellfish poisoning in humans), Pseudo-nitzschia species (amnesic shellfish poisoning in humans, also called domoic acid poisoning), Dinophysis species (diarrhetic shellfish poisoning in humans), and cyanobacteria.

HABs appear to be increasing in many areas of the world. In Alaska, warming waters, less sea ice cover, and longer growing seasons may increase the occurrence of HABs. HABs are an active area of research among scientists worldwide, and as research continues, hopefully more will be learned about the effects of these toxic algae and scientists will be better able to forecast HABs in coastal areas.

How to Report an Illness from Toxic Algae

Report any cases of suspected and confirmed illness from toxic algae!

Causes of HABs in Alaska

Alexandrium Species—Paralytic Shellfish Poisoning

Pseudo-nitzschia Species—Amnesic Shellfish Poisoning

Dinophysis Species—Diarrhetic Shellfish Poisoning


If you or someone you know experiences symptoms of harmful algae poisoning please report it (see Harmful Algal Blooms Facts for symptoms). To report it to the Alaska Section of Epidemiology, call 907-269-8000 (Mon–Fri 8 am–5 pm) or 1-800-478-0084 (after hours).

If one case occurs, there will likely be others and reporting even mild cases can save others from becoming ill and may save lives! Reporting cases helps with forecasting toxic blooms and issuing public health notices about the possibility of toxic shellfish. Health care providers and citizens should report any suspected case of algae poisoning immediately to the Alaska Section of Epidemiology.

For More Information

Harmful algal blooms

Harmful algae monitoring programs