Patterns of blooms of certain types of algae in California coastal waters have been changing. While no trend is evident, these harmful algal blooms are influenced by warming ocean temperatures.

Algae in ocean waters are a vital part of the marine food web. However, when colonies of algae grow out of control, these harmful algal blooms can lead to adverse effects on people, fish, shellfish, marine mammals, and birds. In waters off the California coast, the toxin domoic acid, associated with blooms of the diatom Pseudo-nitzschia, has had significant impacts on marine wildlife and fisheries. For more information, download the Marine Harmful Algal Blooms chapter.

Harmful algal blooms in ocean waters produce toxins that can move up the food chain. When levels of toxin are unsafe for human consumption, commercial fisheries are closed. Marine mammals and birds are also affected. Poisoned sea lions can exhibit seizures, head weaving, and erratic behavior. Although not an indication of toxicity, visible discoloration of ocean waters (sometimes called “red tides”) can occur during blooms, and it is important to check for health advisories.  

Credit: (left), NOAA Fisheries (middle), UC San Diego (right)

The heatmaps present results of weekly sampling for Pseudo-nitzchia at three of the nine stations that are part of the California Harmful Algal Bloom Monitoring Alert Program. The colors represent the relative abundance index – the percentage of Pseudo-nitzchia compared to all other phytoplankton species in a given sample.

Sources: Radan, 2021 and Kudela pers. comm., 2021 (Santa Cruz);
HABMAP/SCCOOS, 2021 (Stearns Wharf and Newport Beach Pier)

• There is no trend in the abundance of Pseudo-nitzschia at the three monitoring locations. These diatoms occurred most frequently and at the highest abundance levels at Stearns Wharf.
• Across eight monitoring stations along the California coast, cell counts of Pseudo-nitzschia and concentrations of domoic acid were lowest in the winter (December to February). For most locations, the highest cell counts occurred in the spring and summer, and the highest toxin concentrations in the spring (March to May).

• The toxins associated with harmful algal blooms can accumulate in fish and shellfish, causing illness or even death when these are consumed by sea birds, marine mammals or humans.
• When these toxins occur at unsafe levels in seafood, California fisheries are closed, resulting in economic loss. For example, the delayed 2015/2016 Dungeness crab season due to a West Coast-wide algae bloom is estimated to have cost over $40 million in lost revenue
• Ecological impacts of harmful algal blooms can include clogging or lacerating fish gills, bloom-derived seafoam destroying the waterproofing of seabird feathers, and depleting dissolved oxygen and reducing light penetration in ocean waters.

• Warming ocean temperatures, including marine heat waves, have contributed to changes in the distribution and abundance of marine harmful algal blooms and in toxin levels in recent years. Warmer water conditions generally favor blooms.
• Other factors include wind direction and intensity, seasonal upwelling of deep waters to the surface, sunlight, nutrients, and large scale ocean circulation patterns, such as El Niño.

• California Harmful Algal Bloom Monitoring and Alert Program, HABMAP
• California Department of Fish and Wildlife, Health Advisories and Closures for California Finfish, Shellfish and Crustaceans
• California Department of Public Health, Marine Biotoxin Monitoring Program
• California Office of Environmental Health Hazard Assessment, Domoic Acid (a marine biotoxin) in fish and shellfish
• Centers for Disease Control and Prevention, One Health Harmful Algal Bloom Systems (OHHABS) and Harmful Algal Bloom (HAB)-Associated Illness
• National Oceanic and Atmospheric Administration, Environmental Research Division's Data Access Program (ERDDAP), Harmful Algal Bloom Monitoring and Alert Program Data
• Sea Grant California, Harmful Algal Blooms and Wild Caught Seafood in California