Phytoplankton & Algal Blooms

Phytoplankton are microscopic, single-celled plants that live in the ocean. Most species are not harmful and are primary producers that form the base of the ocean’s food web. Like land plants, phytoplankton use the sun’s energy and carbon dioxide to produce food and oxygen through photosynthesis. Phytoplankton contain chlorophyll and need sunlight and nutrients to grow. Virtually all marine phytoplankton are buoyant and live in the upper part of the water column, called the photic zone, where sunlight is available.

Algal blooms occur when environmental conditions allow exponential growth of phytoplankton that create very dense clouds. Algal blooms need a combination of environmental factors to form: sunlight, sufficient nutrients and calm waters. These blooms are commonly called red tides but can be a number of different colors depending on the type(s) of algae present. Not all blooms are visible to the naked eye.

In central and northern California, many blooms are caused by natural shifts in ocean currents and wind circulation. In the spring through the early fall, a pattern of strong winds causes surface waters to move away from the shoreline, causing colder, nutrient-rich deeper water to rise . This phenomenon is known as coastal upwelling and the nutrients from the deeper water allow algae to grow and support the coastal food web. Algae can grow to excessive numbers after an upwelling ends, winds die down and the surface waters warm and stratify. When the waters stratify, phytoplankton are trapped near the surface, forming a bloom.

Other regional bloom events may be linked to overfeeding — that is, when excess nutrients like phosphorus, nitrogen and carbon from farms and lawns flow downriver to the ocean and build up at a rate that stimulates excessive algae growth beyond healthy levels. This is called eutrophication.

The vast majority of the approximately 5,000 known species of phytoplankton are not harmful and serve as the base of the food web. Only several dozen species — harmful algae — are known to produce toxins that can kill fish, shellfish, mammals, birds and even humans.


Harmful Algal Blooms (HABs) produce biotoxins that can bioaccumulate in the marine food web similar to mercury. Bioaccumulation happens when toxins build up in an organism at a rate faster than they can be broken down.

Sometimes organisms are not affected by the toxins themselves, but act as vectors and transport the toxins up the food web into higher level organisms such as fish, seabirds, manatees, sea lions, turtles and dolphins. In addition to impacts of single HAB events, the effects of chronic exposure to HAB toxins on these animals can lead to changes in overall health, reproductive failure and behavior changes. These types of impacts are only beginning to be understood, but impacts from chronic exposure could be significant on protected and endangered species, as well as humans.

Human illness or death from HABs occur when people eat contaminated fish and shellfish. Coastal state governments closely monitor select toxin-producing species of algae to provide as much advance notice to, and regulation of, the seafood industry as possible.

In California, the two main illnesses of concern are paralytic shellfish poisoning (PSP) and amnesic shellfish poisoning (ASP).

  • PSP is caused by neurotoxins produced by a dinoflagellate algal bloom, and its symptoms are neurological — causing paralysis or death by slowing respiration. PSP-producing blooms have been sporadic in history and location, but tend to occur annually along the coast north of San Francisco Bay and along the San Luis Obispo County coast. Tens of thousands of shellfish deaths have been attributed to PSP poisoning.
  • ASP is caused by blooms from the Pseudo-nitzschia genus. Several species produce domoic acid, the toxin responsible for ASP. Amnesic shellfish poisoning results in gastrointestinal and neurological disorders, including memory loss or even death. Domoic acid-producing blooms occur nearly every year in California, and ASP has been linked to hundreds of marine mammal and bird deaths since 2003. Domoic acid is regularly detected in harvested shellfish during peak HABs season, crossing seafood safety thresholds for human consumption.

California’s Department of Public Health closely monitors toxin levels in seafood that are associated with PSP and ASP and if toxins accumulate above regulatory limits, both commercial and recreational harvesting is closed to prevent the spread of these illnesses to humans. Sport-harvested mussels are routinely closed from May-October to prevent human cases of PSP and ASP. The Department of Fish and Wildlife also tracks exposure of marine birds and mammals to these toxins.

In addition to PSP and ASP, there are several new illnesses emerging as threats to the marine ecosystem and human health in California:

  • Several species of Dinophysis produce okadaic acid which can bioaccumulate in shellfish, causing diarrhetic shellfish poisoning (DSP) in humans. Toxic effects from Dinophysis algae have only recently become an issue on the west coast.
  • Yessotoxins are also produced by some dinoflagellate blooms and were first detected in 2004. A yessotoxin is suspected to have caused massive abalone and sea urchin deaths in a 2011 event off the Sonoma County coast.
  • Another emerging issue is the increase in HABs in waters at the land-sea interface — estuaries and rivers. Since the 1990s, blooms of blue-green algae (also called cyanobacteria) have increased in frequency and abundance, sometimes producing toxins called microcystins. These have had lethal effects on fish, shellfish and even sea otters.

Researchers estimate that blooms of harmful algae cost the United States at least $80 million per year from wildlife mortality; the cost of medical treatment for human and animal populations; fisheries closures; reduction in seafood sales; monitoring costs and reduced tourism. HABs can also contaminate seawater used to produce freshwater for humans; climate change is expected to exacerbate this problem.

Even nontoxic algae can cause problems for marine ecosystems in other ways:

  • Blooms can clog the gills of fish and invertebrates or smother corals and underwater aquatic vegetation;
  • They can discolor water, pollute beaches or cause drinking water and fish to taste foul;
  • Excessive algal decay can result in low oxygen conditions, killing bottom-dwelling animals that cannot easily move to more oxygenated waters;
  • Dense blooms can also block sunlight from reaching beneficial algae and seagrasses.