phytoplankton image

Phytoplankton, or microalgae, are microscopic, single-celled plants that live in the ocean. Most phytoplankton are not harmful and are primary producers that form the base of the ocean’s food web. Phytoplankton, like land plants, use the energy from the sun and carbon dioxide to produce food and oxygen through a process called photosynthesis. Like terrestrial plants, 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 harmful blooms are visible to the naked eye.

In central and northern California, many blooms are the result of 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 from below and replace the surface water. This phenomenon is known as coastal “upwelling” and allows algae to grow and support the coastal food web. Algae can grow to excessive numbers after an upwelling event ends and winds relax—allowing the surface waters to warm up and stratify. When the waters stratify, phytoplankton are trapped near the surface, forming a bloom.

Other regional bloom events may be linked to “overfeeding.” This occurs 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 effect is known as eutrophication.

The vast majority of the approximately 5,000 known species of phytoplankton (microalgae) are not harmful (only several dozen are known to produce toxins). Ocean ecosystems rely on benign phytoplankton to serve as the base of the food chain. However, a small number of algal species produce toxic chemicals that can kill fish, shellfish, mammals, birds, and even humans.
Photo of pelican killed by harmful algal bloom

Harmful Algal Blooms (known as HABs) cause harm in two ways.

The first is the production of toxins that spread into the marine ecosystem.

The second category includes a variety of environmental impacts that arise from having excessive algal populations (at least one million algae per liter of seawater).

  • Some algae are nontoxic but can cause problems for marine ecosystems in other ways.
  • Some blooms clog the gills of fish and invertebrates or smother corals and underwater aquatic vegetation.
  • Other algae discolor water, pollute beaches, or cause drinking water and fish to taste foul.
  • Some blooms of algae are not inherently harmful but may result in severe environmental impacts.
    • For example, low oxygen conditions may result from the decay of excessive amounts of algal growth caused by nutrient pollution.
    • Dense blooms can also block sunlight for beneficial algae and seagrasses.