Harmful Algal Blooms


Periodically, populations of oceanic phytoplankton can grow and multiply rapidly, producing a “bloom” visible to the naked eye. Harmful Algal Blooms (HABs) are worldwide phenomena, but also occur in the Monterey Bay National Marine Sanctuary and throughout the coastal waters of California. HABs occur when microscopic algae (phytoplankton, often single cells) grow very rapidly under favorable conditions. Phytoplankton require light and nutrients to grow, and algal blooms are often triggered by an increase in nutrients. These nutrients can be naturally derived from the environment, or can be increased dramatically though anthropogenic (human) sources, such as runoff from land. Not all algal blooms (ABs) are harmful (HABs). However, HABs are noteworthy, in part, because some algal species have the ability produce toxins when they bloom, causing illness in marine mammals and humans alike, and can even have detrimental economic impacts. As a term, HABs has been defined by society, not by science.

Monterey Bay HABs web site.
California Monitoring and Alert Program web site. This site can generate plots of select HAB species cell counts over time.
CDHP Toxic Phytoplankton weekly map layers.

What are Phytoplankton?

In the most basic terms phytoplankton are very small organisms somewhat similar to plants. The term phytoplankton encompasses all microalgae, some of which are protists and some of which are bacteria. Phytoplankton are single celled organisms that photosynthesize, using sunlight to convert dissolved carbon dioxide and water into the organic building blocks of life. Phytoplankton have the ability to bloom when they are in favorable growth conditions. Different conditions are favorable to different types of species. For example, some species prefer warmer temperature and lower water velocity, while other species do not. Multiple environmental factors contribute to blooms, and many of these interactions between the environment and the phytoplankton are still poorly understood.

What are Cyanobacteria?

Cyanobacteria, more commonly called blue-green algae, are prokaryotic phytoplankton that obtain their energy through photosynthesis. CyanoHABs are harmful blooms of cyanobacteria. Cyanobacteria can be found in freshwater, brackish, and also marine waters.

What is a Red Tide?

When phytoplankton in the water are found in vast quantity they have the ability to color the water. This is called a “red tide. ” Most red tides are not harmful. Most commonly red tides are caused by a type of phytoplankton called dinoflagellates, however there are other red tide producing species such as diatoms. Red Tide producing species found in Monterey Bay:

Alexandrium catenella (dinoflagellate)
Pseudo-nitzschia spp. (diatoms)
Cochlodinium fulvescens (dinoflagellate)
Lingulodinium polyedrum (dinoflagellate)
Akashiwo sanguinea (dinoflagellate)
Dinophysis spp. (dinoflagellate)
Ceratium spp. (dinoflagellate)

A normal year in Monterey Bay

During the year, light, temperature, phytoplankton, zooplankton abundances vary with the seasons. At the beginning of the year (winter) mid-latitude areas like Monterey Bay are characterized by heavy rainfall and also high surf activity. During this time average daily light input is low (short days and high cloud cover) and inorganic nutrients are high, but because phytoplankton need nutrients and light their densities are low. When phytoplankton densities are low the zooplankton species that graze them are also low. In the spring the wind is elevated and upwelling begins. The upwelling brings cold water and high nutrients. Light levels increase (days grow longer) and surface temperatures warm. This combination of light and nutrients fuels phytoplankton growth and they are able to bloom, leading to a peak in phytoplankton densities.

Subsequently the zooplankton grazers are able to eat the phytoplankton and peak shortly thereafter. Later in spring the nutrients begin to decline and in response the phytoplankton also decline. At this time, zooplankton grazing is another contributing factor to the phytoplankton decline, and the zooplankton numbers drop in response. During the end of the summer season light levels begin to decrease and nutrients increase due to a change in density stratification in the water column. Because of these increased nutrients, there is often a small phytoplankton bloom in fall and a corresponding increase in zooplankton. This bloom will lessen as light levels decrease into the late fall.

The Land to Sea Connection

There is a connection between the land and sea. What we do on land often has a direct effect on the ocean. This is especially true for harmful algal blooms. Nutrients carried by runoff from land to sea can cause naturally occurring algae to bloom, and sometimes these algae produce toxin. Land-derived nutrients come from all types of sources including things like sewage waste, farm animals, and fertilizers used for agriculture, lawns, and golf courses. In addition to contributing to marine algal blooms in the ocean, these nutrients also contribute to freshwater algal blooms in watersheds. Recently, a freshwater algal toxin has been found in the marine environment of the Monterey Bay National Marine Sanctuary. This toxin is called microcystin, and the alga that produces the toxin is called Microcystis aeruginosa. Microcystin is a hepatotoxin, which means it affects the liver. Blooms of M. aeruginosa that produce microcystin can be considered cyanoHABs. If freshwater blooms are large enough, and enough toxin is produced by the bloom, the toxin can be transported into the marine environment through the watershed. The magnitude of the bloom is influenced by the amount of nutrients added to the system. Microcystin has recently impacted sea otters in Monterey Bay National Marine Sanctuary. Filter feeding organisms, like clams and mussels, can accumulate microcystin toxin in their tissues, and if the contaminated shellfish are eaten by higher trophic level organisms like sea otters, the otters can become sick or die.

More Information

UUSGS Open File Report on Cyanobacterial harmful algal blooms and USGS capabilities
The Science of Harmful Algae Blooms web site
Diatoms vs. dinoflagellates: what is the difference? Images comparing the species
HABs vs. Red Tides: what is the difference? Images comparing the two type of events
What are the toxigenic phytoplankton species on the US West Coast? Images of toxigenic phytoplankton
Who is monitoring for bloom activity in Monterey Bay?
Miller et al. (2010): Evidence for a novel marine Harmful Algal Bloom: Cyanotoxin (Microcystin) transfer from land to sea otters.
Gibble and Kudela (2104): Detection of persistent microcystin toxins at the land-sea interface in Monterey Bay, California.