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Harmful Algae & Red Tide Regional Monitoring Program

Map View About HAB News What Are HABs? HAB Species

What are Harmful Algal Blooms?

Aerial photograph of a dense bloom of Lingulodinium polyedrum off southern California in the summer of 2005. The dark red discoloration of the water is why this is often called a "Red Tide." Photo taken by Eddie Kisfaludy.
Dense concentrations of these cells, Lingulodinium polyedrum, cause the discoloration of the ocean. This species also bioluminescences at night when the cells are agitated.

Occasionally phytoplankton can grow very fast and form very dense populations or "blooms". Some of the blooms can be harmful to their environment by various means and are commonly referred to as Harmful Algal Blooms, ("HABs").

Phytoplankton blooms can form populations so dense that when they decay they deplete the oxygen from the water which can be harmful for fish and invertebrates. These types of blooms are not known to produce toxins however, the negative effects of oxygen depleted waters categorizes them as HABs.

What is a Red Tide?

Red Tide is a common name for a phenomenon where blooms of certain algal species, which contain red-brown pigments, cause the water to appear to be colored red.

Not all harmful algal blooms discolor the water and some HABs are formed by species of phytoplankton which can produce potent toxins that can be transferred through the food web where they affect zooplankton, shellfish, fish, birds, marine mammals, and humans that feed either directly or indirectly on them. This type of HAB does not necessarily require the same dense populations as the prior to show harmful effects.

What are phytoplankton?

Phytoplankton are microscopic, single celled plants that live in the ocean. Most species of phytoplankton are not harmful and are primary producers at the base of the food web. Phytoplankton, like land plants, use the energy from the sun and carbon dioxide to produce sugar and oxygen through a process called photosynthesis. This process requires special photosynthetic pigments to capture the solar energy. Chlorophyll-a is the most common pigment found in phytoplankton and is used to as a proxy to determine the biomass of primary producers.

There are several thousand species of phytoplankton. The two major groups that have representative HAB species in California are diatoms and dinoflagellates. Diatoms are unicellular organisms that have yellow-brown chloroplasts and their cell walls are made of silica almost like a glass house. Dinoflagellates are unicellular algae with two flagella which enable them to swim. Dinoflagellates can be autotrophic (photosynthetic) and/or heterotrophic (eat other organisms).

SoCal Map