Background: Harmful Algal Blooms

An algal bloom only becomes harmful when it negatively affects the environment, coastal economies, or human health. Some algal blooms, called red tides, cause the water to appear red or brownish red in color.

Algal blooms, like the red tide shown here, can be benign or harmful depending on the species.

Research on harmful algal blooms (HABs) is needed in order to learn about effective ways to mitigate or avoid their potentially serious impacts. There are several mechanisms by which algal blooms can be classified as harmful, if they create: mechanical damage, physical barriers (viscosity and gelatinous barriers), anoxia/hypoxia (the depletion of oxygen), and/or toxins.

Marine toxins are widespread throughout the world's coastal oceans and can cause marine mammal, bird, and fish mortality, as well as human illness and fatalities. Many of the negative impacts to humans and marine organisms from HABs are related to toxic shellfish ingestion. Toxins produced by algal blooms often accumulate in filter feeding organisms, such as bivalve shellfish and filter feeding fishes. Shellfish and fish can act as carriers of the toxin (with limited or no effects); as carriers, they are considered a 'vector' transfering the toxin to organisms in higher trophic levels. As a result, the California Department of Public Health conducts the Preharvest Shellfish Protection and Marine Biotoxin Monitoring Program, which monitors shellfish growing areas in California.

For more information on Background: Harmful Algal Blooms, contact Meredith Howard at meredithh@sccwrp.org (714) 755-3263.

This page was last updated on: 1/21/2010