Frequently Asked Questions

What are HABs (Harmful Algal Blooms)?

Among the thousands of species of microscopic algae at the base of the marine food chain are a few dozen which produce potent toxins.  These species make their presence known in many ways, ranging from massive "red tides" or blooms of cells that appear to discolor the water, to dilute, inconspicuous concentrations of cells noticed only because of the harm caused by their highly potent toxins. Blooms of non-toxic micro- and macroalgae (seaweeds) also cause harm due either to indirect effects of biomass accumulation (such as anoxia or habitat alteration) or to physical features (such as spines which lodge in fish gill tissue).

HABs have one unique feature in common – they cause harm, due either to their production of toxins or to the manner in which the cells’ physical structure or accumulated biomass affect co-occurring organisms and alter food-web dynamics.

HABs result from a combination of physical, chemical, and biological mechanisms and their interactions.

What is the difference between a "red tide" and a "harmful algal bloom (HAB)"?

Blooms are commonly called red tides, but scientists prefer the term "harmful algal blooms" (or HABs). The term red tide erroneously includes many blooms that discolor the water but cause no harm, and also excludes blooms of highly toxic cells that cause problems at low (and essentially invisible) cell concentrations. Therefore, harmful algal bloom is a more appropriate descriptor.

Where do HABs occur?

HABs occur in most coastal countries.  Many countries are faced with a bewildering array of toxic or harmful species and impacts, as well as disturbing trends of increasing bloom incidence, larger areas affected, more fisheries resources impacted, and higher economic losses.  It is now clear that the global expansion of HAB phenomena is in part a reflection of our ability to better define the boundaries of the problem – the nature and extent of toxic or harmful species and their impacts.  The fact that part of the expansion is simply because of increased scientific awareness and detection capabilities should not temper our concern.

Harmful algal blooms are not limited to marine waters.  Recent years have seen an expansion of toxic blooms in freshwater ecosystems.  These include not only the small ‘farm-ponds’ but also large bodies of water such as the Laurentian Great Lakes:  Lake Erie and Lake Ontario.

» Distribution of HABs in the U.S.
» Distribution of HABs throughout the world

Can I get sick from eating seafood or being near a HAB?

Yes.  Human illness resulting from HABs can occur in a variety of forms.  This can occur when toxic phytoplankton are filtered from the ocean as food by shellfish such as clams, mussels, oysters, or scallops, accumulating the algal toxins to levels that can cause illness or even be lethal to shellfish consumers including humans.  Fish can also become poisonous, either through the direct ingestion of toxic algae and their grazers or via food web transfer of the toxins through multiple trophic levels.

In addition to their accumulation in shellfish, some HAB toxins can be released directly into the water or air, either naturally, or following cell disruption by turbulence or through human activities such as water treatment processing.  For example, in the Gulf of Mexico, neurotoxins from Karenia brevis can be delivered to local residents and beachgoers via sea spray.  This leads to respiratory irritation, coughing, and other ailments.

Excessive growth of freshwater cyanobacteria can lead to blooms that cause severe neuro-, cyto- and hepatotoxicity in a variety of mammals (including humans).  Exposure to these toxins can cause an array of adverse health effects ranging from rashes, to allergies, to devastating liver damage in susceptible populations.

» Impacts of HABs on Human Health

Massachusetts Red Tide fact sheet
Red tide fact sheet from the Massachusetts Department of Health and Human Services.  Click here to access a spanish language version of the fact sheet.

Gulf of Mexico HAB primer
Common questions and answers for stakeholders, decision makers, coastal managers, and the education community.

California red tide leaflet
Seafood Network Information Center, California:  Red Tide:  Questions and Answers (PDF file)

California natural marine toxins
Natural Marine Toxins:  PSP and Domoic Acid (PDF format); sponsored by California Sea Grant, NOAA, and California State Resources Agency.

Florida Red Tide Fact Card
Florida red tide facts, including health and seafood safety tips.  PDF format.

Lyngbya newsletter
A newsletter from the Lyngbya ECOHAB project

Lyngbya fact sheet
A fact sheet from the Lyngbya ECOHAB project

Texas Golden Alga Facts
Golden alga facts including tips for recognition, reporting, and health and safety in PDF format.  To obtain hard copies please e-mail

Texas Red Tide Fact Card
Health tips and seafood safety tips on Texas red tide (PDF format).

NOAA state of the science fact sheet
State of the Science Fact Sheet on HABs, prepared by NOAA.

Dogs and HABs
Dogs and Harmful Algal Blooms, New York Sea Grant

Last updated: February 25, 2016