Subscribe to Diving Deeper

Ocean Service Feeds

What is a Podcast?

A podcast is a an audio file published on the web. The files are usually downloaded onto computers or portable listening devices such as iPods or other players.

Read more about podcasting from

Find other podcasts from the US government

Diving Deeper: Episode 17 (October 7, 2009) —
What is a harmful algal bloom?

HOST: Welcome to Diving Deeper where we interview National Ocean Service scientists on the ocean topics and information that are important to you! I’m your host Kate Nielsen.

Today’s question is….What is a harmful algal bloom?

Harmful algal blooms, also known as HABs, are blooms of species of algae that can have negative impacts on humans, marine and freshwater environments, and coastal economies. These blooms occur when phytoplankton, which are tiny microscopic plants, grow quickly in large quantities while producing toxic or harmful effects on people, fish, shellfish, marine mammals, and birds.

To help us dive a little deeper into this question, we will talk with Allison Sill on harmful algal blooms – what they are, what causes them, and the impacts of harmful algal blooms. Allison is the former coordinator for the Phytoplankton Monitoring Network with the National Centers for Coastal Ocean Science. Hi, Allison, welcome to our show.

ALLISON SILL: Hi Kate, thanks, it’s good to be here.

HOST: Allison, first, what is the difference between a harmful algal bloom and a red tide?

ALLISON SILL: Well, red tides are not always going to be harmful and it’s a general term used to describe blooms. Many tides aren’t red, many of them are actually golden brown, yellow, you can actually have green tides as well. Scientists use the term harmful algal bloom to describe an algal bloom that can have a negative impact on the environment.

HOST: Are all algal blooms harmful?

ALLISON SILL: No, not all algal blooms are actually harmful. And actually less than one percent produce toxins. Blooms can be beneficial. You have to remember that phytoplankton are producers so they’re found at the bottom of the marine food chain and all other life in the ocean relies on this phytoplankton. Blooms can also be a good indicator of environmental change not only in the water, but also on land.

HOST: OK, so not all algal blooms are harmful. Is a bloom only harmful if it produces toxins?

ALLISON SILL: No, a bloom does not have to produce toxins in order to be harmful to the environment. It can also be harmful by causing anoxic conditions where oxygen is depleted from the water. It can block light to organisms lower in the water column or it can clog or harm fish gills. All of these effects can harm or kill plants or fish in the environment.

HOST: Where do harmful algal blooms occur?

ALLISON SILL: Harmful algal blooms can occur along every coast. For example, blooms can occur along the Florida coast and they can occur during any month, but usually they’re more frequent in the summer months. Blooms can also occur along the California coast. Many of the toxic blooms that we see impacting marine mammals and birds occur along the Pacific coast.

HOST: Allison, earlier you mentioned that blooms occur in coastal waters. Do harmful algal blooms only occur in salty, marine waters or can they also occur in freshwater?

ALLISON SILL: Kate, harmful algal blooms can occur in freshwater too and a great example of this would be the Great Lakes where they have reoccurring green blooms and these green blooms can produce toxins. These toxins can be harmful to humans so they are monitored carefully and toxins are removed by the suppliers of public drinking water in that area.

HOST: What causes harmful algal blooms?

ALLISON SILL: There are a variety of factors that can cause these harmful algal blooms some of which are natural and some of which are not natural. The cause also is dependent on the species type and what that species needs in order to actually bloom. When conditions such as salinity – which is the amount of salt in the water, temperature, nutrients - when these factors are optimal, the bloom can occur. And some species are actually good indicators of coastal eutrophication, which is known as nutrient pollution.

HOST: I think we can all understand that in different geographies and at different times of the year, the water conditions such as temperature, light, and salinity that you mentioned, can change in a given area. What causes these excess nutrients to be present in the water?

ALLISON SILL: There are actually both natural and unnatural sources of nutrients in the ocean. Upwelling is a natural occurrence which can be caused by winds and currents, it’s seasonal, and it’s when nutrients are pulled up from the sea floor and suspended near the surface of the water. Nutrient runoff from land is also another cause for these blooms to occur. And we’re finding that this nutrient inflow from land is happening more frequently because more people are moving to the coastline, therefore there’s more human activity along the coast. And the nutrients that are causing problems include things such as fertilizers, pesticides, and detergents which have high levels of phosphorus in them.

HOST: Allison, what are some of the impacts that we see from harmful algal blooms?

ALLISON SILL: Kate, as mentioned before an impact that is very obvious to an observer is actually the discoloration of the water. It might appear red, green, brown. There’s also a great number of impacts on the marine environment. You can have low dissolved oxygen or anoxic conditions which can lead to the death of fish and you might see fish floating on the surface of the water. We also have what are called unusual mortality events and this is when there are unexpected strandings of marine mammals on the beach, typically it can happen in high numbers. This type of event requires a response from certain agencies. And the research is indicating that 26 percent of all unusual mortality events are actually related to marine toxins. There are also human health syndromes, five in particular that are caused by certain species.

HOST: Allison, how do people become sick from these algal toxins?

ALLISON SILL: As mentioned in the previous answer, there are five human health syndromes, four of which can be caused by eating toxic shellfish, one of which can be caused by eating toxic fish. If you were to eat toxic shellfish, this could potentially lead to severe poisoning, possibly even death, but this is very rare because there’s very careful monitoring of algae and shellfish by state agencies. If a toxin is found around shellfish beds, then those shellfish beds are closed to harvesting.

Airborne toxins can be found especially on the beaches of the west coast of Florida and these airborne toxins can cause temporary asthma symptoms in beach goers. This can also have a more long-term effect on residents with respiratory ailments. If these toxins are found on beaches in Florida, then those beaches are closed. But please, do not give up seafood folks because remember less than one percent of all phytoplankton can be toxic, so these syndromes are very rare.

HOST: I know you mentioned earlier how decaying blooms can deplete or remove oxygen in the water killing marine life and that some species can damage or clog fish gills. Are there other impacts of harmful algal blooms on fish and other marine life?

ALLISON SILL: Kate, harmful algal blooms can kill fish and other marine life. And one of the ways in which this can happen is through a process called biomagnification. Biomagnification is when toxins are accumulated within the tissue of animals as you travel up the food chain. So you have very small phytoplankton that have the toxin in them and as those larger animals eat smaller animals that toxin is then increased within the tissue of the animal, therefore harming animals at the top of the food chain. 

In addition to fish and marine life, some blooms can be so thick that it’s hard for light to actually penetrate down below and this can really hurt seagrasses and other aquatic vegetation that rely on light.

HOST: Allison, so it sounds like the impacts from the algal toxins from some blooms can not only present possible threats to human health and the health of marine life, but could also cause economic impacts to our coastal fisheries.

ALLISON SILL: That’s right Kate. Harmful algal blooms can harm tourism. They can cause beach closures, they can also cause a lot of trouble for commercial and recreational fishing industries, and can even harm restaurants that depend on these fisheries in order to stay open. Approximately $82 million each year is lost to harmful algal blooms and this is actually thought to be a conservative estimate.

HOST: Allison, how are non-coastal residents impacted by harmful algal blooms?

ALLISON SILL: Kate, I think it’s interesting a lot of people forget that people that aren’t along the coast can actually be affected by these blooms as well. And non-coastal residents can be affected by having their vacations disrupted. If the beaches in Florida are closed and that’s where they intend to vacation, then their vacation plans are going to change. Also if they eat seafood and the seafood potentially could be contaminated with some of these toxins, that could cause it to be unavailable to them. Also remember, we’re all connected by our watersheds so those inland areas could also be contributing to the nutrient runoff found in our coastal waters.

HOST: Allison, what is the role of the National Ocean Service in studying harmful algal blooms?

ALLISON SILL: Well Kate, research conducted by NOAA’s National Ocean Service is just one piece or one component to increase understanding of harmful algal blooms and their impacts. There are actually many different offices within NOAA working on harmful algal bloom issues. One of which is the Phytoplankton Monitoring Network, who I’m involved with, and that is where citizen and student volunteers are trained on how to collect and identify phytoplankton in their coastal waters. This data is reported to NOAA scientists so it’s great to get these people involved in real-world research.

NOAA supports an Analytical Response Team and this team confirms the presence of specific algae and toxins during the blooms and in marine mammal poisonings. Remember the unusual mortality events that I was talking about earlier where 26 percent are actually caused by marine toxins, this is the team that works with those events. NOAA oversees a Harmful Algal Bloom Forecasting System which is extremely beneficial and it uses satellite data and imagery which can provides regular bulletins to coastal managers and even the general public. And it forecasts the location, extent, and potential development of blooms. This in turn can help scientists and researchers see where these blooms might be going and how it might impact the coastline.

HOST: Allison, it’s great to hear that all of these resources are dedicated to improving the understanding and possibly even being able to forecast these blooms to further minimize human health impacts and economic losses. Do you have any final, closing words for our listeners today?

ALLISON SILL: Well, I want to thank you for inviting me here today Kate because I think this is an important topic for people to learn more about. And NOAA research on harmful algal blooms is placing a very important emphasis on understanding the relationship between oceans and human health. How can you help? The Phytoplankton Monitoring Network is a great way to volunteer to monitor your coastal waters for harmful algal blooms. This can increase the number of eyes on our coastal waters and help us to understand better what is happening with harmful algal blooms.

HOST: Thanks Allison for joining us on today’s episode of Diving Deeper and exploring what harmful algal blooms are and what causes them. To learn more about harmful algal blooms, please visit the Phytoplankton Monitoring Network Web site at

That’s all for this week’s show.