Harmful Algal Blooms

In Full Bloom

Compare concentrations of harmful algal blooms using NOAA's Coastal Services Center Harmful Algal Bloom Forecasting (HABF) Project data.

Harmful Algal Blooms Spring is upon us and flowers are beginning to bloom, but along with the warmer temperatures and April showers can come a bloom of a different color. Harmful algal blooms, also known as HABs or red tides, occur when there is a population explosion of potentially harmful phytoplankton such as certain diatoms, dinoflagellates, and cyanobacteria. Often a bloom is caused by one of these troublesome species being brought inshore from offshore areas by currents. If the conditions such as water temperature and nutrient concentration in the inshore waters are right, a bloom can happen.

Harmful algal blooms can impact the health of marine organisms and humans in a number of ways. If a toxin-producing species (of which there are only a few dozen) is ingested by a marine organism, those toxins can be passed through the food web affecting fish, marine mammals and even humans. Some of the illnesses contracted this way include amnesiac shellfish poisoning (ASP), ciguatera fish poisoning (CFP), diarrhetic shellfish poisoning (DSP), neurotoxic shellfish poisoning (NSP), and paralytic shellfish poisoning (PSP). Other ways that HABs can impact organisms are:

  • Predation - Example: Pfiesteria piscicida eats through the skin of a fish causing lesions which can lead to death by Pfiesteria or by secondary infection.
  • Injury or Irritation - Example: Spiny diatoms like Chaetoceros species can get stuck in fish gills causing irritation and eventually death.
  • Starvation - Example: Aureococcus, which causes brown tides, ingested by a scallop can inhibit the scallop's ingestion of other food.
  • Anoxia - Example: After harmful algal blooms die, large numbers of the dead algae decompose which requires oxygen and causes anoxic conditions.

Harmful algal blooms have always occured, and affect almost all coastal U.S. waters. Scientists are concerned that HABs are increasing in number and diversity causing not only health problems but economic problems to the tune of $100 million a year for fisheries and tourism. And while some of the increase in number of blooms each year may simply be a result of improved detection methods, human influences including exotic species introduction through ballast water, global warming and increased nutrient runoff may also be big contributors. To aid in the early detection of harmful algal blooms, some states like South Carolina have a citizen volunteer phytoplankton monitoring program.

In this data activity, we will take a look at Florida blooms of the red tide-causing dinoflagellate Karenia brevis (formerly Gymnodinium breve) to see if these HABs are increasing in frequency or severity. K. brevis usually blooms in Florida in the fall or winter and causes fish kills, shellfish poisoning, and respiratory and skin irritations in humans. In 1996, K. brevis caused the death of about 10% of the Florida manatee population.

Data Activity
This month's data source is NOAA's Coastal Services Center Harmful Algal Bloom Forecasting (HABF) Project. For the activity, print out the following graphs of G. breve (K. brevis) concentrations in southwest Florida:

These bar graphs show the concentration of G. breve during a two-week sampling period. Within each white or colored section of the bar is the number of samples found to have that concentration level of G. breve. The concentration levels for G. breve are divided into not present, present (1999-2000 only), very low, low, medium, and high. Add the numbers of each section of a given bar to find the total number of samples taken for that time period.

For each of the three graphs, compute the total percentage of samples at each concentration level. For example in 1997-1998, there were 363 total samples, 239 (~66%) of which had no G. breve present. (Check your calculations.) Compare the percentages over time. Is the percentage of samples containing G. breve increasing or decreasing over time? How about the severity of the bloom? Are the percentages for higher concentration levels (low, medium, high) increasing or decreasing? From your observations, do you think that the G. breve blooms in southwest Florida are worsening? How might you verify this? (Continued monitoring, increased sampling). What could you do to prevent or lessen the impact of these blooms?

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Read Me

Lisa Ayers Lawrence, Virginia Sea Grant, Virginia Institute of Marine Science


Grade Level



Lesson Time

1- 1.5 hrs.



  • List 3 kinds of harmful phytoplankton and ways that these algae impact organisms.
  • Compare concentrations (percentages) of G.breve over time.
  • Analyze the severity of HAB blooms.


Harmful algal bloom (HAB), Phytoplankton, Diatoms, Dinoflagellates, Cyanobacteria


Materials Required

1997-1998 graph, 1998-1999 graph, 1999-2000 graph


Natl. Science Standards

IK-1 IK-2 L5-4 L9-4 PS5-2 P9-3 PS9-4 PS9-5


NOAA's Coastal Services Center Harmful Algal Bloom Forecasting (HABF) Project


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