"Building a Bloom"

Note to teachers: Click here to see the printable version of this content.

What is an algal bloom? An algal bloom is a rapid increase in the number of algal cells such that the algae dominate the planktonic community.

What causes an algal bloom? That question isn’t as easy to answer. Like land plants, algae need certain things to grow – water, carbon dioxide, sunlight and nutrients. Carbon dioxide is plentiful in the marine environment, but sunlight and nutrients can be scarce. Sunlight is available near the surface of the water, so algae grow readily when they can remain near the surface. Nutrients are abundant in areas of run-off where water flows over land and picks up minerals that are then carried to the sea. Nutrients are also plentiful in areas where cool, deep, nutrient-rich waters are brought to the surface due to upwelling, tides or wind-driven mixing. When the algal cells get everything they need to grow, they can divide very rapidly and potentially create a bloom.

Of course, it is a little more complicated than just getting sunlight and nutrients. Phytoplankton live in a very dynamic environment. The ocean is always moving and algae and nutrients move with it. Not only can regional circulation patterns affect the nutrient concentrations in the water, but they can also actually physically concentrate or disperse algal cells. In addition, many algal species thrive under only certain temperature and salinity conditions.

And finally, for algae to bloom successfully, there must be a limited number of grazers in the area. After all, if there were enough grazers to keep the algal population under control, there wouldn’t be a bloom.

Click here to see some common HAB species and to learn about their optimal growth conditions.

Baden, D.G. 1998. Center for Marine Science Research, University of North Carolina Wilmington, Wilmington, NC. In: Bushaw-Newton, K.L. and Sellner, K.G. 1999 (on-line). Harmful Algal Blooms - NOAA's State of the Coast Report. Silver Spring, MD: National Oceanic and Atmospheric Administration.

After reading about all that can affect the occurrence of an algal bloom, you might think that it would be rare for all of the conditions to be right and for a bloom to occur. However, as you can see by the red markers on the maps to the left, observations of reported harmful algal blooms are increasing in frequency. Click here to see what types of HABs occur in different areas of the U.S.

Why are the reports of HABs increasing? Scientists have offered numerous explanations. One possible explanation is that we are introducing exotic species through ballast water and aquaculture. The exotic species may be able to thrive in the new area, but may not have a grazer in that area to keep its population under control. A second possible explanation is that certain algae are inhibiting grazers, so the grazers are physically unable to control the algal population. Another possible explanation that has been proposed is that our global climate changes are creating conditions that favor the blooms. It has also been suggested that human activity has played an integral role in an increase of harmful algal blooms by increasing the amount of pollution and nutrients released into the environment, by degrading habitat, and by modifying water flow. Lastly, it has been proposed that the frequency of blooms is not increasing, but that we have developed better detection and monitoring methods so we are noticing more of the blooms that have always been occurring.

Of course, as you have probably already realized, it is possible that every single one of the proposed explanations is correct at some level. One thing that scientists are doing now is studying what factors are contributing to the causes of different blooms.

  • Understand the requirements for algal growth
  • Understand the factors that influence the occurrence and duration of algal blooms
  • Appreciate the diversity of algae in the marine environment

To receive the greatest benefit from the activity, students should have a basic knowledge of algae and their role in the food web. This can be accomplished by having the students read the above content and participate in "Fitting Algae Into the Food Web." Other valuable resources include NOAA's State of the Coast Report on Harmful Algal Blooms and Bigelow's "Toxic and Harmful Algal Blooms."

Top of the Page

Making Algae Bloom

Note to teachers: Please dispose of your algal cultures responsibly. Do not allow your students to pour their algal cultures down the drain without treating the cultures first. There are a couple of ways to dispose of algal cultures. (1) Microwave the cultures (in a microwave-safe container) until they boil. This can be done in a regular microwave used for food preparation. Although it may not smell pleasant, it will not contaminate your microwave. After the microwaved cultures have cooled, they can be poured down the drain. (2) Add bleach to the cultures to make a 1% solution. After allowing the bleached cultures to sit overnight, pour them down the drain with plenty of running water.

Students will devise their own experiments to test the effects of nutrients on algal growth. Remind students that they need to have controls and replicates in their experiments. Make sure the students formulate hypotheses before their experiments commence. A simple example experiment is described below. (Note to teachers: Click here to see the printable version of this example experiment).

Example Experiment:

Materials: algal culture (can be obtained from a scientific or aquarium supplier or even from a classroom aquarium), water (fresh or salt depending on the algae used), 250-500mL clear glass or plastic containers, aluminum foil, available light source (sunny window is fine), droppers or pipettes, liquid plant fertilizer, wax pencils


1. Provide each lab group with two containers. (The replication of the treatments is across the entire class). Label the containers with the wax pencil:
Group #, +Nutrients
Group #, Control

2. Add an equal amount of water to each container.

3. Add an equal amount of algal culture (a few drops) to each container.

4. Cover the top of the container labeled “Control” with a piece of foil.

5. Add an equal amount of liquid fertilizer to each “+Nutrients” container.

6. Cover the top of the container labeled “+Nutrients” with a piece of foil.

7. Place all containers in a sunny window where they can receive equal amounts of light and they can be exposed to the same temperatures (don’t place some on a heater and others near the open window).

8. Over the course of 1-2 weeks, make observations (e.g. color, opacity, smell) on the algal cultures in each of the containers.

9. Write up the observations as a short report. Create a graph to illustrate the findings with algal growth (determined by color) on the y-axis versus number of days on the x-axis. Click here to see an example graph. In the report, make sure to describe what was seen, whether or not the hypotheses were supported and why, and how to improve the experiment next time. At the end of your report, write a paragraph about how algal blooms like the ones in the experiment might occur in lakes and oceans in the real world.

Suggestions for More Complex Experiments:

o Keep track of algal density with cell counts in addition to color detection. Use microscopes, slides and cover slips to do cell counts of the cultures over time. Create graphs of cell density (determined from your counts) over time.

o Add treatments to test different concentrations of fertilizer. Create graphs of cell density versus fertilizer concentration for each sample time.

o Use nutrient mixes rather than a commercial fertilizer. This will allow students to test the effects of specific nutrients on the growth of their alga. Create graphs of final cell density versus nutrient concentration for each nutrient treatment.

o Add treatments to test the effect of sunlight/darkness on algal growth. Create graphs of cell density over time for the different light treatments.

o Add treatments to test different types of detergents – with and without phosphates. Create graphs of cell density over time for different detergent treatments.

o Use more than one species of algae (grown separately) to compare the growth of different species under similar conditions.

o Add treatments to test the effect of temperature or salinity on algal growth. Create graphs of final cell density versus temperature or salinity.

Top of the Page

Phosphates in Detergents

Note to teachers: Click here to see the printable version of this extension.

Since the 1970’s, many states have enacted detergent phosphate bans or restrictions in the attempt to minimize eutrophication of lakes. (For more information see the EPA's web site or download the EPA's publication, The Quality of Our Nation's Water, or visit the U.S. Geological Survey's Water Science for Schools web site. Investigate the detergent phosphate regulations in your state and report your findings to your class. Things you might want to discuss include:

o history of phosphate usage in your state
o legislation that has been proposed and/or passed to regulate the use of phosphates in detergent
o phosphate levels found in your local lakes (historic and recent)
o sources of phosphate in your community today

Top of the Page


Plankton Identification

Phytoplankton can be found in every water body around. Using plastic containers, collect water from a local lake or bay. Filter the water through filter paper to concentrate the cells, and wash what was caught on the filter paper into a shallow glass container or petri dish. Use some of the filtered water for your wash. Look at the cells you collected with a dissecting microscope. Record your observations. Include sketches with your written descriptions. Do you think you captured more than one species of plankton? Can you tell the difference between the phytoplankton and the zooplankton? Are all of the cells solitary, or are some in colonies? Do any cells have spines or hair-like appendages? Use books, the internet, or Phytopia to help identify the types of plankton you found.

Top of the Page



Phytopia is an educational CD-ROM that consists of a searchable database of many important planktonic protists, including many that are harmful, in addition to a Bloom Activation Tool. This interactive tool uses satellite-based images to help students understand how changes in environmental factors – sunlight, ocean temperature, wind and ocean currents – affect phytoplankton productivity and ocean health. This CD-ROM, developed by Bigelow Laboratory for Ocean Sciences, the University of New England, and NASA’s Jet Propulsion Laboratory, is available to educators through Bigelow's web site.

Top of the Page

This table summarizes the McREL science standards that are met through this lesson. To see a detailed list of standards that this lesson addresses, please click here.

Grade Level
Primary (K-2)  
Elementary (3-5)  
Middle (6-8)
High School (9-12)  

Return Home