Nitrates and Phosphates and Algae, Oh My!

The lab activity described here was created by Erin Bardar and Sarah L. Hill of TERC for the EarthLabs project.

Summary and Learning Objectives

Human-induced nutrient loading of the world's oceans has been linked to increased and prolonged algae blooms, sometimes with potentially deadly consequences. In this investigation, students will create their own algal blooms, analyze satellite images of chlorophyll concentrations in the Sea of Cortés, and learn about two alarming consequences of excessive algae growthdead zones and harmful algae blooms (HABs).

After completing this investigation, students will be able to:

  • understand the conditions required for algae growth;
  • understand the factors that influence the occurrence and duration of algal blooms;
  • interpret satellite images of chlorophyll concentrations (ocean color); and
  • explain the potentially harmful consequences of algal blooms including dead zones.

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Context for Use

In the previous investigation, Lab 6 - Aquaculture, students learned that one of the drawbacks of coastal aquaculture facilities is the transfer of additional waste and nutrients to the surrounding water. In this investigation, students learn about other ways in which humans are increasing the nutrient content of coastal waters and about the ensuing consequences for land and sea. The investigation requires laboratory space and equipment, which should be procured ahead of time. For the laboratory portion of the investigation, students should be assigned to small groups of 3-5 students.

Activity Overview and Teaching Materials

In Part A, students make their own algal blooms and test what factors influence the occurrence and duration of the blooms. The following materials are needed for each group of 3-5 students:

  • algal culture (available from a scientific or aquarium supplier or even from a classroom aquarium)
  • water (fresh or salt depending on the type of algae)
  • two 250-500 mL clear glass or plastic containers
  • light source (sunny window will work)
  • droppers or pipettes
  • liquid plant fertilizer
  • wax pencil, tape, or stickers to label containers
  • microscope (10x or greater magnification)

In Part B, students use data from NASA's Aqua MODIS satellite to look for correlations between phytoplankton blooms in the Sea of Cortés and the addition of nutrients through agricultural runoff from the surrounding land. Students watch a time-series animation of chlorophyll concentration over a two-year period. They also generate a chlorophyll concentration data plot showing the change in chlorophyll a concentration in the Sea over the same two-year time period. The map animation is produced using NOAA View Global Data Explorer. The scatter plot is created using NOAA's ERDDAP site.

Students conclude Part B by reading the article Researchers discover direct link between agricultural runoff and massive algal blooms in the sea, which explains the connections between the algal blooms observed in the satellite data and agricultural runoff from Mexico's Yaqui Valley.

In Part C, students learn about dead zones and harmful algae blooms (HABs). They first watch a short video about fishing in the Gulf of Mexico during a dead zone. They examine a graphic about the stages of dead zones and then read the article Dead Zones in the Ocean. Students watch a short film about an Iowa farmer discussing changes he's made to keep nutrients on his farm and then read a handout looking at ways to control dead zones.

The final portion of this investigation introduces students to HABs. They view a short video introducing them to Red Tide in Florida and visit a 5-part NOAA website to learn more about harmful algae blooms (HABs). Finally, they learn about Human Illness Associated with Harmful Algae.

Printable Materials

  • Activity Sheet ( and )
  • Student reading: Steps for Restoring Healthy Ecosystem Function to Control Dead Zones (Acrobat (PDF) 1.4MB Aug10 20)

Teaching Notes and Tips

Be sure to dispose of algal cultures responsibly. Do not allow students to pour cultures down the drain without treating them first. To prepare cultures for safe disposal, do one of the following: (1) Add bleach to the culture to make a 1% solution; leave the bleached cultures to sit overnight and then pour down the drain with running water. (2) Bring the cultures to a boil in a microwave-safe container in a microwave oven. Although this may be accompanied by an unpleasant smell, it will not contaminate your microwave or make it unsafe for future food preparation.

Possible Extensions for Part A:

  • Use more than one species of algae (grown separately) to compare the growth of different species under similar conditions.
  • Add treatments to test the effect of temperature or salinity on algal growth. Create graphs of final cell density versus temperature or salinity.

When creating the scatterplot in Part B, the location of 27N, 111W is not far from the outlet of the river running through the Yaqui Valley referred to in the reading and Stop & Think question 3. If you would like students to input other locations within the Sea of Cortés, the graphs will differ widely but will open up opportunities for discussion.


You can assess student understanding of topics addressed in this Investigation by grading their responses to the Stop and Think questions.

State and National Science Teaching Standards

Additional Resources

Background Information

  • For background information about algal blooms, read the Wikipedia entry: Algal Blooms.
  • For background information about dead zones, read the Wikipedia entry: Dead Zone (ecology).
  • For background information about harmful algae blooms, visit The Harmful Algae Page, supported by a National Oceanic and Atmospheric Administration Center for Sponsored Coastal Ocean Research Coastal Ocean Program (NOAA/CSCOR/COP) grant.
  • Eutrophication and dead zones also occur in bodies of fresh water such as Lake Erie. Check out the Environmental Protection Agency's informational page on the Lake Erie Dead Zone.
  • Although the addition of fertilizer to the world's oceans is typically unintentional, some scientists believe that deliberate addition of certain types of fertilizer may help combat global warming. For more information about this topic, read the Science Daily article Controversial Spewed Iron Experiment Succeeds as Carbon Sink.