Nitrates and Phosphates and Algae, Oh My!

Part C: Dead Zones and Harmful Algal Blooms (HABs)

Algal blooms can have devastating effects on surrounding marine environments. Intense algal blooms can cause hypoxia, or oxygen depletion. Some species of algae produce toxins, which can kill marine life, contaminate water, and make humans sick.

Dead Zones


  1. Learn about how dead zones are formed by watching the visualization Birth of a Dead Zone from the L.A. Times Series, Altered Oceans.
    1. Go to the multimedia presentation portion of Altered Oceans: Part 1.
    2. From the left menu, choose Graphics and then The birth of a "dead zone".
    3. Use the numbered buttons to step through the stages of dead zone formation.

    Checking In

    Answer the following questions to check your understanding of dead zone formation.

    • What are the five phases of dead zone formation?
    • How is the oxygen removed from the water?
  2. Read the Earth Policy Institute article, Dead Zones Increasing in World's Coastal Waters.

    Checking In

    Answer the following questions to check your understanding of the information provided in the article.

    • How many dead zones have been found in the world's oceans? How has this number changed over the last several decades?
    • Where do the highest concentrations of dead zones occur?
    • What steps can be taken to help restore healthy ecosystem function where dead zones occur?

Harmful Algal Blooms (HABs)

Harmful algal bloom. Image courtesy of NOAA

In addition to their ability to suffocate the oceans, some species of algae produce dangerous toxins that can be deadly to both sea and land life, including humans. The term "red tide" is often used to describe these phenomena, however scientists now prefer the term harmful algal bloom (HAB), since:

  • toxic blooms don't necessarily appear red,
  • blooms are not tied to tides, and
  • phytoplankton species within so-called "red tides" may or may not be toxic.

  1. Learn about harmful algal blooms (HABs) and their consequences from the L.A. Times Series, Altered Oceans.
    1. Go to the multimedia presentation portion of Altered Oceans: Part 3.
    2. From the left menu, choose Graphics and then Harmful algal blooms and their consequences.
    3. Use the red bar at the bottom of the window to navigate through the different types of blooms and their consequences.

    Checking In

    Check your understanding of algae-related human illnesses.

    • What four types of toxic algae-related human illnesses are described in the graphic? What are their associated symptoms?
  2. Read the accompanying L.A. Times article Dark Tides, Ill Winds about harmful algal blooms and their effects on land and in the sea.

    Checking In

    Answer the following questions about the information presented in the L.A. Times article.

    • How often do harmful red tides now occur on the Florida Gulf Coast? How does this compare to the frequency of red tides in the past?
    • How are HABs affecting the shellfish industry?

    Stop and Think

    4:In your own words, summarize the importance of reducing the number of human-induced algal blooms in the world's oceans.