EarthLabs > Fisheries > Lab 7: Nitrates and Phosphates and Algae, Oh My! > 7C: Dead Zones and Harmful Algal Blooms (HABs)

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

These images show how ocean color changes from winter to summer in the Gulf of Mexico. Summertime satellite observations of ocean color from MODIS/Aqua show waters which may include large blooms of phytoplankton extending from the mouth of the Mississippi River all the way to the Texas coast. When these blooms die and sink to the bottom, bacterial decomposition strips oxygen from the surrounding water, creating an environment very difficult for marine life to survive in. Reds and oranges represent high concentrations of phytoplankton and river sediment. Image courtesy of NASA.
  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?
      1. Nutrient-rich runoff
      2. Thermal zones
      3. Algae blooms
      4. Organic rain
      5. Flee or die
    • How is the oxygen removed from the water?
      As algae die, the mass of cells and other debris sink to the seafloor. Bacteria break down this organic matter, consuming most of the oxygen in 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?
      Worldwide, there are some 146 dead zones-areas of water that are too low in dissolved oxygen to sustain life. Since the 1960s, the number of dead zones has doubled each decade.
    • Where do the highest concentrations of dead zones occur?
      Most occur in temperate waters, concentrated off the east coast of the United States and in the seas of Europe. Others have appeared off the coasts of China, Japan, Brazil, Australia, and New Zealand.
    • What steps can be taken to help restore healthy ecosystem function where dead zones occur?
      • cutting emissions from wastewater treatment plants and industry
      • reestablishment of coastal wetlands
      • reductions of fertilizer use by farmers
      • curbing fossil fuel use through efficiency improvements, conservation, and a move toward renewable energy
      • preventing erosion through conservation tillage and changing crop rotations

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:


  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?
      1. Paralytic shellfish poisoning: Tingling, burning, numbness, drowsiness, incoherent speech and, in some cases, respiratory paralysis that can lead to death.
      2. Amnesic shellfish poisoning: Vomiting, diarrhea, abdominal pain, confusion and short-term memory loss. In severe cases, seizures lead to coma and death.
      3. Neurotoxic shellfish poisoning: Numbness of lips, tongue, and throat; nausea, diarrhea, vomiting, and headache. Airborne toxins can irritate eyes, nose, and throat and cause sinus infections or other respiratory illnesses.
      4. Ciguatera fish poisoning: Abdominal pain, nausea, vomiting, diarrhea, anxiety, sweating, tingling or numbness of fingers or toes, and in severe cases paralysis or death. Some symptoms can recur for years, including depression.

  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?
      People who have spent many years on Little Gasparilla Island and in other Florida Gulf Coast communities say red tides used to show up once in a decade. Now, they occur almost every year and persist for months.
    • How are HABs affecting the shellfish industry?
      Shellfish beds in New England are being closed because of toxic algae that cause paralytic shellfish poisoning. California and other West Coast states periodically have banned shellfish harvesting because of toxic algae.

    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.

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