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. Watch this film to learn about fishing in the Gulf of Mexico and the 2019 dead zone.

Gulf of Mexico Dead Zone. If video does not play, visit VOA News.

2. Study the image below to learn about the 6 stages of dead zones.

Checking In

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

How is the oxygen removed from the water?

3. Visit this site to read more about Dead Zones in the Ocean.

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 are dead zones classified?

Since 1960, the number of global dead zones have nearly doubled every decade. Consider the image at the left. The image shows the drainage basins for the Mississippi River, which empties into the Gulf. Consider how much of that area contains cities and urban areas (highlighted in red) and farmland (in green), both sources of nutrient runoff that feeds the algal blooms.

Stop and Think

4: From what you know about the Gulf of Mexico dead zone, would you consider it naturally or human caused? How would you classify it according to its duration using the categories from the article you read? Give evidence to support your answer.

4. Watch this video to learn about one Iowa farmer who is using a variety of methods to keep nutrients from his farm from entering the Mississippi watershed.

Healthy Farms, Cleaner Water. If video does not play, visit This American Land.

5. Read the following handout Steps for Restoring Healthy Ecosystem Function to Control Dead Zones (Acrobat (PDF) 1.4MB Aug10 20) to learn more about this farmer's methods and other ways to prevent eutrophication.

The passage of Hurricane Barry over the Gulf in early July 2019 helped mix the water column over the Louisiana shelf, temporarily disrupting the hypoxic zone, preventing it from becoming the massive dead zone predicted earlier in the season. The dead zone in the Gulf of Mexico has been studied since 1985. The 2019 hypoxic zone measured in as the 8th largest in the Gulf at 18,005 km2.

Checking In

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

What steps can be taken to help restore healthy ecosystem function where dead zones occur?

Harmful Algal Blooms (HABs)

Harmful algal bloom. Image by Peter Franks, Scripps Inst, 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. Watch this 2018 video on HABs.

Florida's Red Tide from the SciShow

2. Visit the NOAA website to learn more about harmful algae blooms (HABs). Be sure to read through the five parts (What, Why, Forecasting, When, Where).

Checking In

Check your understanding of algae-related human illnesses.

What are some factors that contribute to HABs?

What are some methods used to track HABs?

HABs cause economic distress to the recreation, tourism, and seafood industries. During HABs, beaches are often shut down to protect the public health. Recreational fishing boats need to travel farther in order to avoid algal waters. Moratoriums are placed on shellfish harvesting. Shellfish, being filter feeders, ingest the algae responsible for HABs, but in doing so, become toxic themselves. Cooking cannot destroy the toxins.

3. Read here about five types of syndromes caused by the different types of toxic algae.

Stop and Think

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

Optional Extensions

Further Reading

Marine Dead Zones in Gulf of Mexico Are Expected To Last Decades

The Ocean Is Losing Its Breath.

View Dead Zones in Google Earth - Open the Oceans layer and click to view Dead Zones. They will appear as fish skeletons.


Happening Now: Dead Zone in the Gulf 2019

What is a Wetland?

Scientists in Gulf of Mexico discover world's biggest 'dead zone'

Predicting HABs