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The Dead Zone

A Marine Horror Story

Summary
Track the Dead Zone using water quality data from the Nutrient Enhanced Coastal Ocean Productivity (NECOP) Program.

Introduction
If they can't swim, they die. Fish and shrimp are able to escape the suffocating grasp of the hypoxic (low-oxygen) waters in the Gulf of Mexico, but animals like clams, snails, and worms are trapped in the "dead zone," an area which grew larger than the state of New Jersey in 2001 (approximately 8,000 square miles). It's a marine horror scene, and we should all take notice because it's happening in other coastal areas, too. More than half of the estuaries in the country experience oxygen depletion during the summer, and a third experience a complete loss of oxygen. The hypoxia in the Gulf, however, is the most dramatic case.

To date, scientists trace the cause of the problem to high levels of nutrients, particularly nitrogen. About two-thirds of the nation's harvested cropland and the treated sewage of 27% of the U.S. population empty into the Mississippi River, and eventually into the Gulf. The nutrients discharged feed algal blooms in the spring and summer, which periodically die and sink to the bottom. Large amounts of oxygen are then consumed by the bacteria that decompose the dead algae. Sometimes, so much oxygen is consumed that there is not enough left in the water to sustain life. This dreadful condition persists until an event such as a tropical storm or a cold front mixes the oxygen-depleted lower waters with the upper waters and brings relief.

Scientists first noted the annual formation of the Gulf of Mexico dead zone in the 1980s. In 2001, a multilevel government task force led by the U.S. Environmental Protection Agency set a goal of reducing the Gulf of Mexico dead zone to a five-year running average of less than 2,000 square miles. The figure represents conditions that were likely to be typical in the early 1970s, and a significant reduction from the current conditions. To achieve this goal, a 30 percent reduction in nitrogen load from the Mississippi and Atchafalaya Rivers is necessary. The effort focuses on refining agricultural practices in the watershed. Tactics include better timing of fertilizer applications, plowing more efficiently, restoring natural etlands, and improving manure management.

Data Activity
Take a look at some of the data collected by scientists who have been monitoring the dead zone for many years. We have chosen a data set for you based on type and consistency of data collected and ease of reading. Go to the Nutrient Enhanced Coastal Ocean Productivity (NECOP) Program's home page. Scroll down to the table of "NECOP Projects by Year" and click on 1994. Go to the "Transect C" section and click on "View Transect C Data". A transect line is an imaginary line perpendicular to the coastline, recorded using latitude and longitude measurements. This data set contains water quality measurements taken at certain stations along the transect line on 9 different dates between March and October of 1994.

We are going to look at one particular station, station 6B, on each of the dates. Divide your class into 9 groups, and assign each one a date. Ask each group to graph dissolved oxygen (mg/L) versus depth (meters). Dissolved oxygen is the tenth column from the left and Depth is the ninth column from the left. Each graph will contain 21-32 data points. Click here for step by step Microsoft Excel graphing instructions.

Questions

1) Looking at all nine graphs, what observations can you make?

Generally, the minimum oxygen level required to sustain aquatic life is 5 ppm (parts per million). Between 5 ppm and 2 ppm, marine life is severely stressed. Below 2 ppm, anything that can swim leaves the area, and anything that can’t relocate dies.

2) What percentage of the graphs had dissolved oxygen readings of less than 5 ppm? How about less than 2 ppm?

3) What do you think happened between the July 1 and August 17 measurements?

4) How do you explain the September 13 measurements?

Your graphs should serve as a dramatic illustration of the hypoxia problem. There is a need to tackle this problem and prevent more of our precious marine environment from becoming barren underwater wasteland and unproductive fisheries.

5) Can you think of potential solutions to the Gulf of Mexico hypoxia problem?

For more classroom activities, visit Gulf of Mexico Program's Educator and Student Resources and The Watershed Game (for grades 4-8).

Read Me

Author
Laura Rose, Virginia Sea Grant, Virginia Institute of Marine Science

 

Grade Level

9-12

 

Lesson Time

1-1.5 hr

 

Objectives

  • Define nutrient loading and hypoxia, and identify the causes of these phenomena
  • Explain the relationship between dissolved oxygen, water temperature, and nitrogen in marine systems
  • Construct a graph illustrating the "dead zone" in the Gulf of Mexico using dissolved oxygen data from the NECOP research database
  • Practice data manipulation using Microsoft Excel
  • Recommend solutions for hypoxia

Vocabulary

Hypoxic, Transect line, Dissolved oxygen

 

Materials Required

Graphing paper (if you choose not to use Excel)

 

Natl. Science Standards

Ik-1 IK-2 PS5-2 PS9-4

 

Related Bridge Resources

Chemical oceanography, Pollution, Physical oceanography

 

Credits

 

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