Download The Dead Zone

The Dead Zone
A Marine Horror Story
Written by: Laura Rose, Virginia Sea Grant,
Virginia Institute of Marine Science
Summary
Track the Dead Zone using water quality data from the Nutrient Enhanced Coastal Ocean Productivity
(NECOP) Program.
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
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. This data is from the Nutrient Enhanced Coastal Ocean Productivity (NECOP) Program's home
page. If you wanted to access it yourself, you could 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 have done the preliminary work for you to get the excel
data downloaded and imported on the teacher server in the classroom.
We are going to look at one particular station, station 6B, on each of the dates. You will work in
pairs at your table. Each table is responsible for graphing two dates (one per person). Each group is 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. See GRAPHING
page for step by step Microsoft Excel and plotting 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) In order to better illustrate this, take a red pencil or marker and draw a straight line at the 2ppm line, and
draw a yellow line at the 5ppm line. HINT: they should be vertical lines.
3) What percentage of the graphs had dissolved oxygen readings of less than 5 ppm? How about less than 2
ppm?
4) What do you think happened between the July 1 and August 17 measurements?
5) 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.
6) Can you think of potential solutions to the Gulf of Mexico hypoxia problem?