Catch Your Breath?
A Tale of Trying to Catch Critters Trying to Catch Their Breath

The DEAD ZONE … No, we’re not talking about the latest fictional Hollywood thriller, but we are referring to something that is very real and very scary to many organisms that live in the northern Gulf of Mexico.

The Dead Zone is an area of the northern Gulf of Mexico where oxygen levels in the bottom water drop below the amount needed to support most forms of life. This condition is known as hypoxia, and it typically happens between late spring and early fall every year, although the size and exact location of the zone can vary.

Hypoxia is bad news for animals in the bottom waters that need oxygen to breathe. Animals that aren’t mobile (mussels, corals) or can only move slowly or short distances (clams, sea cucumbers, sea stars) can quickly die if the hypoxia lasts too long. Fish and shrimp, however, can usually swim to another area where oxygen levels are higher, but that doesn’t necessarily mean they are safe.

In addition to directly impacting the animals, the presence of the Dead Zone also has impacts on the commercial and recreational fishing industries, including the men and women who make their livelihoods from catching, processing and selling fish and shrimp in the northern Gulf of Mexico.

Scientists know that the intensity of hypoxic events has increased during the past 50 years, and mapping of the Dead Zone has shown that the average size since 1993 has also increased compared to previous years.

In this activity, you will act as a scientist to:

  • review actual data to understand and explain the factors that cause hypoxia and control the size of the Dead Zone each year
  • consider the impacts this annual phenomenon might have on the fish and shrimp fisheries in the northern Gulf of Mexico
  • consider what measures can be taken to minimize the size of the Dead Zone in the future

Activity List:

Record your responses to the questions in the activity on your Student Worksheet (PDF)

Get in the Zone

Beginning in 1985, scientists have mapped the size of the Dead Zone each year by measuring the amount of oxygen dissolved in the water at fixed locations. Hypoxia occurs in areas where the amount of dissolved oxygen falls below 2.0 milligrams per liter of water (2.0 mg/l).

The six maps below show the size of the Dead Zone each year from 2000-2005. The black dots indicate where oxygen samples are taken each year, and the grey shaded areas indicate the extent of the Dead Zone (where hypoxic conditions were detected).

Compare the Dead Zone areas of each map and rank the years based on the size of the Dead Zone, starting with the largest and ending with the smallest.

Go With the Flow

You can see from the maps that the Dead Zone occurs in the same general area each year where the Mississippi River and Atchafalaya River (a tributary of the Mississippi River) empty into the northern Gulf of Mexico.

Scientists know that the amount of water flowing from these rivers (river flow) and the amount of nitrogen being carried in the water (nitrogen flux) both have an influence on the size of the Dead Zone each year.

Review the graphs of river flow and nitrogen flux data for the Mississippi River from 2000-2005, and answer the following questions.

  1. Describe the pattern or trend you see over the course of the year in the data for each graph. Are there any similarities in the trends between the two graphs?
  2. Describe how the trends in the data might relate to the Dead Zone, knowing that hypoxia in the northern Gulf of Mexico typically occurs during the summer.
  3. Compare the data for each year to the Dead Zone size map for that year. Does the data support your ranking of the Dead Zone sizes from largest to smallest?

Mixing it Up (or Down?)

When scientists sampled the size of the Dead Zone in 2003 they noticed it was much smaller than they might have expected based on the river flow and nitrogen data. This led them to believe that oxygen from the surface water had somehow gotten into the bottom water and disrupted the Dead Zone. But here’s the mystery - the surface water and bottom water are usually prevented from mixing in the summer because of density differences caused by temperature and salinity- so how could this be?

Write down any ideas you have that might explain how the surface water could be mixed down to the bottom for such a large area of the northern Gulf of Mexico during the summer.

Follow the link below to see the history of tropical storms in the Gulf of Mexico. Click on a year, scroll down the page, and click to enlarge the map. Look at the storm paths and dates for for 2003, and see if any major storms crossed through or near the northern Gulf of Mexico that may have affected the Dead Zone.

Hint: Tropical storms can disrupt the Dead Zone any time between early summer and late fall. But keep in mind that the scientists who measure the amount of oxygen in the water and map the Dead Zone each year usually take samples only once in late July. So, the effects of any storms on the Dead Zone after the sampling and mapping in late July would not show up on the Dead Zone maps.

National Hurricane Center - http://www.nhc.noaa.gov/pastall.shtml


Did any tropical storms have an impact on the Dead Zone during 2000-2005?

Back to the Beginning

You already know that the size of the Dead Zone can vary each year, and that factors such as river flow and nitrogen flux influence hypoxia, but what actually causes hypoxia and the Dead Zone?

Use your new knowledge and scientific reasoning to put the following statements in order to describe how hypoxia and the Dead Zone develop (example – 3, 1, 6, 5, 2, 7, 4):

  1. As bacteria feed on the decomposing phytoplankton they respire (breathe) and use up oxygen in the water, leading to hypoxia
  2. Some of the phytoplankton are eaten by other microscopic animals (zooplankton), but others die from natural or other causes and sink towards the bottom
  3. Hypoxic bottom water is prevented from mixing with oxygen-rich surface water in the summer because of density differences created by temperature and/or salinity
  4. High levels of nutrients, such as nitrogen and phosphorus, are carried by the Mississippi River into the Gulf of Mexico
  5. Phytoplankton (microscopic single-celled algae) in the surface waters of the Gulf of Mexico use the abundant nutrients to conduct photosynthesis and quickly grow and multiply
  6. Tropical storms and the cooling of surface water in the winter can lead to mixing of the surface and bottom waters and disruption of the Dead Zone
  7. Bacteria in the bottom waters of the Gulf of Mexico feed on decomposing phytoplankton as they sink to the bottom

Go Fish….or Shrimp

Obviously, hypoxia can affect the fish and shrimp that normally live in the northern Gulf of Mexico, but what about the effects on the many people that rely on catching these animals for their livelihoods?

Mobile animals like fish and shrimp can move to areas that have higher oxygen levels as hypoxia develops. In fact, some research has shown that fish and shrimp that move out of the Dead Zone in search of oxygen tend to concentrate along the edges of the zone.

How might the behavior of fish and shrimp gathering at the edges of the Dead Zone impact the multi-million dollar fishing industry? Try to provide at least one positive and one negative example.

Read the following Science News article from 2004 to see how your ideas compare with what scientists already know about the Dead Zone:

Science News - http://www.sciencenews.org/articles/20040605/bob9.asp

Slow the Flow

Scientists now know that increased nutrients (especially nitrogen) carried by the Mississippi River are a major factor in causing hypoxia and the Dead Zone. The sources of these nutrients include soil erosion, runoff from developed land, air pollution, sewage discharges, and (perhaps most importantly) fertilizers from agriculture.

Farmers typically apply fertilizers (which are often high in nitrogen) to their soil in the spring and summer to try to efficiently produce as much crop as possible on their land. One problem is that there often aren’t regulations about how much fertilizer a farmer can use, and even responsible farmers may not know how much fertilizer is enough. Not to mention that applying fertilizer can be expensive. Most farmers would like to find a balance between applying enough fertilizer to help their crops, while also saving money by minimizing how much they have to use. It’s not easy!

Write down three ideas you have that might help farmers to more effectively use fertilizer on their crops, or to help filter out nutrients from the Mississippi River and it’s tributaries before they reach the Gulf of Mexico.

Read the following Science News article from 2004 to see how your ideas compare with those of scientists and resource managers:

Science News - http://www.sciencenews.org/articles/20040612/bob9.asp

Acknowledgements

This activity was developed through a collaboration of education staff members, a graduate student and a research faculty member at the Institute of Marine and Coastal Sciences at Rutgers University, and the Centers for Ocean Sciences Education Excellence – Mid-Atlantic (COSEE-MA)

Authors: Eric Simms, Dr. Jim Ammerman, Jason Sylvan

Contributing Writers: Janice McDonnell, Sage Lichtenwalner, Corinne Dalelio