The Dead Zone: Ecology and Oceanography in the Gulf of Mexico - Case Teaching Notes - Case Study Collection - National Center for Case Study Teaching in Science

Introduction / Background

This case study examines the Dead Zone, the area of aquatic hypoxia in the Gulf of Mexico, and shows students how careful data gathering and analysis reveals the root causes. The case illustrates the practical value of understanding basic ecological and oceanographic principles in solving real-world problems. It also gives students exposure to the give and take of the science process, how scientific consensus is eventually reached, and how scientific information moves into the realm of public policy.

This case could be used in an introductory biology course following lectures on the basic structure of food webs. Since many ecology texts are primarily oriented towards terrestrial systems, the case could also be used in introductory ecology courses to strengthen exposure to marine food webs and the role of the physical properties of the water column. It could also be used in environmental science courses as a concrete example of an environmental problem and its public policy outcome. In an oceanography course, this practical example could be used to illustrate the need for an interdisciplinary approach to oceanographic problems.

Objectives

The overall objective of the case is to show the close integration of biological and physical influences on an aquatic environment and the outcome when nutrient inputs are elevated. Specific objectives include:

Understanding the structure of an aquatic food web.
Understanding the role of the microbial loop.
Understanding the role of salinity and temperature in creating water column density structure.
Understanding how the interaction between biological processes and water column structure can cause hypoxia.
Reading and interpreting graphical data.

Classroom Management

This is an interrupted case in which students are given discrete pieces of information and data in a progressive disclosure format. The case takes approximately three 75-minute class periods to complete. It could be used in three 60-minute class periods if time is managed very closely. It could also be spread out over a longer period, with lecture interspersed between the case parts. Students work in small groups of 4 or 5 to develop answers to each set of questions, followed by whole-class discussion. The case could be adapted to large auditorium classes with students working in pairs.

Overview and Description of the Parts of the Case

The case, which has eight sections, or parts, begins with a statement of the problem presented in Part I followed by Part II, “What Affects the Dissolved Oxygen Content of Water?” Part III, “How Do the Gulf Waters Change with the Seasons?” examines how the attributes of the Gulf waters change with the seasons. In this section, students learn how the features of the water column vary and how data showing those features are typically displayed. Part IV, “How Do the Organisms Affect Dissolved Oxygen Concentration?” looks in some detail at the aquatic food web and how the organisms in the Gulf affect oxygen content, while Part V, “Why Does the Phytoplankton Population Increase?” looks at the nutritional inputs that lead to the phytoplankton population explosion. Part VI, “Why Is the Dead Zone a Seasonal Phenomenon?” leads students through some of the physical influences that affect water column stability, and Part VII, “Where Does the Nitrogen Come From?” reveals the source of nitrogen inputs into the Gulf. Part VIII, the “Conclusion” ends the case with a look at public policy outcomes.

Part I (The Problem) & Part II (What Affects the Dissolved Oxygen Content of Water?)

Students are given Parts I and II to read in class. Small groups can work on the questions associated with Part II, followed by whole-class discussion, which all together takes about 20–30 minutes of class time. Most students will collectively come up with the answers without the need for background material (see the Detailed Analysis for answers). Answers to the questions should generate a list of possible contributors to the low oxygen levels in the Dead Zone.

Part III (How Do the Gulf Waters Change with the Seasons?)

After finishing the whole-class discussion, Part III is handed out and students read the text and work on the questions in small groups. This section presents data from Gulf station profiles and illustrates how the water column properties vary with depth. It also demonstrates the seasonal changes in the water. It should take about 30 minutes to work through this section: 10–15 minutes in small groups, with 10–15 minutes of whole-class discussion. Parts I–III could be completed in one class period.

After finishing Part III, students should have discovered two important facts about hypoxia in the Gulf—namely, that it is a characteristic of the bottom of the water column and it is seasonal. Later parts of the case develop material to explain these characteristics. The questions and group discussions should provide students with an understanding of why the salinity is low in spring in the surface waters, and gradually increases as the summer wears on. They should also notice that in spring the deepest water is hypoxic, and that hypoxic water reaches higher and higher in the water column through August, and then by mid-September the stark difference in hypoxic vs. oxygenated water has disappeared.

If desired, this part could be omitted and replaced with a short lecture or assigned reading that covers the key points.

The station profiles of the water column are in a graphical format that will not be familiar to many students. This format is the standard in the field, however, and if the instructor takes a few minutes to explain the axes and to illustrate how to read the profile, students should be able to move forward in analyzing the data.

Part IV (How Do the Organisms Affect Dissolved Oxygen Concentration?)

Part IV introduces the marine food web and the microbial loop component of that web. Students should have a basic grasp of food webs in general before beginning this part of the case. Instructors of upper-level courses might consider including material on energy budgets. Students will need to consult various resources to be able to answer the questions, so this part works especially well as a homework assignment. Part IV could be handed out at the end of class and each group might distribute the questions among the group members for research outside of class. Information about marine food webs is available at several good websites listed in the Useful Resources section of Part IV. It is essential that students be introduced to the microbial loop and the role it plays in the marine ecosystem.

Instructors might wish to point out that the units for the x-axis in Figures 4 and 5 in this part of the case are months of the year, and that the first month for which there are data is different for each of those graphs.

On day two of the case, students could be given some time (about 10–15 minutes) in their small groups to pool their answers and share what they have discovered. Following this, the whole-class discussion can be used to reinforce understanding of the marine food web and in particular the role the microbial loop plays. Much of what is available in print about the depletion of oxygen associates it somewhat vaguely with the phytoplankton. Instructors should make clear that while phytoplankton provide the food supply, the vast amount of oxygen consumed is by aerobic bacteria, not the phytoplankton themselves. Class discussion of this material might take 20–30 minutes; together with the initial small group work, this part of the case can take up to 30–45 minutes to complete.

Students may ask about the effect of summer temperatures on respiration rates. Instructors might pursue this question themselves. They should note from the station profiles in Figure 2 (presented in Part III) that water temperatures are highest in September, when respiration rates drop to their lowest. Temperature can certainly have some effect, but clearly the greatest stimulus to respiration is the available food supply.

Part V (Why Does the Phytoplankton Population Increase?)

Part V works through why the phytoplankton populations increase so dramatically by considering the most important limits on phytoplankton growth, and then illustrating the seasonal fluctuation of nitrogen. Student groups should be able to work through the questions relatively quickly or, if desired, the questions can be addressed by the class as a whole. This part should take about 10 minutes. Parts IV and V together could fill most of a class period.

Part VI (Why Is the Dead Zone a Seasonal Phenomenon?)

Part VI examines why the Dead Zone dissipates in the fall, and takes students through some basic principles of oceanography. In class discussion, students and the instructor may come up with a working definition of what stability means in the ocean. If students do not understand density, this is the place to address it. Then have students read the scenario and work in their groups on Questions 1 and 2, followed by class discussion. At this point, students should understand that mixing must occur if bottom water is to be re-oxygenated. They should be able to look at Figure 2 in Part III of the case and see how the changes in salinity, temperature, and dissolved oxygen concentration indicate that mixing occurs sometime between August 12^th and September 19^th.

The film clip available on the SALMON (Sea-Air-Land Modeling and Observing Network) Project website (see References below) is used in this section to illustrate the structure of the water column. The SALMON Project, based at the University of Alaska in Fairbanks, is providing continuous real-time or near real-time observations of physical oceanographic properties and linking these with models to provide ocean forecasts in much the same way that weather forecasts are made. The film clip is best shown with the sound turned off, making students think through what they see. Instructors may choose to show it again, with the sound on, at the end of this section. The film clip should be played up to the point where the tank, with the barrier in, has been filled up. Before the barrier is removed, have students in their groups work on Question 3. A short class discussion where students explain their choices works well. Then show the clip. After the students have observed the film clip, instructors may queue it up to one of the end frames. By choosing one that clearly shows that the two waters remain distinct, students have visual evidence that little mixing occurred. Then have students answer Questions 4–6. This part of the case should end with a group discussion that ties together changes in water column stability with seasonal processes and dissipation of the hypoxia. When finished with this part, students should understand that it is not easy to mix ocean water. Mixing is made easier when seasonal cooling occurs. The energy for mixing comes from wind and wind strength tends to be seasonal.

Part VI should take about 30 minutes: 10 minutes for the initial class discussion and Questions 1 and 2, 15 minutes for the film clip and the rest of the questions, and 5 minutes for a wrap-up discussion.

Part VII (Where Does the Nitrogen Come From?) & Part VIII (Conclusion)

Part VII covers the source of the nitrogen, showing that changes in land use practice have changed the amount of nitrogen delivered to the Gulf. This part of the case illustrates the science process in action. Before consensus is reached, research results may be interpreted in very different ways or even be in conflict. This is a continual frustration for the general public—when an issue is of particular concern, the popular press reports results of new studies as they are published without giving a clear sense that the scientific jury is still out. Part VII will give students a sense of scientific consensus arising from the accumulation of studies consistent with a particular interpretation. The conclusion (Part VIII) makes that point explicitly, and goes on to describe how government agencies responded to scientific consensus with formal policies and action steps.

Parts VII and VIII can be covered in about 15 minutes. If desired, more time can be spent with the class discussing the various government commissions and agencies involved in taking action on the problem.

Together with Part VI, Parts VII and VIII could fill most of a third class period.

Detailed Analysis

Detailed case analysis is provided in a separate file that is password-protected. To access this information, go to the detailed case analysis. You will be prompted for a username and password. If you have not yet registered with us, you can see whether you are eligible for an account by reviewing our password policy and then apply online or write to answerkey@sciencecases.org.

Student Resources

For Part IV

Lalli, C. M., and Parsons, T.R. 1993. Biological Oceanography, An Introduction. New York: Pergamon Press.
The Habitable Planet.: Available on-line at http://www.learner.org/channel/courses/envsci/unit/
Cycling Through the Food Web. The Bigelow Laboratory for Ocean Sciences.: Available on-line at http://www.bigelow.org/bacteria/
Our Ocean Planet—Oceanography in the 21^st Century. Robert Stewart.: Available on-line at http://oceanworld.tamu.edu/resources/oceanography book/contents2.htm

For Part VI

“Ocean World,” Ch. 6 Temperature, salinity and density.: Available online at http://oceanworld.tamu.edu/resources/ocng_textbook/chapter06/chapter06_04.htm
Chamberlin, W.S., and T.D. Dickey. 2008. Exploring the World Ocean. New York: McGraw-Hill.

References

Since some of these references reveal the details of how the Dead Zone forms, they are not included here (except for the reference to the video clip below), but are in the password-protected Detailed Analysis for the case. They include material from popular science magazine sources as well as government reports, news releases, research articles, and review articles.

SALMON (Sea-Air-Land Modeling and Observing Network) Education Guide to Teaching Physical Oceanography, Module 11- Internal Wave II: Annapolis Wave Generator. School of Fisheries and Ocean Sciences, University of Alaska Fairbanks.: http://www.ims.uaf.edu/salmon/education/lesson%20plans/mod11.html Accessed: May 27, 2008.; Note: To access the film clip from the above website, scroll down to where it says: “Demo Movie: Now see the real lab demonstration as it happens.”

Go back to the case

Acknowledgements: The authors were participants of the annual Case Studies in Science Workshop at the University of Buffalo in 2007. This case was published with support from the National Science Foundation under CCLI Award #0341279. Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

Date Posted: May 27, 2009.

Originally published at http://www.sciencecases.org/dead_zone/notes.asp

Case Teaching Notesfor “The Dead Zone: Ecology and Oceanography in the Gulf of Mexico”