Case Study: The Dead ZoneCase Study
Read the following case study, about the Ecology and Oceanography in the Gulf of Mexico. The case study is broken into eight parts. After each part, you will answer a few questions related to the reading. Submit a MS Word document or *.pdf file with your responses for each question to the assignment dropbox. All answers should be in your own words (do not copy and paste definitions). Answer each question in at least two complete sentences (upwards of 27-42 words per answer), some answers may require more explanation than others. Note: You may need to do some additional research on the internet and cite your sources. There are 15 questions, questions vary from 2 to 4 points. You will be deducted points for short and incomplete answers.
Part I. The ProblemThe ProblemBill sat at the kitchen table, adding up last month’s expenses from running his fishing boat. With his trawler, he fishes for bottom fish from his home base in Terrebone Bay, Louisiana—snapper and grouper mostly. For the last few summers, he has had to boat further and further out from the Louisiana shore to get to decent fishing grounds. The additional fuel costs were killing him. He rubbed his tired eyes and tried running the numbers through his calculator again.
“Hi Dad, ” said his daughter Sue, walking into the kitchen. “How does it look this month? ”
“Not so good, ” said Bill, tossing his pencil onto the table. “The fuel bills were higher than ever this summer. It is going to be tight for our finances. I wish I knew why the fish disappear near shore in the summer. ”
He privately worried about how he was going to be able to afford Sue’s college tuition this fall. Maybe it was time to get out of the fishing business, except fishing was all he knew. In addition, all his money was tied up in his fishing boat and gear. Who would buy it now that fishing in the Gulf was so problematic?
Sue sat down at the table and toyed with the pencil. She knew about her dad’s worries. “You know, Dad, ” she said, “I’ve been thinking. Let me talk with some of my professors at the university. Maybe I can get some information from them about what causes the fish to disappear, and whether anyone is working on a solution. Someone at school ought to know something. ”
Bill smiled at his daughter, even though he was not hopeful. “Good idea, kiddo, ” he said. “Maybe more people are working on this than we know. See what the professors can tell you. ”
Sue hurried across campus. She had an appointment with Professor Gracia in the biology department, and she was late. She rushed up the stairs of the biology building and knocked at his door.
“Come in, ” Professor Gracia called out. “You must be Sue. I am glad you could make it before I had to leave. You are right in thinking that a number of scientists must be working on the problem you described, ” he said as he handed her a map of the United States (Figure 1a).
“Look here, ” he said, pointing to a region of the Gulf of Mexico just below Louisiana and eastern Texas (Figure 2a). “See that shaded area? We call that the Dead Zone. During the summer there is very little in the way of marine macro-organisms there. ”
“Wow, I had no idea it was so big! ” said Sue. “Do the fish actually die there? ”
Professor Gracia started gathering up materials for his next class. “Some fish may die. Most of the fish and crustaceans that can leave the Dead Zone do so. It’s called the Dead Zone because the dissolved oxygen levels in the water get so depleted the water can’t support life. ”
Sue could see that the far edge of the Dead Zone (Figure 1b) corresponded with the distance her dad had to boat to get to good fishing grounds. “Is anyone working on why the Dead Zone forms? ” she asked.
“A lot of people are very concerned and are actively collecting data to help get to the bottom of the cause, ” said Professor Gracia. “I’ve got to get to class right now, but let’s meet again. I have more information and data to share with you. ”
“That’s sounds great, ” said Sue. “I’ll talk to you soon. ”
Figure 1a. Map of the U.S. showing area of the Dead Zone in the Gulf coastal waters of Louisiana and Texas.Figure 1b. Detail of the Dead Zone (shaded) in the Gulf of Mexico.QuestionsThere are no questions for Part I.
Part IIWhat Affects the Dissolved Oxygen Content of Water?”Hey guys, look at these maps (Figure 1a and 1b) of the Dead Zone I got from Professor Gracia,” said Sue, walking up to her friends sitting at the lunch table in the student cafeteria. “What I don’t get is why this particular area should have such low dissolved oxygen concentrations.”
Sue handed the maps (Figure 1a and 1b) to her friend Paula, a physics major. Paula stopped eating her sandwich long enough to give them a look.
“There must be some physical cause, ” Paula said. “I can’t imagine anything else that could affect the dissolved oxygen content of water so dramatically. ”
“Oh, come on, ” said Sue’s friend Zack. “Living organisms should have a huge impact—aren’t all the fish busy consuming oxygen in the water? ”
“Well, of course you would think of that, you’re a biology major, ” said Sue. “But let’s be systematic. What are all the physical and biological influences we can think of that could affect how much oxygen is dissolved in the water? ”
What physical forces or conditions affect the dissolved oxygen content of water? (2 points)
What are the biological processes that can affect dissolved oxygen concentration? (2 points)
What might cause each identified condition to fluctuate over the seasons? (2 points)
Part IIIHow Do the Gulf Waters Change with the Seasons?”Thanks for meeting with me again, ” said Sue, shaking hands with Professor Gracia. “I’m hoping you can help me understand what’s going on in the Gulf, what people think causes the low oxygen levels. ”
“I can sure get you started, ” replied Professor Gracia, pulling out some papers from the pile on his desk. “You know, the Gulf waters are very dynamic, changing dramatically with the seasons, and from the surface to the bottom. For example, the Atchafalaya and Mississippi Rivers carry enormous amounts of fresh water into the Gulf and the volume fluctuates with the season. Because the river water is fresh, it is less dense than the seawater, and tends to stay on the surface. The prevailing current near shore in the Dead Zone is from east to west, so the river water is carried from where the river empties towards western Louisiana and Texas.
“Here are some data that will be useful for you to look at showing some of the seasonal changes in temperature, salinity, and dissolved oxygen concentration. Scientists measure the temperature, salinity, and dissolved oxygen concentration of water by using a probe. The probe continuously measures these properties as it is lowered to the sea floor. The data are presented in graphs called station profiles. Here are some taken at different times from a station just off Terrebonne Bay. ”
Professor Gracia handed several station profiles and a water discharge graph to Sue, and then glanced at his watch. “I’m afraid I’ve got to head off to a meeting, but why don’t you take these profiles and spend some time with them. See what you can glean from the data. ”
“Thanks, ” said Sue. “I will. ”
Figure 2.Salinity (triangles), temperature (squares), and dissolved oxygen concentration (circles) at various depths, in meters (0 = surface). A station oﬀ Terrebonne Bay, Gulf of Mexico. Modified from N.N. Rabalais et al., 2002.Figure 3. Average monthly water discharge from the Mississippi River for the years 1930–1992. Modified from Walker, 1994.Questions
What is the average temperature of water in the top 5 meters in April? In August? How do those values compare to the average temperatures at 15–20 meters for those months? (3 points)
Typical seawater has a salinity of 35 psu (practical salinity units). In which month is the diﬀerence in salinity of surface and bottom waters the greatest? Why do you think the diﬀerence is the greatest at this time of year? (3 points)
Water that contains 2 mg oxygen per liter or less is termed hypoxic, since at that concentration many aquatic aerobic organisms are unable to survive. How does the depth at which hypoxia is observed change over time? (2 points)
Part IVHow Do the Organisms Affect Dissolved Oxygen Concentration?After spending time looking over Professor Gracia’s station profiles, Sue felt like she had a much better sense of the seasonal changes in the Gulf and the effects of freshwater on the salinity at different depths. Her friend Zack’s comments about fish using up oxygen made her wonder just how much living organisms can affect the oxygen concentration in such a large body of water. She wondered about what organisms are present besides fish, shrimp, and seaweeds—organisms she already knew about. Sue had learned about food webs in her introductory biology course, so she was comfortable with the idea of primary producers, primary consumers, and predators. But what organisms were playing these roles in the Gulf, and could they realistically affect the oxygen concentration in the water?
She decided to do some legwork to figure out who the key players are, where they reside in the water column, and how much respiration they carry out using the basic ideas of a generalized food web to guide her. She listed these questions for herself (and you):
QuestionsWhy does the primary productivity in the Gulf of Mexico ﬂuctuate over the year (see Figure 4 below)?
Case Study: The Dead ZoneCase Study