task 7

9/6/2019 Case Study - The Dead Zone

https://bconline.broward.edu/content/Courses/Oceanography/OCE1001/assignments/assignment6.html 1/11

Case Study: The Dead Zone

CASE 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 PROBLEM

The Problem

Bill 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. "

9/6/2019 Case Study - The Dead Zone

https://bconline.broward.edu/content/Courses/Oceanography/OCE1001/assignments/assignment6.html 2/11

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.

9/6/2019 Case Study - The Dead Zone

https://bconline.broward.edu/content/Courses/Oceanography/OCE1001/assignments/assignment6.html 3/11

‘ Figure 1b. Detail of the Dead Zone (shaded) in the Gulf of Mexico. Questions

There are no questions for Part I.

PART II

What 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? "

Questions

1. What physical forces or conditions affect the dissolved oxygen content of water? (2 points)

2. What are the biological processes that can affect dissolved oxygen concentration? (2 points)

9/6/2019 Case Study - The Dead Zone

https://bconline.broward.edu/content/Courses/Oceanography/OCE1001/assignments/assignment6.html 4/11

3. What might cause each identified condition to fluctuate over the seasons? (2 points)

PART III

How 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 o Terrebonne Bay, Gulf of Mexico. Modified from N.N. Rabalais et al., 2002.

9/6/2019 Case Study - The Dead Zone

https://bconline.broward.edu/content/Courses/Oceanography/OCE1001/assignments/assignment6.html 5/11

‘ Figure 3. Average monthly water discharge from the Mississippi River for the years 1930–1992. Modified from Walker, 1994.

Questions

4. 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)

5. Typical seawater has a salinity of 35 psu (practical salinity units). In which month is the di erence in salinity of surface and bottom waters the greatest? Why do you think the di erence is the greatest at this time of year? (3 points)

6. 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 IV

How 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):

Questions

7. Why does the primary productivity in the Gulf of Mexico fluctuate over the year (see Figure 4 below)? (3 points)

9/6/2019 Case Study - The Dead Zone

https://bconline.broward.edu/content/Courses/Oceanography/OCE1001/assignments/assignment6.html 6/11

‘ Figure 4. Primary production in the Gulf of Mexico in mg carbon assimilated per cubic meter per day. No data were collected for July. (Modified from Sklar & Turner, 1981.)

8. At what time of the year does respiration rate peak (see Figure 5 below)? How does that compare to peak times of primary production? (2 points)

‘

Figure 5. Respiration rates in the Gulf of Mexico at 18–20 m, transect C6b. (Modified from Dortch et al., 1994.)

9/6/2019 Case Study - The Dead Zone

https://bconline.broward.edu/content/Courses/Oceanography/OCE1001/assignments/assignment6.html 7/11

PART V

Why Does The Phytoplankton Population Increase?

Sue could see that photosynthesis by the phytoplankton population increased sharply in the spring months, suggesting their

populations had soared. Following the spring peak, photosynthetic rates declined as the population members died and

respiration rates in the water rose dramatically. It made sense to Sue that the organic matter from dead phytoplankton was

sinking, providing a rich food source for marine bacteria populations at the base of the water column. As the bacterial

populations climbed, they depleted the oxygen available in the water, especially at the bottom of the water column.

But why did the phytoplankton population explode in the first place? Sue knew that light and nutrients were the things most

likely to limit growth. Day length was increasing in April and May when the populations climbed, but since the population

crashed when day length continued to increase in June, light did not seem to be the cause. Nutrients seemed more likely to

be the culprit, carried into the Gulf by the Mississippi River. This also made sense knowing that the prevailing current would

carry nutrient-rich water from the river to the west of the Mississippi's mouth, exactly where the Dead Zone was located.

Nitrogen is commonly the limiting nutrient so Sue decided to confirm her suspicions by tracking down data on the monthly

change in the nitrogen concentration of the Mississippi River (Figure 6).

‘ Figure 6. Monthly discharge of nitrate and nitrogen from the Mississippi River into the Gulf of Mexico. Values are normalized for the average annual value from 1978 to 1995. (Modified from Turner, et al., 2005.)

Questions

9. What are the peak months for nitrate-nitrogen discharge from the Mississippi River into the Gulf? Why do you think this is? (3 points)

10. How do the peak months for nitrate-nitrogen discharge compare to the peak months for phytoplankton primary production? (3 points)

PART VI

9/6/2019 Case Study - The Dead Zone

https://bconline.broward.edu/content/Courses/Oceanography/OCE1001/assignments/assignment6.html 8/11

Why Is The Dead Zone A Seasonal Phenomenon?

"Hey Sue, can I join you?" asked Paula. Sue was sitting in the cafeteria, digging into her lunch.

"Sure, Paula. I'm excited—I actually have made some headway on my project to figure out what's happening in the Dead

Zone," said Sue. "It looks like the Mississippi River water is carrying nutrients like nitrogen into the Gulf, and that in turn

promotes a population explosion of photosynthetic plankton. Excretion of organic compounds from the phytoplankton, plus

their dead cells when they die, sink down, providing a rich source of food for heterotrophic aerobic bacteria. It's the bacteria

that use up the oxygen in the water."

Paula picked at her salad. "Um, ok. But the Mississippi River flows constantly—why does the Dead Zone occur only in the

summer? Your dad is able to fish much closer to shore in the late fall and into the winter. What makes the Dead Zone

disappear then; what restores oxygen to the water?"

"Ah, Paula, you always manage to get right to the part I haven't figured out yet," said Sue. "I understand why the oxygen

depletion happens, but I really don't know what restores the oxygen levels once they are low."

Sue thought back on the ways that dissolved oxygen enters the ocean. One or more of these processes must re-oxygenate the

water for the Dead Zone to disappear.

Questions

11. Does it seem likely that any of the seasonal changes noted in Part II, Question 3, re-oxygenate the bottom waters of the Dead Zone in the autumn and winter? (2 points)

12. Recall that in the summer the water column in the zone of hypoxia is layered. Figure 2 in Part III shows that the river plume occupied the upper water column. This resulted in a low salinity surface layer, made warm by solar irradiance. Beneath the river plume was the Gulf water. This water had a higher salinity and was cooler. How does temperature and salinity a ect the density of water? (3 points)

13. What makes the hypoxia disappear in the fall and winter? (2 points)

PART VII

Where Does The Nitrogen Come From?

"I'm impressed—you've put together most of the pieces of the Dead Zone puzzle," said Professor Gracia. Sue blushed, but

nodded, as they walked together towards his office.

"Well, it really matters to my family, so I had a pretty strong motivation," she said. "I think I understand now why the Dead

Zone is temporary, getting flushed out by the turbulence of fall and winter storms. The data on nitrogen carried in by the

Mississippi show a sizeable increase in early spring when the river discharge rate is at its peak. That supports the bloom of

phytoplankton, and the eventual population explosion of marine aerobic bacteria. They deplete the oxygen available in

deeper waters, and form the Dead Zone in early summer. The one thing I haven't figured out is where the nutrients in the

Mississippi River water are coming from, and why the Dead Zone was not a problem many years ago. "

"I think your second concern, why hypoxia is a relatively recent phenomenon, is explained by this graph," said Professor

Gracia. "What do you notice about nitrogen discharge in the sixties versus the last decade? "

9/6/2019 Case Study - The Dead Zone

https://bconline.broward.edu/content/Courses/Oceanography/OCE1001/assignments/assignment6.html 9/11

‘ Figure 7. Nitrate and nitrogen (in millions of tons per year) discharged into the Gulf of Mexico from the Mississippi River. (Modified from Goolsby, 2001.)

"I can see that nitrogen has increased, but what is the source of the nitrogen? " said Sue.

"That's probably the most controversial part of this whole problem," said Professor Gracia, opening his office door. He pushed

some books o a chair and motioned towards it. "Here, have a seat. Most researchers, such as Nancy Rabalais and Don

Goolsby, have argued that the most important source of nitrogen comes from the fertilization of farms on the land that

drains into the Mississippi River. Nutrients not used by crops are washed into the river system and are carried downstream.

The farming interests argue that you can not rule out the possibility that it comes from other sources. Here, take a look at

this letter to the editor of the journal Science by Cli ord Snyder, the Midsouth Director of the Potash and Phosphate

Institute. " Professor Gracia pulled out a folder from his fling cabinet, and handed Sue a sheet.

Sue read the letter quickly:

‘ "There are also no conclusive data that identify the sources of the nitrate and nitrogen that enter theMississippi River and ultimately reach the Gulf. In the White House Committee on the Environment and Natural Resources (CENR) Topic 3 report, Don Goolsby and others used a statistical model to conclude that agriculture was the major source of nitrogen to the Mississippi River basin. Their conclusion was not surprising, since inputs to the model were based on the assumption that many non-agricultural sources (for example, urban runo and geological nitrate) were insignificant. Their estimates of discharge are simply proportionate to the tonnage of each input source in sub-basins of the Mississippi River basin. River monitoring clearly indicates that major nitrogen loads come from the geographic area of the Corn Belt, but the sources remain unclear. This geographic area contains naturally rich soils of the prairies, as well as agriculture. The proportion of the nitrogen that comes from agriculture and the proportion of the agricultural nitrogen that arises from fertilizer use remain uncertain. " (Snyder, 2001)

"Snyder seems to be saying that while it's true the nitrogen is coming from the Corn Belt, it could be originating from the

prairie soils, that we can't know how much is coming from the natural soil and how much is coming from added fertilizer. I

guess it would be hard to tell exactly where a nitrogen molecule came from," Sue said.

"Ok," said Professor Gracia. "Now take a look at this data from Don Goolsby and his colleagues, and see what you think. "

9/6/2019 Case Study - The Dead Zone

https://bconline.broward.edu/content/Courses/Oceanography/OCE1001/assignments/assignment6.html 10/11

‘ Figure 8. Nitrogen inputs to the 20-state region of the Mississippi River basin, in kilograms of nitrogen per hectare per year. Fixation refers to the nitrogen fixation that occurs in legume crops. (Modified from McIsaac et al., 2002.)

Questions

14. Which nitrogen source has added the greatest amount of nitrogen to the land in the Mississippi River basin in the years since 1970? Give a reasoning why this source is on the rise. (4 points)

15. Refer to Figure 7, which shows the increase in nitrogen carried by the Mississippi into the Gulf, and to Figure 8 above. Are the data in these graphs consistent with the idea that nitrogen naturally present in rich prairie soils is the source of nitrogen carried into the Gulf of Mexico? Why or why not? (4 points)

Note: Did you remember to answer all 15 questions with at least two complete sentences (27-42 words per answer)?

PART VIII

Conclusion

When important scientific claims are made there is typically a flurry of research and experiments, and a welter of di erent

conclusions and interpretations. As data from di erent sources accumulates, however, it becomes clearer, which

interpretations are consistently supported, and which are not. Although a small minority of scientists continues to argue that

the source of the nitrogen increase has not been proven, most scientists examining the data from numerous studies have

concluded that fertilizer application in farms of the Mississippi River basin is the source of nitrogen that leads to the Dead

Zone. The problem has moved slowly from scientific debate, to scientific consensus and into the arena of public policy and

government action. The United States Geologic Survey released a report in January 2008. Here is an excerpt from the

accompanying news release, which states unequivocally:

‘ "Nine states in the Mississippi River Basin contribute the majority of nutrients to the Northern Gulf of Mexico,threatening the economic and ecological health of one of the nation's largest and most productive fisheries. " ‘ "Excessive nutrients have resulted in a zone of low dissolved oxygen or hypoxia, caused by the growth of largeamounts of algae. This can stress and cause death in bottom-dwelling organisms in the Gulf." (Alexander, et al., 2008)

9/6/2019 Case Study - The Dead Zone

https://bconline.broward.edu/content/Courses/Oceanography/OCE1001/assignments/assignment6.html 11/11

Now that the cause of the Dead Zone has been determined, state and federal agencies are looking for ways to reduce

nutrient pollution from farmlands before it enters the Mississippi River system. Voluntary reduction in fertilizer application,

restoring wetlands along waterways and establishment of vegetated bu ering zones to absorb nutrients before they enter

rivers and streams are among the action strategies being pursued. The goal is to reduce the area of the Dead Zone to 5,000

square kilometers by 2015. This represents about 1/3 the average square area of the Dead zone between 2003–2007.

Achieving this goal will require overcoming many challenges, including reluctance of farmers to alter farming practices. In

the meantime, small fishing operations near the Dead Zone will continue their struggle to survive.

Questions

There are no questions for Part VII.

ROLLING CREDITS...

References

Lalli, C.M., and T.R. Parsons. 1993. Biological Oceanography, An Introduction. New York: Pergamon Press. The Habitable Planet. Cycling Trough the Food Web. The Bigelow Laboratory for Ocean Sciences. Our Ocean Planet—Oceanography in the 21st Century. Robert Stewart. Ocean World, Ch. 6 Temperature, salinity and density. Chamberlin, W.S., and T.D. Dickey. Exploring the World Ocean. 2008. New York: McGraw-Hill.

Citations

Case Study - The Dead Zone was brought you to by Kathleen Archer and Lauren Sahl, 2009, The Dead Zone: Ecology and Oceanography in the Gulf of Mexico, National Center for Case Study Teaching in Science, New York, p.1-13. Figure 1: Goolsby, D.A., and W.A. Battaglin. 2000. Nitrogen in the Mississippi Basin—Estimating sources and predicting flux to the Gulf of Mexico. USGS Fact Sheet 135-00. Last accessed: Nov 24, 2016. Figure 2: Rabalais, N.N., R.E. Turner, and D. Scavia. 2002. Beyond science into policy: Gulf of Mexico hypoxia and the Mississippi River. BioScience 52: 129–142. Figure 3: Walker, N.D. 1994. Satellite-based assessment of the Mississippi River discharge plume's spatial structure and temporal variability. OCS Study MMS 94-0053. U.S. Dept. of the Interior, Minerals Management Service, Gulf of Mexico OCS Region, New Orleans, La., 56 pp.

Figure 4: Dortch, Q., N.N. Rabalais, R.E. Turner, and G.T. Rowe. 1994. Respiration rates and hypoxia on the Louisiana shelf. Estuaries 17(4): 862–872. Figure 5: Sklar, F.H., and R.E. Turner. 1981. Characteristics of phytoplankton production o Bataria Bay in an area influenced by the Mississippi River. Contributions in Marine Science 24:93–106. Figure 6: Turner, R.E., N.N. Rabalais, E.M. Swenson, M. Kasprzak, and T. Romaire. 2005. Summer hypoxia in the northern Gulf of Mexico and its prediction from 1978 to 1995. Marine Environmental Research 59(1): 65–77. Figure 7: Goolsby, D.A., and Battaglin, W.A. 2001. Long-term changes in concentrations and flux of nitrogen in the Mississippi River Basin, USA. Hydrological Processes 15:1209–1226. Figure 8: McIsaac, G.F., David, M.B., Gertner, G.Z, and Goolsby, D.A. 2002. Relating net nitrogen input in the Mississippi River basin to nitrate flux in the lower Mississippi River: A comparison of approaches. Journal of Environmental Quality 31: 1610–1622.

Introductory Oceanography - OCE1001 | Broward College