The Dead Zone:Ecology and Oceanography in the Gulf of Mexico “Hi Dad,” said his

Question

The Dead Zone:Ecology and Oceanography in the Gulf of Mexico

“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. “Th e fuel bills were higher than ever this summer. It’s going to be tight for our fi nances. I wish I knew why the fi sh disappear near shore in the summer.”

Bill smiled at his daughter, even though he wasn’t 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’m 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.

“Look here,” he said, pointing to a region of the Gulf of Mexico just below Louisiana and eastern Texas. “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 fi sh actually die there?”

Professor Gracia started gathering up materials for his next class. “Some fi sh may die. Most of the fi sh 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 corresponded with the distance her dad had to boat to get to good fi shing 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.”

Part II – What Aff ects the Dissolved Oxygen Content of Water?

“Hey guys, look at these maps 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 to her friend Paula, a physics major. Paula stopped eating her sandwich long enough to give them a look.

“Th ere must be some physical cause,” Paula said. “I can’t imagine anything else that could aff ect the dissolved oxygen content of water so dramatically.”

“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 infl uences we can think of that could aff ect how much oxygen is dissolved in the water?”

Part III – How Do the Gulf Waters Change with the Seasons?

“Th anks 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 fl uctuates with the season. Because the river water is fresh, it’s less dense than the seawater, and tends to stay on the surface. Th e 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. Th e probe continuously measures these properties as it is lowered to the sea fl oor. Th e data are presented in graphs called station profi les. Here are some taken at diff erent times from a station just off Terrebonne Bay.”

Professor Gracia handed several station profi les and a water discharge graph to Sue, then glanced at his watch. “I’m afraid I’ve got to head off to a meeting, but why don’t you take these profi les and spend some time with them.

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?

Typical seawater has a salinity of 35 psu (practical salinity units). In which month is the diff erence in salinity of surface and bottom waters the greatest? Why do you think the diff erence is the greatest at

this time of year?

In which month is the salinity diff erence between surface and bottom waters the least? What reasons can you think of to explain why the surface and bottom water salinities become more uniform at that time?

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?

Part IV – How Do the Organisms Aff ect Dissolved Oxygen Concentration?

After spending time looking over Professor Gracia’s station profi les, Sue felt like she had a much better sense of the seasonal changes in the Gulf and the eff ects of freshwater on the salinity at diff erent depths. Her friend Zack’s comments about fi sh using up oxygen made her wonder just how much living organisms can aff ect the oxygen concentration in such a large body of water. She wondered about what organisms are present besides

fi sh, 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 aff ect the oxygen concentration in the water?

She decided to do some legwork to fi gure 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:

Questions

What group of organisms are the most important primary producers in the marine aquatic food web? How deep down in the water column can they be found?

What factors are the most important for controlling the growth of these organisms? Th at is, what limits their growth?

Why does the primary productivity in the Gulf of Mexico fl uctuate over the year (see Figure 4 below)?

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

What are the major consumers in the Gulf of Mexico food web?

What are the remaining components of the food web in this area?

What groups are responsible for the greatest total amount of respiration (consumption of oxygen)?

At what time of the year does respiration rate peak (see Figure 5 below)? How does that compare to peak times of primary production? Why is there a lag between these two?

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 fi rst 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.

Th is 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 confi rm 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. (Modifi ed from

Turner, et al., 2005.)

Questions

What are the peak months for nitrate-nitrogen discharge from the Mississippi River into the Gulf?

How do the peak months for nitrate-nitrogen discharge compare to the peak months for phytoplankton primary production?

Part VI – 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 fi gure 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 fl ows constantly—why does the Dead Zone occur only in the summer? Your dad is able to fi sh 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 fi gured 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

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?

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. Th is resulted in a low salinity surface

layer, made warm by solar irradiance. Beneath the river plume was the Gulf water. Th is water had a higher salinity and was cooler. How does temperature and salinity aff ect the density of water? How does this aff ect the stability of the water?

Let’s check your answers with a demonstration. Your instructor will queue up a fi lm clip. Predict what will happen to the water when the barrier is removed from the tank, and explain why.

Observe the fi lm clip. Did it confi rm your prediction? If not, what did happen and why?

To mix a stable water column requires kinetic energy. Can you think of any processes that might supply this energy? Do any of these processes change in intensity with the seasons?

What makes the hypoxia disappear in the fall and winter?

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 offi ce.

“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 fl ushed out by the turbulence of fall and winter storms. Th e data

on nitrogen carried in by the Mississippi show a sizeable increase in early spring when the river discharge rate is at its peak. Th at supports the bloom of phytoplankton, and the eventual population explosion of marine aerobic bacteria. Th ey deplete the oxygen available in deeper waters, and form the Dead Zone in early summer. Th e one thing I haven’t fi gured 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 phenomena, is explained by this graph” said

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

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

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

“Th at’s probably the most controversial part of this whole problem,” said Professor Gracia, opening his offi ce door. He pushed some books off 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. Th e farming interests argue that you can’t 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 Cliff ord Snyder, the Midsouth Director of the Potash and Phosphate Institute.” Professor Gracia pulled out a folder from his fi ling cabinet, and handed Sue a sheet.

“Th ere are also no conclusive data that identify the sources of the nitrate and nitrogen that enter the Mississippi 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. Th eir conclusion was not surprising, since inputs to the model were based on the

assumption that many non-agricultural sources (for example, urban runoff and geological nitrate) were insignifi cant. Th eir 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. Th is geographic area contains naturally rich soils of the prairies, as well as agriculture. Th e proportion of the nitrogen that comes from agriculture and the proportion of the agricultural nitrogen that arises from fertilizer use remain uncertain.” (Snyder, 2001)

Sue read the letter quickly.

“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.”

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 fi xation that occurs in legume crops. (Modifi ed from McIsaac et al., 2002.)

Which nitrogen source has added the greatest amount of nitrogen to the land in the Mississippi River basin in the years since 1970?

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?

Explanation / Answer

Part II

Answer

1. The physical and biological forces that affect the dissolved oxygen content of water are as follows:

Physical forces are temperature, the surface area of the water, and salinity. The cooler the temperature, the more the dissolved oxygen will be available in the water. If the surface area is large, then more oxygen can be absorbed by the body of the water. If the salinity is high, then the dissolved oxygen will be in less concentration. As the salinity increases the concentration of dissolved oxygen also decreases.

Biological forces include diffusion, aeration, respiration and decomposition, and photosynthesis.

Part III

1.In April, the water temperature in the top 5 meters ranges between 18oC to 19oC and in August, the temperature at 15 to 20 meters range from 29.5oC at the top to 26oC.

2. In April and September, the difference in salinity at top and bottom waters was greatest. The difference is greater during this period because many events take place like the continuous flowing of river water into the sea, precipitation, and evaporation of water and snow, and melting of ice. These fluctuation events will increase the salinity of seawater.

3. In September, the difference in the salinity between the surface and bottom waters is least. The river water is carried to the ocean and as the river water is fresh it is less dense and stays at the surface resulting in fewer fluctuations.

4. Hypoxia is a condition where the oxygen levels are low. Generally, the water has 2 mg oxygen per liter but if the amount of oxygen is less than 1.43 milliliters of oxygen per liter of seawater, then it will result in saturation where sea animals cannot survive and die.

Part IV

Answer

1. Seaweed and other forms of algae like phytoplankton are the primary producers in the marine aquatic food web. Since, these organisms require sunlight and the sunlight reach as far as 200 meters below the surface, these organism are found throughout this zone which is referred as Euphotic zone.

2. The important limiting factors that affect the growth and succession of phytoplankton and seaweed are light, temperature, organic and inorganic micronutrients, salinity, oxygen concentration, and buoyancy regulation.

3. The fluctuations in the primary productivity in the Gulf of Mexico over the year are due to the dynamic seasonal changes and the effect of fresh water on the salinity at different depths. Since, the density of fresh water is less than sea water, it remains on the surface. Seasonal changes can highly influence the salinity, dissolved oxygen, and temperature of the water at various levels.

4. The major consumers in the Gulf of Mexico food web are sea snail, longhead turtle, sea urchin, red start fish, and surgeonfish.

5.The other components of Gulf of Mexico food web include the primary consumers like jelly fish, small fish, squid and baleen whales which feed on zooplankton, and the predators like large fish, dolphins, shark that feed on smaller animals.

6. The algae are responsible for greatest total amount of respiration. The available nutrients provides favorable conditions for algae which utilize the water’s oxygen supply for respiration.

7. The respiration rates are peak in the months of July and August. When compared with primary production there is no evident data recoded in the month of July and in the month of August the primary production rate is low. This is due to replenishment of nutrients from deep waters to the surface water. The mixing of river water causes the algae and phytoplankton to move toward the surface so the reducing the respiration rate exceeds primary production.

Seaweed and other forms of algae like phytoplankton are the primary producers in the marine aquatic food web. Since these organisms require sunlight and the sunlight reach as far as 200 meters below the surface, these organisms are found throughout this zone which is referred as a Euphotic zone.

The important limiting factors that affect the growth and succession of phytoplankton and seaweed are light, temperature, organic and inorganic micronutrients, salinity, oxygen concentration, and buoyancy regulation.

The fluctuations in the primary productivity in the Gulf of Mexico over the year are due to the dynamic seasonal changes and the effect of fresh water on the salinity at different depths. Since the density of fresh water is less than seawater, it remains on the surface. Seasonal changes can highly influence the salinity, dissolved oxygen, and temperature of the water at various levels.

The major consumers in the Gulf of Mexico food web are sea snail, long head turtle, sea urchin, redstart fish, and surgeonfish.

The other components of Gulf of Mexico food web include the primary consumers like jellyfish, small fish, squid and baleen whales which feed on zooplankton, and the predators like large fish, dolphins, a shark that feed on smaller animals.

The algae are responsible for a greatest total amount of respiration. The available nutrients provide favorable conditions for algae which utilize the water’s oxygen supply for respiration.

The respiration rates are the peak in the months of July and August. When compared with primary production there is no evident data recorded in the month of July and in the month of August the primary production rate is low. This is due to replenishment of nutrients from deep waters to the surface water. The mixing of river water causes the algae and phytoplankton to move toward the surface so the reducing the respiration rate exceeds primary production.

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