PART I. THE PROBLEM

Questions

There are no questions for Part I.

PART II : What Affects The Dissolved Oxygen Content Of Water?

Questions

1. What physical forces or conditions affect the dissolved oxygen content of water?

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

3. What might cause each identified condition to fluctuate over the seasons?

PART III : How Do The Gulf Waters Change With The Seasons?

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?

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?

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?

PART IV : How Do The Organisms Affect Dissolved Oxygen Concentration?

Questions

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

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?

PART V : Why Does The Phytoplankton Population Increase?

Questions

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

10. 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?

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?

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?

13. What makes the hypoxia disappear in the fall and winter?

PART VII : Where Does The Nitrogen Come From?

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.

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?

PART VIII : Conclusion

Questions

There are no questions for Part VII.