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 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
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. 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 affect the density of water? How does this affect the stability of the water?
Let’s check your answers with a demonstration. Your instructor will queue up a film clip. Predict what will happen to the water when the barrier is removed from the tank, and explain why.
Observe the film clip. Did it confirm 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?
Useful Resources
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Ocean World, Ch. 6 Temperature, salinity and density. At http://oceanworld.tamu.edu/resources/ocng_textbook/chapter06/chapter06_04.htm.
Chamberlin, W.S., and T.D. Dickey. Exploring the World Ocean. 2008. New York: McGraw-Hill.
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