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’s 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, 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?
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?
In which month is the salinity difference 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?
Figure References
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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.
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.
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