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:
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? That is, what limits their growth?
Why does the primary productivity in the Gulf of Mexico fluctuate 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. (Modified 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?
Figure 5. Respiration rates in the Gulf of Mexico at 18–20 m, transect C6b. (Modified from Dortch et al., 1994.)
Useful Resources
Lalli, C.M., and T.R. Parsons. 1993. Biological Oceanography, An Introduction. New York: Pergamon Press.
The Habitable Planet. Available online at http://www.learner.org/channel/courses/envsci/unit/.
Cycling Through the Food Web. Available online at http://www.bigelow.org/bacteria/. The Bigelow Laboratory for Ocean Sciences.
Our Ocean Planet—Oceanography in the 21st Century. Robert Stewart. Available online at http://oceanworld.tamu.edu/resources/oceanography-book/contents2.htm.
Figure References
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.
Sklar, F.H., and R.E. Turner. 1981. Characteristics of phytoplankton production off Bataria Bay in an area influenced by the Mississippi River. Contributions in Marine Science 24:93–106.
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