Lecture 17: Exam Questions

1. What is limnology?
Answer: Limnology is the study of inland fresh waters. It can be thought of as "oceanography for fresh water".

2. What are different methods of lake origination? Give examples of some.
Answer:

  • Tectonic lakes are formed by the movement of tectonic plates. These lakes are old and very deep, such as Lake Baikal in Russia.
  • Kettle lakes form when ice left from retreating glaciers is buried in till and then melts. There are a number of kettle lakes in the plains of Wisconsin.
  • Glacial lakes are formed from a variety of glacial processes, including glacial ice scour, morainal dams, and cirque lakes. Glacial lakes tend to be small but numerous.
  • Landslide lakes form when a landslide dams a stream or river. Landslide lakes are similar to man-made reservoirs. An example is Lake Sarez in Tajikistan.
  • Volcanic-caldera lakes form when a volcanic explosion leaves a large crater that is then filled with water. An example is Crater Lake in Oregon.
  • Dissolution lakes (or cenote lakes) form in karst topography from the dissolution of limestone bedrock. They tend to be small with steep sides. These are commonly found in Florida.
  • **Oxbow lakes (or floodplain lakes, or billabong, in Australia!) form when a river bend is pinched off and leaves a lake behind. These tend to be shallow, narrow, and flooded on a seasonal basis. Lake Chicot is the largest oxbow lake in North America, and oxbow lakes on the Rhine are an excellent example of lake formation caused by artificial straightening of a river.

3. What is thermal stratification and why does it occur?
Answer:Thermal stratification is the phenomenon where you get layering of water in lakes. It occurs because of differences in water density caused by differences in temperature or salinity. Water is most dense at 4 degrees Celsius. Large temperature differentials cause large resistance to water layers mixing, causing the stratification. Thermal stratification is important because it can "cut off" or create barriers between upper and lower layers within a lake. This impacts interaction between species and causes boundaries within the ecosystem. Additionally, it impacts the nutrient cycling in the system. Typically, lakes are not stratified in the winter, mix in the spring and fall, and are stratified in the summer.

4.Why do you get mist coming off lakes in the fall?
Answer: Water "holds" temperature better and longer than air. Thus, in the fall, the air temperature cools down more quickly than water temperature. This relatively warm water compared to air temperature results in the creation of mist, as heat transfers from the water to the air.

5. What is the connection between thermoclines in oceans and carbon capture and sequestration?
Answer: People are exploring "storing" or "trapping" captured carbon dioxide in the cold water in oceans located beneath the thermocline. The theory here is that the thermoclines in certain oceans are continual (do not go away seasonally) and create strong barriers to the movement of deep cold water. Thus, if carbon dioxide can be mixed into that deep, cold water, it is unlikely to be released into the atmosphere because of the thermocline barrier.

6. What is the “Paradox of the Plankton”?
Answer: The Paradox of the Plankton has to do with the paradox between the observed large diversity of plankton and the limited resources available to plankton in the aquatic environment. Gause's Law argues that in situations with limiting resources, one species will "win out" and other species will become rare or extinct. Plankton all require a specific set of nutrients (nitrogen, phosphorus, etc.) to survive. According to Gause's Law, the limiting nature of these nutrients should result in a limited diversity of plankton. However, the large diversity of plankton in many systems seems to contradict Gause's Law.

7. What is the Redfield ratio? Why is it important?
Answer: The Redfield Ratio is the ratio of carbon to nitrogen to phosphorous that is most often found in algae (106:16:1). This implies that this is the ratio of those nutrients that algae need to survive. Based upon this, you can determine what is the limiting nutrient in a given system. For example, if the ratio is 40:16:1, you would know that carbon is the limiting factor in that specific system.

8. What is one benefit of "bottle experiments"?
Answer: They can help you to determine what is the limiting nutrient in a system. This is easier than doing such an experiment in a larger system (such as the lake and algae bloom example in class).

9. What are hypoxia and anoxia? Why are they important?
Answer: Hypoxia is when oxygen levels in water are less than 2 mg/L. Anoxia is when no oxygen is present in water. They are important because they are effectively "dead zones" where aerobic organisms can not live (and thus microbes dominate in these areas).

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