An Oyster Growers Dilemma
Research Question: Where along the Thames River are dissolved oxygen levels best for oyster growing? Make sure you consider how conditions will change seasonally.
Part A: Background
Oxygen
Dissolved oxygen, or D.O., is essential for the survival of living things in the estuary. Animals, plants, even some bacteria, all need a sufficient amount of oxygen in order to survive. One way oxygen gets into the water is through diffusion. That means the air that is in contact with the surface of the water naturally mixes into the water. Wave action and wind also help bring atmospheric oxygen into the water. The second way oxygen gets into the water is through plants. As plants, seaweed, and phytoplankton perform photosynthesis, oxygen is produced as a by-product of this process.
It is normal for dissolved oxygen levels to fluctuate in the estuary. For example, the temperature and salinity of the water affect the amount of oxygen that is able to dissolve. Cold water can hold more oxygen than warm water, and fresh water can hold more oxygen than salt water. This means that oxygen levels in water fluctuate seasonally. Oxygen can also be depleted from the water on shorter time scales. In estuaries, oxygen levels are often higher during the day when algae and plants are photosynthesizing, but low at night when the photosynthesis stops and the oxygen gets used up by animals.
Nutrients like phosphates and nitrates can also affect the oxygen levels by disrupting the normal food web. When there is an overabundance of nutrients in the water, an algal bloom can occur. The algae quickly use up all the nutrients, then die and decompose, which in turn leads to oxygen levels dipping to lethal levels. This hypoxic layer forms at the bottom of the water column.
Oysters and Oxygen
Oysters need water with dissolved oxygen levels of at least 3.2 milligrams per liter, but 5.5 mg/l or more is best for survival and growth. Oysters exposed to low oxygen may also be more vulnerable to infection from parasites. Scientists working in the Chesapeake Bay studied oysters in areas where the oxygen levels sometimes dipped down to very low levels. The oysters could survive brief exposure to low oxygen, but they grew more slowly and also had higher rates of disease. Read more about their study here.
The graph below shows that in both years of the study, oysters exposed to lower oxygen levels once a day were much more likely to be infected by parasites.
Graph caption: Data from Breitburg et al 2014, PLoS ONE 10(2):e0116223
Part B: Prediction and Reasoning
If you haven’t already, take the Google Earth tour and think about how the Thames River might vary from its headwaters (where it starts, in Norwich) to its mouth.
Write the answers to the following questions on your sheet of paper.
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Make a prediction. Where along the Thames River do you expect the dissolved oxygen levels on the bottom to be the highest in the summer? Specify a sampling location (labelled A-F on the map).
2. At the location you chose above, what season of the year do you expect the dissolved oxygen to be the highest?
3. Explain your reasoning. WHY do you think the dissolved oxygen will be highest at that location/season?
Part C: Analyze the Data
Look at the dataset below. On your piece of paper, illustrate the data by making a graph. Your graph should have clear labels on both the x-axis and the y-axis. The type of graph (scatterplot, column graph, etc) is up to you.
This table shows the mean dissolved oxygen we measured at each location at the river bottom at different times of year. Each number is an average of many trips.
Part D: Interpret the Results and Make Arguments from Evidence
Write answers to the following prompts on your sheet of paper.
1. Make a claim that answers the research question at the top of this section (one sentence).
2. What evidence was used to write your claim? Reference specific parts of your graph.
3. Explain your reasoning. Make sure to connect your answer to what you have learned about oyster biology, dissolved oxygen, and seasonal change.
4. Was your prediction supported by the results? Use evidence to explain why or why not.
5. Are these readings within the normal range for this area? Standard Ranges for Water Chemistry
6. Revisit the prediction you made at the very beginning, about where might be the best place for an oyster farm. Has your choice of location changed based on what you have learned about oysters, dissolved oxygen, and seasonal change? Explain why or why not.
7. How do dissolved oxygen levels change with seasons in the Thames River, and how would you expect these seasonal changes to affect oysters?
8. How would you follow up to learn more about the potential impacts of dissolved oxygen on your oyster farm? Describe a new question that should be investigated to build on these results, and what future data should be collected to answer your question.
Congratulations! Your final analysis should include the following components:
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Your initial prediction and reasoning about the best location for an oyster farm.
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A statement of the research question that you chose/were assigned
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Your prediction and your reasoning about the research question
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Your labeled graph
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Your answers to the results questions
Share your results with your teacher!