An Oyster Growers Dilemma
Research Question: Where along the Thames River are nitrate levels best for oyster growing? Make sure you consider how conditions will change seasonally.
Part A: Background
Nitrates
Nitrogen is one of the most common elements in the environment, and is essential for plant and animal life. Nitrates are nitrogen compounds and are formed from nitrogen in human and animal wastes, and are used as components of fertilizers and explosives. From natural sources, nitrates enter waterways through runoff from the watershed, and from biological processes in the water itself. Nitrogen is critically important for photosynthesis, so nitrates are commonly used to improve crop yields or lawn growth. These nutrients can be transported from land sources like farms and lawns by heavy rains that drain to streams and rivers. Nitrates can also escape in the discharge of wastewater treatment facilities, particularly ones with combined sewer overflow systems. When nitrates enter water, they are taken up by plants and enter the food web, are carried by moving water, and eventually sink to the bottom.
Nitrates are a normal part of healthy ecosystems, but when they get too high, it can be a problem because high nitrates can cause algal blooms. Nitrate levels in run-off often fluctuate seasonally, as a result of changes in human activities. For example, fertilizers with nitrate in them are typically applied only at certain times of year.
Nitrate Concentrations (NO₃⁻)
Fresh Water
Healthy
High (cause for concern)
Enriched (could cause an algal bloom)
0-2.5 mg/L
2.5-3.7 mg/L
>3.7 mg/L
Long Island Sound
Healthy
High (cause for concern)
Enriched (could cause an algal bloom)
0-0.5 mg/L
0.5- 1.0 mg/L
>1.0 mg/L
Oysters and Nitrates
Oysters are filter-feeders, which means they take in water (and all the suspended particles in it!) to eat. They eat phytoplankton, other organic matter, and nutrients like nitrates and phosphates. Oysters significantly reduce the amount of these nutrients in the water, but they will excrete any they can’t use for growth. Since oyster tissues store phosphates and nitrates, when humans harvest them for food, those nutrients are removed from the environment. Read more about the role of oysters in removing nitrates from the environment here.
Under normal levels of nitrates (and phosphates) in the water, aquatic plants and phytoplankton use the nutrients for growth, and are in turn eaten by the oysters. When there are too many nutrients, for example as a result of run-off from fertilizers, phytoplankton grow quickly and become super-dense. This is called an algal bloom. The phytoplankton quickly use up all the nutrients, then die and decompose, which in turn leads to oxygen levels dipping to lethal levels. This problem is worse at the bottom of the water column, where oysters live. If oxygen levels get too low, oysters will grow more slowly, be more vulnerable to diseases, or even die.
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 nitrate levels 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 nitrate levels to be the highest?
3. Explain your reasoning. WHY do you think the nitrate levels 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 nitrate level we measured at each location in spring and summer. Project Oceanology does not have data on nitrate levels in the fall and winter. 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, nitrates, and seasonality.
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, nitrates, and seasonality? Explain why or why not.
7. How do nitrate 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 nitrates 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!