Introduction to Oceanography: Abiotic
Color & Clarity
Research Question: How and why do the water color and clarity vary at different locations in an estuary?
Part A: Background Information
Water Clarity is a measure of how clear or cloudy the water is. This is important because the water clarity will determine how much light is able to penetrate into the water. Sunlight is essential for photosynthesizing marine plants and phytoplankton. It also allows animals that depend on their eyesight to find food or to avoid predators. The more transparent (clear) the water is, the deeper the light will reach.
Water Color is an indication of what is suspended in the water - this can include phytoplankton, suspended sediment, dissolved organic material and even pollution.
Water color and water clarity are related. Typically open ocean waters are an indigo blue color because the water is clearer - sediment that washes down rivers mostly settles out before the water reaches the open ocean, and open ocean water has less nutrients and therefore less phytoplankton than coastal waters. In an estuary like Long Island Sound, brownish-green colors are common because these waters are nutrient-rich, abounding in phytoplankton, and may also contain sediments from rivers. Water color in coastal areas like Long Island Sound is highly influenced by weather, because storms can dramatically increase the amount of sediment traveling down rivers.
Part B: Sampling Method
Source: Wikimedia Commons
The secchi disk is a tool used to measure the clarity or turbidity of the water. The black and white disk is weighted to help it sink. It is lowered into the water until the black and white pattern on the disk is no longer visible. This depth is then recorded to the nearest quarter of a meter.
Try a fun Secchi Disk Simulation by clicking here. You will be able to practice lowering the secchi disk into the water until the black and white pattern disappears!
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The Forel Ule color scale is a tool used to help quantify the color of the water. Each standard color from blue (I) to green (VII) to yellow (XIV) to brown (XXII) and everything in between is given a roman numeral. As you hold the color scale over the water you match the color of the water observed against the white background of the secchi disk, pictured above.
To learn more about the secchi disk and Forel Ule Color Scale watch this brief video:
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Source: http://forel-ule-scale.com/
Here are a few examples of water color variation. Which areas do you think would be the best for a fish that eats algae?
Part C: Prediction and Reasoning
Study the background information provided on clarity and color (above), and the map of sample locations with information (below). Write answers to the following prompts on your sheet of paper.
1. Make some predictions: A group of students sampled the water color and clarity (secchi depth) at three different locations in the Long Island Sound estuary. Site A is in the Thames River, Site B is at the mouth of the Thames River, and Site C is near the mouth of Long Island Sound, where it meets the Atlantic Ocean.
Prediction 1: Which location do you think will have the clearest water?
Prediction 2: Remember, water in coastal locations can range from blue to green to brown. What color do you predict the water will be at each of the three locations?
2. Explain your reasoning. WHY do you think the water will be clearer in the location you chose? For each location, why do you think the water will be that color?
Site A is in the Thames River, Site B is at the mouth of the Thames River, and Site C is near the mouth of Long Island Sound, where it meets the Atlantic Ocean.
Part D: Analyze the Data
Look at the data below. Record the results on your piece of paper.
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Part E: 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 (one sentence).
2. What evidence was used to write your claim? Make sure to reference specific data from the data table
3. Explain your reasoning. Make sure to connect your answer to what you have learned about the water color and clarity.
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. How would you follow up? Describe a research question that should be investigated to build on what the students have found, and what future data should be collected to answer your question.
7. Extension: Organisms within an estuary can be significantly affected by the color and clarity of the water. Check out this website for information about seagrass beds, and this one about the animals who live there. Write a paragraph that (1) identifies at least one factor that would cause the water color or clarity of an estuary to change, (2) explains how changes in water clarity would affect sea grasses, and (3) explains how this affects (or is affected by) people.
Congratulations! Your final analysis should include the following components:
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A statement of the research question that you chose/were assigned
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Your prediction and your reasoning
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Your answers to the results questions
Share your results with your teacher!