Introduction to Oceanography: Abiotic
Density
Research Question: How and why does the density of the estuary change with depth?
Part A: Background Information
The density of a substance equals its weight (in grams) divided by its volume (in cubic centimeters). The density of water varies according to its temperature and salinity. Cold water is more dense than warm water, and salty water is more dense than freshwater. A cubic centimeter (cm) of distilled water at 4 degrees Celsius has a density of 1.000 g/cm³. Ocean water, being more saline, has a density of about 1.030 g/cm³.
Dense water will sink below less dense water. Therefore, the ocean is often composed of layers of water of different densities. For example, in an estuary where a river meets the ocean, fresh water flows out over the more dense salty ocean water. Layering also occurs where cold deep ocean water lies beneath the warm surface water. The zone where the two layers of different water densities meet is called the pycnocline.
Part B: Sampling Method
Source: Project Oceanology
Van Dorn Bottle set up (left) Hydrometer (right)
The Van Dorn Bottle allows water to be sampled from any depth in the water column. It consists of a narrow plastic tube, with rubber plungers that allow it to close. To deploy, the Van Dorn Bottle plungers are held open with wire, and the apparatus is lowered into the water using a marked line and winch. When the bottle is at the desired depth, a “messenger” weight is dropped along the line to a mechanism which releases the plungers, closing the bottle. This keeps the sample purely from the desired depth, without mixing with other water depths while being retrieved.
Once the water samples have been collected a hydrometer is used to measure the density from each sample depth. The water sample is simply poured into the hydrometer and it is set on a flat surface. The more dense the water the higher the needle points. The instrument is read on the inner scale labeled specific gravity. The scale ranges from 1.000 to 1.032.
To learn more about the Van Dorn Bottle watch this brief video:
Part C: Prediction and Reasoning
Study the background information provided on density (above),and look at the field notes (below).
Field Notes
Sample Location: Mouth of the Thames River estuary. (View map here)
Time of Year: Early May
Weather Conditions: Day time air temperatures are about 61 Fahrenheit and evening air temperatures are
Sample Depth: Density samples were collected at the surface (0 meters), the middle of the water column
(4 meters) and the bottom (9 meters). This is called a depth profile.
about 48 Fahrenheit.
Write answers to the following prompts on your sheet of paper.
1. Make a prediction: Make a prediction. How do you expect the density to change from top to bottom?
2. Explain Your Reasoning: WHY do you think the density will look this way?
Part D: Analyze the Data
Look at the dataset below. On your piece of paper, illustrate the data by making a special kind of graph called a depth profile. The axes look a little different than you might be used to; simply plot column 1 numbers on the vertical axis and column 2 on the horizontal. The origin is still (0,0). Connect your points with a curved line. Remember good graphs have a title, a labeled x-axis (including units), a labeled y-axis (including units), and an appropriate scale.
This table shows the density in the water column.
Here is an example of how to set up your depth profile graph.
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? Reference specific parts of your graph.
3. Explain your reasoning. Make sure to connect your answer to what you have learned about the relationship between depth and density.
4. Was your prediction supported by the results? Use evidence to explain why or why not.
5. What do these numbers mean for the health of Long Island Sound? Are these readings within the normal range for this area? Standard Ranges
6. How would you follow up? Describe a new question that should be investigated to build on these results, and what future data should be collected to answer your question.
7. Extension: Organisms within an estuary can be significantly affected by the density of their environment. Check out this article from LIVE SCIENCE about how animals (big and small) may help to mix up ocean waters of different densities. Write a paragraph and answers to the following: (1) One example from the article that supports how animals mix the density layers that exist in the ocean; (2) one example why some scientists don’t believe animals can help mix the ocean; (3) Why do you think all scientists don’t agree on this topic?; and (4) What is one way that marine organisms can be affected by changing densities?
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 labeled graph
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Your answers to the results questions
Share your results with your teacher!