Introduction to Oceanography: Abiotic
Dissolved Oxygen
Research Question: How and why does the dissolved oxygen concentration change from surface to bottom?
Part A: Background Information
Dissolved Oxygen is essential for the survival of living things in the estuary. Animals like crabs, lobsters, fish and clams all need a sufficient amount of oxygen in order to survive. So how does dissolved oxygen enter into the estuary anyway? 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 Long Island Sound. 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. Healthy levels of oxygen range from about 5-15 mg/L.
There are times when the oxygen levels get to unhealthy levels. Rapid changes in temperature and salinity may decrease oxygen levels. Excess nutrients and entering the estuary can cause large blooms of phytoplankton, which may sound like a good thing. The problem arises when these phytoplankton die. Bacteria and other decomposers use up oxygen to break down the organic matter. These low oxygen conditions are called hypoxia which is a stressful amount of oxygen for most organisms (0-3 mg/L). There are times when the estuary experiences dead zones in which no organisms are able to live because it’s a lethal level of oxygen in the water.
To learn more about low oxygen conditions in LIS watch the video below about dead zones.
You can also click here to check out more information about hypoxia from the Long Island Sound Study.
This is a visual representation of “eutrophication,” or oxygen-depletion caused by nutrient runoff from the watershed.
Graphic Source: https://longislandsoundstudy.net/ecosystem-target-indicators/lis-hypoxia/
Part B: Sampling Method
.jpg)
Source: Project Oceanology
The YSI Meter is a very important tool that can measure water temperature, salinity and dissolved oxygen all at the same time. The photo above (left) shows the sensor portion of the YSI meter that is deployed into the water (the weight attached helps the sensor to sink down straight). The photo above (right) shows the handheld computer portion of the YSI meter. Each number on the screen represents the temperature, salinity or dissolved oxygen reading in real time. This sensor is deployed just under the surface of the water for the first reading and then it’s lowered every meter after that until it reaches the bottom of the sample location.
To learn more about the YSI meter watch this brief video:
Part C: Prediction and Reasoning
Study the background information provided on Temperature (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: Temperature samples were collected at the surface (0 meters) and recorded every meter until
reaching the bottom at 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: How do you expect the dissolved oxygen to change from the surface of the water to the bottom? Will it increase, decrease, or stay the same?
2. Explain your reasoning. WHY do you expect the oxygen levels to change (or not) with depth?
Part D: Analyze the Data
Look at the data table below. On your piece of paper, illustrate the data by making a special 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 (y) and column 2 on the horizontal (x). The origin is still (0,0). Connect your points to form a 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 dissolved oxygen measured at each depth from surface to bottom in early May.
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 how dissolved oxygen is distributed in the water column.
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 for Water Chemistry
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 dissolved oxygen level of their environment. Check out this interactive model about how the dissolved oxygen of water could impact blue crabs. Write a paragraph that (1) identifies at least one factor that might cause oxygen in an estuary to change, (2) explains how dissolved oxygen levels affect blue crabs, and (3) explains how this affects (or is affected by) people.
Congratulations! Your final analysis should include the following components:
-
A statement of the research question that you chose/were assigned
-
Your prediction and your reasoning
-
Your labeled graph
-
Your answers to the results questions
Share your results with your teacher!