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On Again, Off Again —
The Dead Zone

Activity #2
Cross Section Map of the Hypoxic Zone by Transect
Student groups

Introduction/ Purpose:
A cross section is a diagram that shows features of a vertical section or a slice of something such as the earth or the water column. A cross section can be constructed by interpolating between data points from each station along a transect. A cross section can show changes and similarities in the data with distance and can be used to determine extent, area or volume of a feature within the cross section. For this activity, students will draw a cross section of the dead zone from the near-shore shallow stations into the deeper stations in the Gulf of Mexico.

Objective:
Students will (in groups):

• Use their profile answers from Activity #1 to compare profile graphs within a transect.
• Use their station profile graphs to draw a cross section of the dead zone for each transect by interpolating between data points.
• Calculate the distance between stations and the total length of the transect.
• Estimate the area of the cross section.
• Compare and discuss all cross sections from each group.

Standards:

• Grades 6-8: SI-M-A4, A-4-M, M-1-M, M-6-M, G-4-M, G-7-M, D-1-M
• Grades 9-12: SI-H-A3, M-2-H, M-3-H, G-2-H, D-7-H
• Skills: Graphing, measuring, estimating, interpolating data, observing, measuring and formulating models

Materials:

* For help Changing Latitude & Longitude to Distance Units, click here.

Procedure:

1. Divide students into groups by transect.
2. Compare profiles within the transect. Discuss differences and similarities among DO, salinity, temperature and relative shape and size of hypoxic area and answer questions #1–9 using the data table and profiles.
3. Determine horizontal scale, length of x axis and size of paper required.
4. Set up graph with x and y axis agreed upon.
5. Determine location of and mark each station location for each transect.
6. Place and tape each profile at the appropriate station, keeping the sea surface constant.
7. Draw the sea floor by interpolating between stations and shade in the area below the sea floor.
8. Mark the upper and lower ends of the hypoxic zone of each station profile in red.
9. Draw the upper boundary and the lower boundary of the hypoxic zone in red by interpolating between stations. The lower boundary may be the same as the gulf bottom in several stations.
10. Complete questions #10-14 below.
11. Estimate the area of the hypoxic zone in the cross section.
12. Report results of your cross section 2-D model to the class
13. Compare all cross sections from each group as a class.

Questions:
As a group:

1. What is the maximum, minimum and average water depth along the cross section?
2. What is the maximum and minimum depth where hypoxic conditions begin (top of hypoxic zone) and end (bottom of hypoxic zone)?
3. Compare the depths where hypoxic conditions occur among the transect profiles.
4. Do hypoxic conditions always occur at similar depths or same location within the water column? What appears to affect the depths of DO along the transect?
5. Where in the water column (top, middle, bottom) do you generally find hypoxic conditions?
6. What is the maximum and minimum percent of a profile that is hypoxic in the transect?
7. Do all stations contain hypoxic conditions?
8. List stations that are hypoxic, and anoxic (use for Activities #4 and 5) and those that are not.
9. In relation to the shoreline, where are the hypoxic stations located? Where are those that are not hypoxic located?
10. Calculate/estimate the distance between each station and the total length of your transect (see “Changing latitude and longitude to distance units” below).
11. Calculate the distance between stations at the sea bottom. Hint: use geometry (rectangles and right triangles and see “Changing latitude and longitude to distance units,” page 8. Is the length of the transect much greater at the bottom than at the top? What does this say about the slope of the continental shelf off the coast of Louisiana?
12. Estimate the area covered by the entire cross section and describe how you determined this.
13. Estimate the area of hypoxic conditions in your cross section.
14. What percent of the cross section is affected by hypoxia?

Go to Activity #3