A Case Study of Local Trends in the Carbon Cycle

Bloom on Lake Erie, April 2000

Image courtesy SeaWiFS and Visible Earth

To investigate the relationship between atmospheric carbon dioxide levels and chlorophyll-a measurements in a local watershed
Grade Level: 10 – 12
Estimated Time for Completing Activity: 50 minutes
Learning Outcomes:
  • Students will compare two data sets to find correlations.
  • Students will be able to explain the importance of carbon dioxide and photosynthetic plants in the carbon cycle.
  • Students will be able to relate global climate change to local effects.
  • Overview of the Carbon Cycle
  • Familiarity with using Excel
  • Computer with Internet access
  • Microsoft Excel or other spreadsheet software
AP Environmental Science Topics
  • Agricultural use of water
  • Atmosphere-ocean interactions
  • Carbon cycle
  • Global climate change
  • Greenhouse gases
  • Green revolution
  • Harmful algal blooms
  • Human impact on natural systems
  • Photosynthesis
  • Water pollution
Lesson Links:

Global Carbon Cycle

Dr. Charles Keeling measured background atmospheric carbon dioxide (CO2) from Mauna Loa Observatory in Hawaii beginning in 1958. Since his death in 2005 the measurements have been continued under the supervision of his son, Ralph Keeling. The Keeling Curve (see Lesson Link) shows the increasing trend of carbon dioxide in the atmosphere, as well as the seasonal cycle of CO2 in the Northern Hemisphere. The seasonal oscillation seen in the graph is due to the increase and decrease of photosynthetic plants that use CO2. Since carbon dioxide is a powerful greenhouse gas, the overall increasing trend is very alarming and has sparked much debate about global climate change.

When ocean phytoplankton bloom, they also use carbon dioxide from the atmosphere for photosynthesis, just as land plants do. The gas is converted to carbohydrates by the phytoplankton. When they die, the carbohydrates are carried deeper into the ocean. The material is decomposed or eaten by other organisms, especially bacteria. The end product is carbon dioxide, ammonia, nitrites, and nitrates in the water below the mixed layer. The end products are mixed back to the surface in winter, and the carbon dioxide is released to the atmosphere. The nutrients feed the next year’s bloom. A very small amount of carbon, less than one percent, reaches the sea floor and is removed from the atmosphere for centuries. This is the biological pump. The sinking of deep water in the Arctic and Antarctic carries carbon dioxide to the deep ocean. This is the physical pump. Scientists are still trying to determine how much carbon dioxide might be removed from such a process that might counter the increases noted by Keeling. Phytoplankton blooms can be measured from space by chlorophyll content, or a discoloration of surface water. The picture above shows the greenness of Lake Erie from satellite in April 2000.

Phytoplankton blooms can naturally increase with more available carbon dioxide, but may also increase when other nutrients drain into bodies of water from anthropogenic activities. These activities may include agricultural runoff, industrial runoff and other human waste influences. Some of these blooms are harmful to local watersheds. While the blooms will temporarily raise dissolved oxygen levels in the water during the daytime when sunlight is available, at nighttime when the phytoplankton respire, there is a noticeable decrease in dissolved oxygen. These swings result in large kills of local aquatic life. Phytoplankton have the ability to be both helpful and harmful to the surrounding environment. While some of these blooms are harmful there are many that are not. As phytoplankton blooms are at the base of the food chain, they serve as an essential food source for a large number of marine organisms. However, Phytoplankton blooms also have the ability to be harmful to marine life.

Local Example

The Maumee River watershed empties into Lake Erie from Ohio, at its furthest southwest point. Many other smaller rivers empty into the Maumee River at points upstream. The St. Marys Watershed, the watershed where St. Marys city school district is located, is one of these rivers. In this lesson, students can explore both global and local carbon cycle issues by examining chlorophyll data from satellite.

For other locations, please see Teachers Notes below.


Pre-Lesson Inquiry Activity. In your notebook hypothesize as to how hurricanes affect phytoplankton blooms. As a follow up to your hypothesis for confirmation, explore how hurricanes may cause or disrupt phytoplankton blooms by using the Lesson Link above ‘Hurricanes and Phytoplankton Blooms’

Click on the Lesson Link above called Excel Workbook. In this workbook, the first sheet contains numerical CO2 data values associated with the Keeling Curve for years 1997-2004. The second sheet contains corresponding time series data from the Live Access Server for chlorophyll-a concentration at a location near the Maunee Bay on Lake Erie.

Review the data on Sheets 1 and 2, then view Sheet 3 which shows an Excel plot with annual average CO2 on the x-axis and annual average Chlorophyll on the y-axis. This x-y type line graph can be used to view the overall relationship between the two data sets. Answer the questions below.


1. What mathematical relationship, if any, is there between chlorophyll-a and carbon dioxide in the Maumee River Watershed (linear, logarithmic, exponential)?
2. What explanation is there for this relationship?
3. What other factors could have an impact on the results?
4. Where did this data come from? Who collects it? How is it collected?
5. How can this affect other characteristics of the local watershed?
6. Describe how this can affect other parts of the world.
7. Is this enough data to make an accurate analysis? Why or why not?


1. Using Excel Workbook Sheet 4, plot Leaf Area Index data, a measure of land vegetation, versus the carbon dioxide data. What trends do you see?

2. Suggestions for teachers in other locations:

– Pick your location and examine either the Leaf Area Index (LAI – land) or Chlorophyll (water) parameter compared to CO2. To do this, click on the lesson link for the Live Access Server (LAS). Follow links to the parameters. LAI is under Land Surface, and Chlorophyll-a is under Oceans. Choose Time Series under Select View. Under Select Output, you may either generate a line graph or a text file for importing to Excel. Then enter your location and desired time range. Click Next to generate the output. Be sure pop-up blockers are turned off.

– Do you come to similar conclusions as this lesson did for Lake Erie? Perhaps this could even be a class project, with groups of students being assigned, or choosing, a variety of different areas where they could examine these relationships. Then they could get back together and compare a number of locations to look for consistent or different trends.

Lesson plan contributed by Tami Golliday, St. Marys, Ohio

Click here for Teachers Notes

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