Grade Level: Secondary

National Science Education Standards

Unifying Concepts & Processes: Constancy, change, and measurements
Science As Inquiry: Understandings about science as inquiry:
Life Science: Matter, energy, and organization in living systems
Earth and Space Science: Geochemical cycles
Science and Technology: Understandings about science and technology:


After this activity, students should be able to:

1. Demonstrate skill in extracting pigments from plants using chromatography.
2. Explain why measuring plant pigments is important and what plant pigments can tell us about the ocean.
3. Use a time series plot to describe how phytoplankton concentrations change with the seasons in the open ocean.
4. Draw a depth profile to describe how phytoplankton concentrations change vertically in the water column.
5. Describe the physical conditions that control changes in phytoplankton abundance.
6. Predict the timing of future phytoplankton blooms.

Oceanographers measure plant pigments to determine the amount of phytoplankton present in their samples. These pigments are also used to identify which taxonomic groups are present at a given location. The abundance and types of plankton present in an area influences the nature of the local food chain, and helps regulate the exchange of carbon dioxide gas between the ocean and atmosphere, which plays a role in the greenhouse effect.

The abundance and distribution of phytoplankton are controlled by the seasonal and spatial availability of the factors they need to survive and reproduce: sunlight and nutrients. Nutrient availability peaks during the spring after winter storms have mixed cold, nutrient-rich waters near the surface. Sunlight is also increasing at this time. The simultaneous presence of ample sunlight and nutrients leads to a rapid growth in phytoplankton populations. This is called the spring bloom.

Plants contain a variety of pigments. Chlorophyll a, found in all land plants, is the major photosynthetic pigment. It captures the energy found in red and blue wavelengths of light and uses it to produce glucose during photosynthesis. Additional pigments such as chlorophyll b and carotenoids, known as "accessory pigments", capture different wavelengths of light and pass the energy to the chlorophyll a for use during photosynthesis. These accessory pigments often have additional functions such as acting as a "sunscreen" to protect plant cells from sunlight damage.

Oceanographers commonly measure the concentration of chlorophyll a to determine the amount of phytoplankton in their samples. In addition, oceanographers study the various accessory pigments found in the phytoplankton. Many of these pigments are found exclusively in one class of phytoplankton and can easily be used to determine if specific groups are present. For example, dinoflagellates, a group of phytoplankton most famous for causing the red tide phenomenon, contain a reddish orange accessory pigment called "peridinin". Diatoms, one of the most numerous groups of phytoplankton, contain a golden brown pigment known as "fucoxanthin". 

Chromatography is the technique used to separate plant pigments. The term comes from Greek for "chroma" (color) and "graphein" (writing). This process uses solvents to extract the pigments from the plant cells, and then separates them according to physical properties of the pigments. In the following hands-on activity, students use simple chromatography to extract the pigments found in common land plants.

Before they perform the following activities, students should have a knowledge of photosynthesis and chlorophyll a, and possibly of accessory pigments (this entire exercise would fit in well with process of learning about photosynthesis).

The teacher should introduce this exercise with an explanation modeled after the Concepts section above. Once the students have completed the hands-on activity, they can begin to explore the BATS data set, specifically the chlorophyll a concentrations.

1. Ask your students to draw a time series plot to show how phytoplankton abundance (as measured by chlorophyll a concentrations) changes at a single depth (such as 40 meters) for each month of the year at an open-ocean location such as the BATS site. Stress that the ocean covers 70% of the Earth's surface, so that seasonal changes in phytoplankton abundance can affect the exchange of carbon dioxide gas between ocean and atmosphere on a global scale. Students can draw their time series plots using the OceanExplorer TimeSeries tool or graph paper.

2. Repeat step 1,  but employ depth profiles to focus on the vertical distribution of phytoplankton in the water column at a single moment in time rather than on their distribution through time. Students can draw their depth profiles using the OceanExplorer Profiler tool or graph paper.

3. Compare student time-series plots and depth profiles against chlorophyll a data from BATS. Discuss the similarities and differences. You can use the BATS time-series plots (Fig. 1) and depth profiles (Fig. 2) shown below, present the students with a plot that you've constructed using OceanExplorer, or let the students use OceanExplorer to create their own plots.

4. Now discuss why phytoplankton abundance peaks during the spring. After discussing the importance of nutrients and sunlight for plant growth, ask your students to investigate the BATS data set to determine when nutrients like phosphate and nitrate are most abundant in the sunlit surface waters where phytoplankton live. Have them use the OceanExplorer to graphically display seasonal changes in nutrient availability.







Figure 1: A time series plot of chlorophyll a concentrations in the upper 40 meters of the water column at BATS between 1990 and 1997. At what time of year do phytoplankton concentrations peak? Why? Click image for larger version.

Figure 2: A depth profile of chlorophyll a concentrations in the upper 500 meters of the water column at BATS during April 1994. At what depth are  phytoplankton concentrations greatest? Why? Click image for larger version.

Hands-on Activity
In this activity, students separate chlorophyll and carotenoid pigments extracted from spinach to demonstrate that plants contain various pigments and aren't "just green." The students then graph the chlorophyll a concentration at the BATS site and look for patterns in seasonal and depth distribution.

1. Cut up plant material, using scissors. The pieces must be fairly small, about 1 mm x 1 cm. Place approximately 5 grams of chopped plant material in each test tube, and cover with 5 ml of solvent. Carefully mix and smash the plant material in the solvent using a glass rod, then make sure all of the chopped pieces are submerged. Tightly stopper the test tube and soak for 30 minutes in a dark place.

2. After extraction, use a capillary tube or Pasteur pipette tip to drop the extract about one inch from the end of a paper towel strip. The spot will spread out on contact with the paper; let each drop dry before adding another. The more drops you add, the more visible your results will be. Once you have a distinctly green area on your towel, let it dry completely.

3. Put small amount of solvent (about 1/4 inch) into the bottom of the jar, then put the glass rod over the mouth of the container. Suspend the paper towel (pigment end down) into the jar so that the end of the towel just barely touches the solvent, then attach with a clothespin or paper clip. Make sure that the paper towel strip does not touch the sides or bottom of the tank. Cover the tank and watch the solvent travel up the towel. When it has traveled approximately two inches, take the strip out of the jar and let it dry.

4. You should see distinct green and yellow bands. The green band is a mixture of the chlorophylls (usually chl a and chl b for land plants) while the yellow band is the various carotenoids.

Fresh spinach (or leaves from any other leafy, non waxy plant: grass, dandelion, tomato leaves) approximately 5 grams per extraction tube
Extraction solvent: Acetone (or ethanol), 5 ml per extraction tube
Test tubes, with stoppers
Glass rod
White paper towels, the thicker the better (or white coffee filters)
Solvent tank (mayonnaise jar or deep beaker)
cardboard cover for tank
capillary tube or glass (Pasteur) pipette
clothespins or paper clips
OceanExplorer Excel workbook

Ask your students to interview a gardener or landscaper about their use of nutrients (fertilizer) to encourage plant growth.

You can evaluate your students´ preconceptions about phytoplankton and nutrient concentrations by examining  the graphs that they draw using the OceanExplorer TimeSeries and Profiler tools. You can then compare graphs that they draw following the activities in this lesson plan to gauge how their understanding of these topics has changed.


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