Overview

Approximately 70% of the Earth is covered by oceans varying in color from deep blue to green. This variation is due to varying populations of tiny aquatic plants called phytoplankton, or algae. Where phytoplankton production is small, such as the Sargasso Sea, the water is deep blue. By contrast, coastal waters rich in phytoplankton are green.
 

Phytoplankton are the base of the food chain in the sea. In this activity you will use data collected by the Coastal Zone Color Scanner (CZCS) on the Nimbus-7 satellite during the period 1978-1986. The CZCS data shows the average phytoplankton pigment concentration (in mg/m^3) for the oceans of the world each month during that period. Additional background information is available here.

Learner Outcomes

By completing this lesson, the learner will:

• use remote sensing data and scientific visualization techniques using NIH Image (download pc or mac version as needed).
• display and interpret images of phytoplankton pigment (chlorophyll) concentrations in the global oceans.
• use NASA scientific data sets and classification schemes
• convert image data to pigment concentration and biomass data.
• compute sea surface pigmentation concentration from satellite image data using a scientific calculator and an exponential equation.
• use pigmentation data and proportional reasoning to estimate the biomass of the food chain at the phytoplankton, zooplankton I, zooplankton II, and fish levels.
• investigate periodic changes in the distribution of phytoplankton pigmentation.
• reflect on the environmental significance of man's activities in and on the sea as they relate to phytoplankton productivity.

Let's make an analysis of the false color image of the world's oceans shown below. A number of clues concerning the contents of this image are encoded in its name, which in general is of the form CyydddI.BRW. This image comes from the SeaWiFs Mission.
 

C 82 120 I . BRW

C indicates that the file contains pigment concentration data  yy indicates the year, in this case 1982  ddd indicates the day of the year (1 - 365) over which the previous month's data was averaged, in this case the 120th day of the year.  I indicates that the data set has been interpolated to fill in missing data points  BRW indicates that this is a browse (small) file. All browse files in this data set are 360 pixels wide and 180 pixels high. Each pixel in this array represents an equal-angle (1° longitude by 1° latitude) section of the planet. These sections are not equal area, however.

- day 59 of 1982 Which month is this data for?
- day 273 of 1982 Which month is this data for?
- day 151 of 1982 Which month is this data for?

Concept Introduction

QUESTION: What is the biomass of the food chain at the phytoplankton, zooplankton I, zooplankton II, and fish levels. How does this change throughout the year?

1. Save the image above as C82120I.BRW, convert to TIF format, and print out these instructions, and start NIH Image.

The data in C82120I.BRW represents the average phytoplankton pigment concentration during the month of April 1982. NIH Image displays that data in a visual format in which the phytoplankton pigment concentrations are color-coded. The value of each pixel (move the mouse around the image), also called the data number or DN, is found in the Info window in the lower left hand corner of the display. These DN range from 0 to 255, the same as the range in colors or gray-scales on your computer. Storing the data in this manner makes the program run faster. Of course, it means that you can't read the pigment concentrations directly from the image. To convert the DN to pigment concentrations, you will have to use the following formula:

 

Average chlorophyll per pixel in mg/m^3 = 10^(.012*DN - 1.4)

Let's explore what this formula means and how it is used. In the corner of your classroom, use 3 meter sticks to mark the edges of an imaginary box 1 meter (m) on a side. The volume of that box is 1 m3. In the corner of that box (where the 3 meter sticks come together) place a smaller cube 10 cm on a side.

7. How many milligrams per cubic meter are there in a high concentration area? How many postage stamps does this equal?

8. Zoom in on each of the following locations and find the pigment concentration of the sea pixel closest to each city. The zoom tool is the magnifying glass found at the upper left hand corner of the Tools window. Click on it and then click on the image near each city. Click several times to keep enlarging the image until you can easily pick out a sea pixel close to the city. The land masses are represented by the DN=255, the ice is represented by the DN=254 and the coastlines are represented by the DN=253. When you choose a pixel close to each city, avoid these DNs.

(X,Y)           City                            DN              Pigment Concentration (104,90)          Singapore 
(302,54)        Buenos Aires 
(116,57)        Perth 
(11,149)        Oslo

Over the course of a year, plants on land cycle through stages of germination, growth, reproduction, and death or dormancy determined in large part by seasonal changes in climate and sunlight. What about phytoplankton? Is it's life cycle also timed by the changing seasons or are there other, more important factors that determine its productivity? One way to find out would be to look at a sequence of pigment concentration images, each representing a different month of the year. The box below contains 12 months of pigment concentration images for 1982 in TIFF format. Images may be saved and examined individually or grouped and then animated, averaged, or printed side by side in a montage using image processing software. Save each of these images to your hard-drive.

From the File pull-down menu, Open all of the pigment concentration files for 1982. As each file is opened, it stacks on top of any previously opened files. When all 12 images for 1982 have been opened, select Windows to Stack from the Stack pull-down menu. Then select Animate from the Stack pull-down menu. All 12 images will be displayed in the order that you opened them ... probably too fast for comfort ... over and over. To control the speed of the animation, type a number from 1 to 9, 1 producing the slowest animation. To step forward or backward through the stack, use the right arrow or back arrow keys. Click on the image with the mouse to end the animation.

11. Where did that production occur, in equatorial or polar waters?

12. What DN is associated with the color red? Compute the pigment concentration for that DN.

In the sea, the base of the food chain is phytoplankton. Little animals called zooplankton eat the phytoplankton. Fish and other animals eat the zooplankton. Mammals like dolphins, seals, and killer whales eat the fish. All of these animals derive their nourishment either directly or indirectly from the phytoplankton. If the phytoplankton decline in number, so must all the creatures that depend on them. Conversely, where the phytoplankton bloom, zooplankton, fish, and sea mammals may eat their fill from the bounty of the sea. In other words, the productivity of the phytoplankton sets a kind of upper limit on the productivity of the entire food chain. By taking note of a few facts, we can estimate the mass of the plants and animals at each level of the food chain.
• About 2.5 % of the organic material in phytoplankton is chlorophyll. The rest of the organic material is related to other cellular structures and functions.
• About 10% of the overall mass of phytoplankton is organic material. The rest is water. The overall mass of the phytoplankton is the mass of the base of the food chain.
• The second level in the food chain, zooplankton 1 animals, has an overall mass of approximately 10% of the phytoplankton population.
• The third level in the food chain, zooplankton 2 animals, has an overall mass of approximately 10% of the zooplankton I population.
• The fourth level of the food chain, fish, has an overall mass of approximately 10% of the zooplankton 2 population.
Using these facts, you can estimate the overall biomass of the food chain as follows:
• For any given DN, the chlorophyll concentration is given by the expression mg/m^3 = 10^(0.12*DN - 1.4 )
• The total organic material per cubic meter (dry biomass) may be computed by dividing the chlorophyll concentration by .025.
• The overall mass (biomass) of the phytoplankton is obtained by dividing the dry biomass by .10. For example, for a DN of 20, the pigment concentration is 10^(.012*20 - 1.4 ) = .069 mg/m^3 of chlorophyll. Dividing this amount by .025 yields 1.38 mg/m3 of dry biomass. Dividing this dry biomass by .10 yields 13.8 mg/m^3 of phytoplankton biomass. For a DN = 20, the base of the food chain is less than one postage stamp of food per cubic meter!

14. How many postage stamps of phytoplankton per cubic meter is this at the base of the food chain? · The biomass of the zooplankton and the fish can be estimated based on the biomass of the phytoplankton. The biomass of the zooplankton 1 is about 10% of the biomass of the phytoplankton. Similarly, the biomass of the zooplankton 2 is 10% of the biomass of the zooplankton 1. The biomass of the fish is 10% of the biomass of the zooplankton 2.

15. Calculate the biomass of the zooplankton and the fish using a DN of 200. · Now, find an area on the image that appears to have a high pigment concentration.

16. Calculate the biomass of the fish in this area.

17. Do the same for an area that appears to have a low pigment concentration. How many mg/m^3 of fish are in this area?

18. Write a few sentences comparing and contrasting the fish biomass in both areas.

Application

By now you should have a better understanding of the importance of this tiny thing called phytoplankton. It is the bottom of the food chain, thus supporting life in the oceans. It is also a powerful force in the global climate. Small can be mighty!

With all 12 images in a stack, it is a simple matter to compute the average phytoplankton productivity for the entire year. From the Stack pull-down window, select Average.

19. Over the course of the entire year, was the production of phytoplankton greatest near the coasts of the continents or in mid-ocean?

20. Is phytoplankton production constant during the year or does it fluctuate?

21. If you were a whale hungry for a giant serving of a tasty little zooplankton called krill, where would you go to eat and when would you want to be there?

22. Many industrial processes produce toxic chemicals as a by-product. If those chemicals are dumped into rivers, they eventually reach the sea. Discuss the possible consequences to phytoplankton production and the oceanic food chain in general.

23. Suppose that dumping of some materials in the ocean is necessary. Where would you recommend dumping? Where would you avoid? Why?

SeaWiFs Image

Topex/Posiden Image