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. Like all plants, phytoplankton
use the pigment chlorophyll to convert sunlight into food. Phytoplankton
differ from land-based plants in that they do not have roots, stems, or
leaves. Where phytoplankton production is small, such as the Sargasso Sea,
the water is deep blue. By contrast, coastal waters rich in phytoplankton
are green.
Scientists study phytoplankton
for many reasons. Apart from bacterial life, phytoplankton are the base
of the food chain in the sea. Healthy, productive phytoplankton turn sunlight
into organic materials for consumption by higher life forms. Phytoplankton
that are starved for nutrients or poisoned by chemicals cannot perform
in the same manner. By studying phytoplankton from space, scientists can
see where ocean currents provide nutrients for plant growth, where pollutants
prevent plant growth, and where subtle changes in the climate (warmer or
colder, more saline or less saline) affect phytoplankton growth. Since
phytoplankton depend upon specific conditions for growth, they often provide
the first evidence of a change in their environment.
In addition to acting as the base of the food chain, phytoplankton play an essential role in the global carbon-cycle. During photosynthesis phytoplankton remove carbon dioxide from sea water, release oxygen as a by-product, and store the carbon in the form of organic materials. This process reduces the concentration of carbon dioxide in sea water. As a result, the oceans absorb additional carbon dioxide from the atmosphere. In a sense, the phytoplankton create a temporary "carbon sink" into which the world can pour its excess atmospheric carbon. Unfortunately, the world produces vast amount of carbon dioxide whether or not the phytoplankton and land plants are healthy enough to absorb it. When they die, phytoplankton sink to the ocean floor, carrying with them much of the carbon stored in their cells. The carbon in the phytoplankton is gradually covered by other materials sinking to the ocean floor. In this way, the oceans act as a kind of global carbon dump. No one yet knows how much carbon the oceans and land can absorb. Nor do we know how the Earth's environment will adjust to increasing amounts of carbon dioxide in the atmosphere. Studying the distribution and changes in global phytoplankton using ocean color and other tools will help scientists find answers to these questions.
Considered individually, these tiny plants may seem insignificant. But
collectively they are very important. Small in size but unimaginably large
in number, the phytoplankton are important to more than the zooplankton
(small aquatic animals) and fish that eat them. They are important to all
life on earth. [See After
the Warming by James
Burke, PBS video]
Nimbus-7 and the Coastal Zone Color Scanner
In this activity students 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 .
Using that data you will estimate the phytoplankton biomass (in g/m^3)
from its chlorophyll content and the biomass of the zooplankton and fish
populations higher up the food chain. You might even discover where whales
go to eat.
Procedures
Below is the picture C82120I.BRW, a false-color image of the world's oceans. A number of clues concerning the contents of the file are encoded in its name, which in general is of the form CyydddI.BRW.
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, use the following formula:
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.
A milligram (mg) is defined to be one-thousandth of a gram. For comparison, a postage stamp has a mass of approximately 20 mg.
In order to compute the pigment concentration for a particular DN, you substitute the DN in the formula and compute the result. For this, a calculator is necessary. For the sake of this example, compute the pigment concentration associated with a DN of 100.
The range in DN values is 0 - 255. Compute the corresponding range in pigment concentrations in mg/m3. Go through the image and find various DNs and convert them to pigment concentrations.
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) OsloOver 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 Phyto Anim folder contains 12 months of pigment concentration images. Images may be examined individually or grouped and then animated, averaged, or printed side by side in a montage.
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.
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.
Using these facts, one can estimate the overall biomass of the food chain as follows:
Suggested Follow-Up Activities