Channel Islands

Channel Islands From Shore to Sea

From January 27-February 7, 2003, the Jason Project conducted its From Shore to Sea expedition to California’s Channel Islands. In support of Jason XIV, NASA provided satellite remote sensing data and the Image Composite Explorer (ICE) tool to help educators and students explore the relationship between sea surface temperature and life in the ocean.

Click to launch ICE; the program will open in a new window displaying sample data collected January 24, 2002. In the right margin of this page the same scene is available for consecutive days beginning January 25, 2003. Because that area is often cloudy and the satellite sensor cannot measure the sea’s surface through clouds, you will notice some black patches indicating “no data.” A true-color browse image is also provided for each day to help orient you. (Note: the browse scenes have not been corrected for the distortion at the edges of the image that is caused by the sensor’s wide viewing area and the curvature of the Earth.)

The MODIS sensors aboard NASA’s Terra and Aqua satellites collect detailed measurements of the ocean’s surface environment every day all over the world. For example, MODIS measures the surface temperature of the ocean accurately to within a quarter of a degree Celsius (0.25°C). Scientists can then produce images showing even small variations in the ocean’s surface temperature. Different temperatures are shown in different colors, and because the images don’t look “natural,” they are called false-color images.

There is often a direct relationship between sea surface temperature and biological activity in the ocean. False-color images from satellite sensors can help us to see and better understand this relationship. Warm surface waters block deeper, colder currents from rising to the surface. But where the surface waters are colder, the deep, nutrient-rich currents can “upwell” bringing nourishment needed to support life. Where nutrients (such as iron) are plentiful in the ocean, so too are blooms of the microscopic plants and animals that form the foundation of the marine food chain. Given ample nutrients, the tiny plants, known as phytoplankton, can quickly “bloom” into very dense populations producing colorful patterns on the ocean’s surface. Satellites help us observe this direct relationship between sea surface temperature and biological activity.

Phytoplankton Micrographs
The astonishing diversity of phytoplankton is visible only under a microscope. One trait all phytoplankton share, however, is chlorophyll—the green pigment that converts energy from the sun into food. (Images copyright Smithsonian Environmental Research Center)

By measuring the color variations of the ocean, scientists can determine where concentrations of phytoplankton are floating at the sea’s surface. (Read What Are Phytoplankton? for details.) Like land-based plants, phytoplankton contain the pigment chlorophyll — used for photosynthesis — that gives them their greenish color. Chlorophyll absorbs red and blue wavelengths of light and reflects green light. From outer space, MODIS can distinguish even slight variations in ocean color that our eyes cannot detect. To MODIS, ocean water with high concentrations of chlorophyll will appear as blue-green or green, depending upon the type and density of the phytoplankton population there. This allows scientists to produce false-color maps showing where there are high and low concentrations of chlorophyll. Knowing how much cholophyll is present gives scientists an idea of how many organisms are present in a phytoplankton bloom.

Chlorophyll Map Fluorescence Map Sea Surface Temperature Map
In this lesson, you will examine images mapping chlorophyll concentrations (left), fluorescence values (middle), and sea surface temperatures (right) for the surface waters surrounding the Channel Islands. These examples are in color, but in the lesson applet all images appear grayscale. ICE allows you to choose a color palette; or you can “probe” each image to find the unit values for each pixel in the scene.

Phytoplankton have relatively short life spans—generally a day or so—in which they are busy photosynthesizing and reproducing. But when they are under stress, or as older generations of a bloom die off, phytoplankton photosynthesize less and re-radiate the sunlight they are absorbing as both heat and fluorescent light. MODIS is sensitive enough to detect this relatively small amount of fluorescent light escaping the sea’s surface. By comparing MODIS’ chlorophyll and fluorescence measurements, scientists hope to estimate how much photosynthetic activity occurs within a given phytoplankton bloom, which in turn will help them to better estimate how much carbon is being drawn down from the atmosphere and fixed in phytoplankton bodies.

Together, these three measurements provide insights into the causes and effects of climatic and environmental change on physical and biological conditions at the ocean’s surface. In this lesson, the measurements are shown as false-color maps so that you can examine the relationships between them.

Quick-start tips:

  • To change which of the three images is displayed in the large window, click the “Step” button.
  • To find out the value of SST, chlorophyll, or fluorescence at a given point in the image, click the “Probe” button and move your mouse to the location.
  • To display an image in color, select a color table choice from the drop-down menu below the large image.
  • To get an analysis of how values change from one part of an image to another, click “Plot transect” and draw a line across an interesting part of the image. You’ll get a graph that has the values of each ocean characteristic (also called a ‘parameter’) along the line. The starting point of the line you drew will appear at the left end of the graph; the end point will be at right.
  • To learn more about what you can do with ICE, read the User’s Guide.

As you interact with the MODIS data over the Channel Islands, think about the following questions:

Do you observe any relationship between areas of warmer and colder sea surface temperature and areas where there is higher and lower chlorophyll concentration? If so, why do you think there is a relationship? If not, why not?

Do you see a relationship between areas of high chlorophyll concentration and where there are high fluorescence values? If so, why do you think there is a relationship? If not, why not?