Part 6—Examine the Bloom in Satellite Images

What does a phytoplankton bloom look like to a NASA satellite orbiting 640 km (400 miles) above Earth's surface? Both NASA's Terra and Aqua satellites carry a MODIS (Moderate Resolution Imaging Spectroradiometer) instrument onboard. These satellites orbit the globe daily, providing images of Earth's land and ocean surfaces. Two MODIS data products that are useful for oceanography are chlorophyll concentration and sea surface temperature images.

The MODIS image below shows ocean chlorophyll on March 22, 2003. The Gulf of Maine is outlined in a red box. White patchy areas over the ocean in the lower right are clouds. The image was taken in early Spring before the phytoplankton bloom.

A month later, on April 23rd, 2003, the Gulf of Maine looked very different in the ocean chlorophyll image below. Note the yellow and red areas indicating higher chlorophyll levels.

The Coastal Ocean Observing Center Data (COOA) at the University of New Hampshire (UNH) provides an array of coastal ocean observing data. The MODIS images used in the previous step are available by clicking the MODIS images link. These satellite data holdings include 8-day composite images of Chlorophyll-A and Sea Surface Temperature (SST) for the years 2000 through the present.

1. Go to the MODIS Ocean Products page and select the ocean parameter labeled 2009 Aqua chlorophyll from the drop-down menu and click Go.
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2. The resulting page shows thumbnails of composite images produced every 8 days. These are composite images; that is, they are averages of all cloud-free measurements acquired during the 8-day period. Since clouds often prevent MODIS from imaging the ocean surface, compositing is a way of eliminating the clouds. Still, there are places where clouds persisted for 8 days. These areas appear white over the ocean on the thumbnail images.
3. Scroll through and examine images around the date you predicted the bloom would occur. Clicking any thumbnail image will bring up the full-size image.
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   This image is from April 23, 2009. 
   

   

   
   
   
4. Use the images that show chlorophyll levels around your predicted bloom date to help you answer the following questions.
   • Can you identify the spring bloom in the chlorophyll images?
   • From the images, when would you say the bloom occurred? (You may be able to assign a beginning, peak, and end date for the bloom).
   • Where in the Gulf of Maine was the bloom most evident? Did the data from the buoy(s) near that area indicate that the bloom might be more pronounced there than in other areas?
   • How long did the bloom appear to last?
   • Based on your understanding of phytoplankton blooms, describe what you would expect to find in the water column in the place where the bloom occurred.

Compare the MODIS satellite images from 2004 to more recent years.

 
Some questions to consider as you compare the images: 
 

• Do the blooms occur at the same time every year?
• How are the blooms alike?
• What differences can you distinguish among the blooms?

This chapter provided an overview of the factors that trigger a phytoplankton bloom as well as an introduction to the data, techniques, and analysis tools used to study these blooms.

The Gulf of Maine is a complex marine ecosystem. Many factors interplay almost simultaneously to create a phytoplankton bloom and our understanding of these factors is incomplete. While we can't know precisely When Dinner is Served in the ocean, using the same data and tools scientists use to study these blooms we can make a reasonable prediction as to the timing, and in the process, gain a better appreciation of this unique oceanographic phenomenon.

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