Part 6: Examine the Bloom in Satellite Images
In any step, click the Show me link to reveal extra information. If you prefer a printout of the full set of instructions for this part, choose Print from the File menu.
Step 1-What Does a Bloom Look Like From Space?
What does a phytoplankton bloom look like to a NASA satellite orbiting 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.
Step 2-Obtain MODIS Images from WebCOAST
The Coastal Ocean Observing Center (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 MODIS images. These satellite data holdings include 8 day composite images of chlorophyll-a and sea surface temperature for the years 2000 through the present.
Go to the MODIS Images page and select the ocean parameter from the drop down menu labeled 'Aqua 2004 chlorophyll-a'.
The resulting page shows thumbnails of images produced every 8 days. These are composite 8 day 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.
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. (Please Note: Your browser may shrink the full-size image to fit in the browser window, rendering it too small to see clearly. If you need to override this automatic resizing, use the tip that appears below the questions.
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.
Does your browser shrink the images to fit in the browser window?
Newer versions of the Netscape and Internet Explorer browsers include a default setting that automatically resizes images to fit in your browser window. Since these MODIS images are quite large (1300 x 1300 pixels) shrinking these images significantly reduces the clarity of the image. Here's how to change the default settings:
Internet Explorer:
Tools Menu: choose Internet Options
Click the Advanced tab
Scroll down to the Multimedia section
Deselect Enable Automatic Image Resizing
Netscape:
Edit Menu: choose Preferences
Click Appearance
Deselect Resize Images to fit in Browser Window
Step 3-Compare MODIS Images for 2002 through 2007
Show me how
HideReturn to the MODIS Images page (most likely you can just use the back arrow on your browser) and select any of the chlorophyll-a links in the left column of the table. You will then see a thumbnail page of MODIS images for the year you've selected. ('Aqua' and 'Terra' are both NASA satellites that house the MODIS instrument)
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?
Step 4-A Final Look at Buoy M
Let's return to GoMOOS Buoy M, located roughly in the middle of the Gulf of Maine. As you'll recall, this buoy was reporting exceptionally high chlorophyll values in mid-April, 2004. We found reasons to suspect that these observations aren't valid and should be disregarded. The MODIS imagery provides an additional check on these questionable data.
Above, we superimposed the location of Buoy M onto the MODIS chlorophyll image taken on April 22, 2004. The image makes it clear that the highest chlorophyll values aren't near Buoy M. The MODIS image of April 22 shows elevated chlorophyll levels throughout much of the region, with the highest levels in a large mass in the south-central region of the Gulf. Around Buoy M, chlorophyll levels appear to be approximately 2-5 mg chlor-a per cubic meter, nowhere near the measurements reported by that Buoy. When we consider all the information that we have about Buoy M, we can safely conclude that data from that buoy after mid-April, 2004 isn't valid and should be ignored.
Step 5-Conclusions
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.
