Week 4: Using Satellite Data to Study the Ocean
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Using Giovanni to Study Chesapeake Bay
- How has the concentration of chlorophyll in Chesapeake Bay changed under different environmental conditions?
- How has the concentration of chlorophyll in Chesapeake Bay changed over time?
No matter where you liveâ€"along the coast, in the heartland, or somewhere in betweenâ€"your life is affected by water quality. In places like Chesapeake Bay, a major water quality concern stems from nutrient loading (increased levels of dissolved nitrogen and phosphorous). As water makes its way through a local watershed (region of land that drains into a body of water) and eventually to the ocean, it is inevitably affected by how people use the land. Runoff from fertilizers applied to agricultural fields, golf courses, and suburban lawns; deposition of nitrogen from the atmosphere; soil erosion; and discharge from aquaculture facilities and sewage treatment plants all contribute to increasing nutrient content in coastal waters. When phytoplankton (a type of algae) are overfed with excess nutrients, their reproduction rates can increase significantly, causing what is known as an algal bloom.
Water discolorations associated with algal blooms can be easily identified from space. These discolorations are largely due to the chlorophyll (the green pigment associated with photosynthesis) contained in the phytoplankton. Other factors such as suspended sediment, colored dissolved organic matter, and bottom reflections can also affect the "color" of the ocean. The Sea-viewing Wide Field-of-view Sensor (SeaWiFS) is an instrument on the Orbview-2 satellite that collects ocean color data (measurements of light intensity at visible wavelengths) that are used to calculate characteristics like chlorophyll-a concentration and water clarity.
Compare Data Under Different Environmental Conditions
We'll use Giovanni to explore SeaWiFS monthly chlorophyll-a concentration data in Chesapeake Bay. A very dry month (April 2002) and a very wet month (April 2003) show how chlorophyll concentration in the Bay is affected by upstream conditions in the Chesapeake Bay watershed. These different conditions cause very apparent changes in the biological properties in the waters of the Bay and along the coast.
NOTE: All of the same operations described below can also be performed with ocean color/chlorophyll data from the MODIS (Moderate Resolution Imaging Spectroradiometer) instrument aboard NASA's Aqua satellite. As the youngest of NASA's ocean color instruments, MODIS-Aqua is likely to be the sensor of choice for 2010 and beyond.
- Click here to open Giovanni in a new window.
- Click and drag your mouse on the map to select the Chesapeake Bay. Use the zoom and pan tools at the top left of the map window to help locate the region. Redraw the selection box as necessary. You may find it easier to enter latitude and longitude coordinates in the appropriate boxes to set the boundaries of your selection box on the map. In this case, enter -78 for the West boundary, 40 for the North boundary, 36 for the South boundary, and -74 for the East boundary.
- Scroll down and select SeaWiFS Chlorophyll a concentration in the Parameters box.
- Choose the Time Period: Begin Date: Year 2002, Month - April; End Date: Year 2002, Month - April.
- Leave the visualization type as Lat-Lon map, Time-averaged. Click the Edit Preferences button and then scroll down. Under the Color Bar options, click the radio button for Custom and enter a minimum value of 0.5 and a maximum value of 10. Then click the Generate Visualization button.
- Examine the resulting visualization. Chlorophyll concentrations are measured in milligrams per cubic meter (mg/m3). As the legend shows, red areas contain the greatest concentration of chlorophyll (and therefore the greatest concentration of phytoplankton), while the purple areas have very low chlorophyll concentrations. It is important to note that chlorophyll concentrations measured by satellite are less accurate near the coast (particularly in shallow waters) than they are farther offshore, but the measurements are still useful for observing changes in water quality.
- Change the Time Period to April of 2003 and generate a new visualization. During the wet month in April of 2003, there was increased water flow to the Bay. This led to increased runoff throughout the watershed and resulted in higher concentrations of nutrients reaching the Bay.
Look at How Chlorophyll Concentrations Change Over One Year
Try looking at how chlorophyll concentrations in the Chesapeake Bay region change from month to month over the course of a year.
- Go back to the main Giovanni page and select your area of interest and the SeaWiFS Chlorophyll a concentration parameter as you did before.
- This time, set your start and end dates to cover one year (e.g., January-December, 2007).
- Instead of choosing the Time-Averaged Lat-Lon map, choose Animation as your visualization type and then click Generate Visualization.
- Use the control buttons at the bottom of the player window to watch the animation. Look for evidence of seasonal changes and periods of relatively high or low chlorophyll concentration.
Export Giovanni Images in an Animation to ImageJ, If You Have Time
A Giovanni animation isn't a single file you can download. Rather, it is a set of separate images displayed in sequence. Furthermore, while Giovanni produces great visualizations, it doesn't offer tools to analyze the visualizations after you've created them.
One of the key themes of this course is integrating the tools you're learning to use. Using a fairly simple procedure, you can copy Giovanni images into ImageJ where you can stack, animate, and analyze them. You can also save your stacks as TIFF or GIF files for distribution.
If you have time and want to give it a try, this movie will show you how. The technique involves a fair amount of window manipulation, so don't worry if it takes you a couple of tries to get the procedure down. Remember - you can't hurt anything!
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How to download movies
- Click the link to go to the SERC media library listing for the movie. The record will open in a new window.
- On the SERC media library page, right-click (Win) or control-click (Mac) the link (below the movie on the Flash version pages) to download the movie file to your hard drive.
Flash Video Versions
Download these version to play on your iPod or iPhone.