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Mobile Bay and the Mississippi River

  Mobile_Bay_title_graphic    Mobile Bay and Mississippi River delta SeaWiFS image   Mobile Bay excerpt

Questions for this Chapter

Where do water and sediments from Mobile Bay go after they enter the Gulf of Mexico?   Does this discharge interact with the much larger discharge of the Mississippi River?

Is there a correlation between discharge and satellite-measured chlorophyll concentrations?

 

Data Frames for this Chapter

Chlorophyll a Concentration (SeaWiFS)

 

Visualizations for this Chapter

Animation (Giovanni)

Longitude-Time Hovmoller Plots (Giovanni)

Area Plot and Time Series (Giovanni)

Time Series (Excel)

Mobile Bay, Alabama, is the fourth largest estuary in the United States, with a surface area of over 1000 square kilometers.   The average discharge of water from Mobile Bay is about 1800 cubic meters per second, though this is influenced by the tidal cycle.  The water from Mobile Bay is frequently laden with sediments, so the discharge from Mobile Bay can frequently be seen as a semi-circular brownish area northeast of the Mississippi River "crow's foot" delta, as seen in the images at the top.    Mobile Bay was the site of a famous naval battle during the Civil War. 

From the earliest settlers, the Mississippi River has been one of the United States most prized natural resources.  Beginning in the 1830s, the Mississippi served as a gateway to the inland of the United States, bringing goods and people on steamboat transportation.  Its flow affected settlement, commerce, and even inspired Mark Twain’s book, “On the Mississippi.”  However, the delta – where the river meets the Gulf of Mexico – affects various processes in the Gulf.  Holding the third largest river basin in the world, the Mississippi River deposits large amounts of nutrients and freshwater into the Gulf of Mexico, causing tremendous implications for the aquatic environment in the Gulf. 

Map and Coordinates

map of mississippi delta

coordinates

 

Animation (Giovanni)

  1. Select the area specified above.
  2. Parameter: Chlorophyll a (SeaWiFS)
  3. Temporal:            Begin Date = 1998, Jan

    End Date = 2007, Dec

  4. Select Visualization: Animation
  5. Generate Plot

map of chlorophyll concentrations

Watch the animation to get a general sense of the movement of the water from Mobile Bay in the Gulf of Mexico.

Top

Longitude-Time Hovmoller plot (Giovanni)

zoom of the delta

coordinates

  1. Select the area specified above. 
  2. Parameter: Chlorophyll a (SeaWiFS)
  3. Temporal:            Begin Date = 1998, Jan

End Date = 2007, Dec

  1. Select Visualization: Longitude-Time Hovmoller Diagram
  2. Generate Plot
  3. Change the color scale to Custom: Min = 0

    Max = 5

longitude-time Hovmoller plot

This graph displays how chlorophyll concentrations are changing over a 3-year time span in the East-West direction.  The discharge from Mobile Bay flows down the center of the graph.  We can see that the sediments from Mobile Bay tend to flow to the west after they reaches the Gulf.  Examine the true-color SeaWiFS image at the top to see if this pattern is present.   

 

During April 2003, there was severe flooding along the lower Mississippi Valley, particularly Jackson, Mississippi.  Compare the ’03 longitude plot with longitude plots from other years to see if the discharge from Mobile Bay was also affected.

  1. Select the area specified above.
  2. Parameter: Chlorophyll (SeaWiFS)
  3. Temporal:            Begin Date = 2003, Jan

End Date = 2003, Dec

  1. Select Visualization: Longitude-Time Hovmoller Diagram
  2. Generate Plot
  3. Change the color scale to Custom: Min = 0

    Max = 5

longitude-time plot for 2003

As we can see in the Longitude-Hovmöller Plot, the water from Mobile Bay tends to flow to the west as it enters the Gulf from the inlet..  By analyzing small plots on either side of the inlet, we can see that location can change the average chlorophyll levels and the variability.  Let’s look at 3 half-degree (0.5° x 0.5° )plots near the Mobile Bay inlet.

 

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Area Plot and Time Series (Giovanni)

West:  coordinates for western plot

Direct: coordinates for plot directly out of river

East:   coordinates for eastern plot

  1. Make plots for each of the coordinates above.  (Click the examples below to view larger versions.)
  2. Parameter: Chlorophyll (SeaWiFS)
  3. Temporal:      Begin Date = 1998, Jan

                                    End Date = 2007, Dec

  1. Select Visualization:         Lat-Lon Map, Time-Averaged

    Time Series

  1. Generate Plot
  2. For the area plots, change the color scale to Custom:

    Min = 0

    Max = 5

  1. For the time series, rescale the y-axis:      
  2. Min = 0

    Max = 7

     chlorophyll-west plot             chlorophyll plot -directly beneath delta                chlorophyll plot - eastern

9-year time series of chlorophyll concentration - west9-year chlorophyll time series - direct9-year chlorophyll time series - east

 

Chlorophyll concentrations usually increase when there is an influx of nitrogen, which is a limiting nutrient for growth.  When the water from Mobile Bay flows into the Gulf, it brings nitrogen from sources such as agriculture, manure, and other physical processes.  Therefore, we could hypothesize that an increase in discharge might also mean an increase in chlorophyll production. 

Go to the website: http://www.mvn.usace.army.mil/eng/edhd/Wcontrol/wcmain.htm.

This website gives the stage data for the Mississippi River at several locations.   The "stage" of a river is the level of the river with respect to a reference level.   A higher stage value indicates that the level of the river is higher, which in turn indicates that the river is carrying more water and the volume of water discharged by the river is greater.

  1. Click on the image in the middle of the screen.  Choose the area close to where the Mississippi River hits the Gulf. 
  2. Select the Venice station. 
  3. Choose the time series:    January 1, 1998

    December 31, 2007

  4. Select Plot

9-year discharge time series

 

Look over these plots to see if the stage – the level of the Mississippi River, an indicator of how much water the Mississippi is discharging into the Gulf of Mexico – exhibits a pattern.   Would you expect the discharge from Mobile Bay to be related to the discharge from the Mississippi River?

Click here to see an extension of a time series comparison. 

 

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Questions

Which way does the water from Mobile Bay flow in the Gulf of Mexico?  How can you tell?  Trace the route through the Gulf.

Mobile Bay is known to have events called "Jubilees" in the summer.   What factors contribute to Jubilees, and what is the main cause of them?

Which way does water from the Mississippi River flow in the Gulf of Mexico?   Examine the chlorophyll data to see if you can see a pattern of movement.   Then examine maps of the hypoxic "dead zone" on the bottom of the northern Gulf of Mexico.  Is there a relationship between these patterns?

After the intense flooding in the Midwest in 2008, what would expect to see as the waters from the Mississippi River flow into the Gulf?

How do the time series differ over the three area plots?

 

 

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