The Chesapeake Bay Dead Zones by K32JY33

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									The Chesapeake Bay Dead Zone

According to the Chesapeake Bay Foundation, the Chesapeake Bay “dead zone,” areas
of water with less than 5 mg/L of dissolved oxygen, comprises roughly a quarter of the
mainstream waters of the bay during the summer months. The short-term low level of
dissolved oxygen either kills fish and blue crabs or drives them from their preferred
habitat. Less mobile species, upon which the fish and crabs feed, become stressed or die.
The low level of dissolved oxygen is a direct result of nutrient pollution, mainly nitrogen.

The figure below shows the dissolved oxygen level throughout the year for several sites
along the bay. In this project we will use the exponential decay model to study the
decrease in dissolved oxygen for the first half of the year for three of these sites. We will
approximate the “best-fit” exponential model using the dissolved oxygen levels for
January and June.




The Exponential Model

An exponential model has the form Y  ab X , where a is the value of Y when X=0
(sometimes called the initial value) and b is the decay factor. The decay rate, r, is
calculated as 1-b and is interpreted as the proportion that is lost or removed during each
time period.

Project

Find and compare the decay rates for the three sites given in the table below, which are
the mean amounts of dissolved oxygen (DO) from the figure above.

          Site                  January DO      June DO      Decay rate
          Maryland Mid-Bay      9.5             .5
          Smith Point           10              .5
          York Split            11              5

Use each of your models to predict dissolved oxygen in May. How good are these
predictions?

What are some possible explanations for the differences in decay rates?

Is the exponential model appropriate for this situation? Why or why not?

What other models could be used?

Additional comparisons: What percentage of the year is the dissolved oxygen below 5
mg/L? How does the percentage change if you consider the ranges instead of the means?


The data and information for this project are from the following web site:

http://www.cbf.org/site/PageServer?pagename=resources_facts_deadzone

Solutions
Maryland Mid-bay:
       y  17 .12 (. 55 ) x    The decay rate is 1-.55 = .45, or 45% each month.
        Prediction for May     y  17 .12 (. 55 ) 5  .86

Smith Point:
       y  18 .21(. 55 ) x     The decay rate is also .45.
        Prediction for May     y  18 .21(.55) 5  .92

York Split:
       y  12 .88 .(85 ) x     The decay rate is .15 or 15% each month.
        Prediction for May     y  12 .88 .(85 ) 5  5.71

Other possible models: logistic for the first half of the year, seasonally adjusted time
series

								
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