Although it seems clear from the graph that chlorophyll a concentrations usually correlate with river discharge data, we have not demonstrated that an actual relationship is statistically valid. To be a valid scientific observation, we can extend this analysis using Excel statistical tools.
- Click on Download Data above the Giovanni Time Series image for the western plot. Use only the 1998 – 1999 data years.
- Scroll down to the bottom. Under Time-Series Rendering, click ASC.
- A new window will pop up with the complete data set. Copy and paste to the Notepad text editor and save as ‘mis_west’
- Open Microsoft Excel. From File, click Open Under Files of Type, scroll to All Files.
- Now, your text files should appear. Select one. Change to Fixed Width.
- In the next step, make sure that Date is switched to MDY under Column Data Format.
- Go back to the stage data for Venice, LA.
- Choose the data for the period Januray 1998- December 1999 and click generate table.
- From the table, follow the same steps above to import the data into Excel.
- Make sure that the guidelines for the data follow the same format: i.e. MDY.
Once these two datasets are imported, copy them onto the same spreadsheet.
- The first step is to average the daily stage data into monthly data.
- To average the data for January, enter ‘=AVERAGE(A1:A31)’ into a new cell
Continue to do this for the rest of the data.
Once you have the full 2-year data, line it up next to the date and chlorophyll data as shown:
Now, we can analyze the data using the Correlation tool:
- To download Data Analysis tools, click Add Ins under Tools.
- Choose Analysis ToolPak as the application to install.
- After installation, go back to the Tools toolbar and select Data Analysis.
- Click Correlation.
- In the next box, enter the chlorophyll data and the stage data under Input Range. Then click OK.
- On a new sheet, the r-value (the degree of correlation) should appear in the B3 cell.
The value .73 is the correlation coefficient or the r-value. It shows how well the points fit a line. R ranges from -1 to 1. 1 signifies a perfect direct correlation; -1 signifies a perfect inverse correlation; and 0 signifies no correlation. .73 means that 73% of the data corresponds to a direct linear equation between chlorophyll concentrations and total discharge.
Given what is known about phytoplankton chlorophyll and nutrients, this seems to makes sense. The Mississippi River brings tons of nutrients - 900,000 metric tons of nitrogen and 35,000 metric tons of phosphorous to be exact - into the Gulf from land sources, such as farms and wastewater. The larger the river discharge (i.e., the higher the stage),, the more nutrients the river carries. Both the total nutrient amount and the nutrient concentrations (due to increased runoff) spike during heavy discharge events such as floods - and for regional events, there will be more discharge from other sources, such as nearby Mobile Bay and Mississippi's Pearl River. When excess nutrients are brought into the system, chlorophyll concentrations rise as phytoplankton grow, due to the nutrient increase. Because the Mississippi River is the dominant source of fresh water to this area of the Gulf of Mexico, chlorophyll concentration and freshwater discharge from the Mississippi show a good correlation here.
Questions for Further Investigation
What does the r-value mean about the relationship between chlorophyll a concentrations and discharge?
If the EPA is able to significantly reduce the nitrogen as it plans to do, what will happen the relationship between chlorophyll and discharge?
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