Part 5: Retrieve and Graph Chlorophyll Data
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Step 1-Obtain and Graph Chlorophyll Data
Chlorophyll is the pigment that plants use for photosynthesis. As such, measurements of chlorophyll concentration can serve as an estimate of the biomass (or quantity) of phytoplankton in the water. When a bloom occurs, chlorophyll concentrations increase dramatically. On a graph of chlorophyll concentration over time, bloom events are represented by vertical spikes in the graph.
Use the GoMOOS Graphing and Downloading pages again to generate graphs of chlorophyll levels for buoys B, E, I, and M. Select Chlorophyll as the data type and Daily Average as the unit of time.
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How did your estimated date(s) of the phytoplankton bloom compare to the actual bloom as indicated in the chlorophyll graphs at each buoy location?
Step 2-A Closer Look at Buoy M
When this chapter was written in July, 2004 we noticed something very striking about the chlorophyll graph for Buoy M. We plotted chlorophyll at all four locations on the same graph.
We can see that chlorophyll levels at Buoy M are dramatically higher (roughly 15 times higher!) on April 24 than the chlorophyll levels reported the week before. These values greatly exceed the chlorophyll levels reported by any of the other buoys. They look suspicious! As you can imagine, moored ocean buoys are subject to a host of environmental influences such as wind gusts, storms, whales, and passing ships. All of these can affect the buoy's ability to record and transmit accurate ocean measurements. A small collision with a ship, for example, might render a buoy inoperable for some length of time. Instruments on the buoy can also malfunction, resulting in erroneous measurements.
In assessing whether data are valid or not, it's useful to consider all the information that's available about the data in question. In this case, we can see that chlorophyll levels at Buoy M increased substantially in a short period of time. We can also see that these levels exceed all other readings at this and other buoys. Probably most importantly though, the buoy stops reporting data at all on May 17th, which suggests it was taken "off-line" at that time, possibly for repairs or replacement. Taken together, the information we have about the buoy suggests we should probably disregard the chlorophyll data reported by Buoy M after mid-April when its values went well outside the normal range
.Science is messy. This example illustrates that every observation must be carefully considered and evaluated within the context of all the information that is known about the data.
If you requested a graph of this same data today, it may look quite different from this example. Data are routinely checked for quality and when problems are identified, corrections are made or the incorrect data are "thrown out." Still, the concept of evaluating all data before drawing any conclusions is a recurrent theme in science and an important concept to keep in mind.
In the next section, we will use satellite images to gain another perspective on ocean phytoplankton. This second set of chlorophyll measurements provides an additional check on questionable data values.
