Lake Water Clarity Over Time

I am ready to meet my Maker. Whether my Maker is prepared for the great ordeal of meeting me is another matter.

— Winston Churchill, epitaph

Figure 1: Seechi Disk (Wikipedia).

Figure 1: Secchi Disk (Wikipedia).

I have a small summer cabin on a lake in northern Minnesota that my family uses for recreation. The health of the lake is very important to me because I plan on leaving the property to my children so that my children and grandchildren (when and if they come) will have a nice place to vacation. One way to assess the health of the lake is by measuring its clarity, which is measured using a Secchi disk (Figure 1).

The Wikipedia has a good description of the disk and the measurement process, which I quote below.

The Secchi disk, as created in 1865 by Angelo Secchi, is a plain white, circular disk (30 cm in diameter or approximately 12 inches) used to measure water transparency in bodies of water. The disc is mounted on a pole or line, and lowered slowly down in the water. The depth at which the disk is no longer visible is taken as a measure of the transparency of the water.

One of the issues with measuring clarity using a Secchi disk is that the testing is performed by volunteers and the results rely on human visual judgement. In recent years, the Secchi data is being augmented with satellite water clarity data.  The state of Minnesota has published a good reference paper on the topic for those of you who wish to know more.

The amount of lake data that is available today is just amazing. For example, Google provides an excellent image with a superimposed lake depth map (Figure 2). The small red area on the map is the lake’s public access area.

Figure 2: Google Map of Eagle Lake in Itasca County.

Figure 2: Google Map of Eagle Lake in Itasca County.

The state of Minnesota provides a “Lake Finder” application that allows you to easily find information about a specific lake. To illustrate how I use the data, my cabin is on Eagle Lake and I recently downloaded the Secchi clarity data from Minnesota’s web site. I then plot the data as shown in Figures 3. The plot on shows data for recent years – the data goes back into the 1980s. I will use this data in discussions with other cabin owners about the lakes health.

Figure 4: Eagle Lake Secchi Disk Data.

Figur3: Eagle Lake Secchi Disk Data.

The data is gathered by volunteers. There is no Secchi data for the months when the lake is frozen, which is indicated on the plot by the dark regions.You can see in the data that the water clarity generally improves over the winter and degrades over the summer as the amount of algae and other plants increase.

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