How Clear the Water?

Today is one of several during open-water season when I observe and measure the clarity of Birch Lake’s water. I do so as a member of Wisconsin’s  Citizen Lake Monitoring Network, a group of volunteers who collect water-quality data on their lakes and share it with the state Department of Natural Resources. I do this with a Secchi (SEKK-ee) disc, a standard tool lake scientists use. It’s a simple device – a flat plate painted with alternating black and white quadrants and suspended from a rope, which has markings one foot apart. I use the disc every 10 to 14 days, from ice-out into October.

I drive the pontoon boat to the deepest spot in the lake (27 feet), then stop, lie down on the boat’s deck, and watch the disc as I lower it into the water, paying out rope slowly, slowly, When I can no longer see the disc, I attach a clothes pin to the rope at the water line. Then I slowly bring the disc back up until I can see it again, and attach a second clothes pin. Finally, I retrieve the disc and determine how far down it was visible, using the markings on the rope. The final Secchi disc reading – the average of the two measurements indicated by the clothes pins – tells how deep sunlight penetrates into the water. That distance is roughly double the reading indicated by the scale on the rope, because the light has to make a round trip – down into the water and back up for the disc to be visible.

The clarity in any lake changes with the season. For example, on Birch Lake in May soon after the ice melts, I can see the disc 15 to 16 feet down. Through spring and summer, as the sun warms the water and algae grow, the clarity declines. Today’s Secchi reading was 7.3 feet. The tint of the water also changes as the months go by – clear in early spring, slightly yellow or brown during summer, then a greenish cast in autumn when the algae are at their peak. By September the Secchi depth will likely be about five feet, or a little less.

Taking a Secchi reading is a pleasant interlude. Looking down into the water brings a  certain intimacy, and a deeper appreciation for the lake one calls home.