Natural Variability or a New Trend?
What’s with our lake clarity?

Although Bills Lake is renowned for its lake transparency, data over the past three decades suggests that our reputation is a bit overblown. According to the Carlson Trophic State  Index, a standard by which lake clarity can be measured and compared, to be placed in the highest category is to have an average Seechi Disk reading of 15 feet or higher. Our average – the depth at which an eight-inch metal disk connected to a measuring tape can be seen before it disappears--  has been a bit over 11 feet over the past 30 years. This puts us toward the top of Carlson’s middle category. We are regarded as a oligotrophic-mesotrophic lake.

Not last year. Residents who pay attention to these things noticed how exceptionally clear our waters were throughout the summer. This was borne out by the measurements. Our average, as compared to previous years was astronomical: almost 20 feet. The best reading was an amazing 36 feet. The worst reading – 10 feet – approached previous years’ averages.

Why? Scientists from the Michigan Lake and Stream Associations (they train the local lake testers, i.e. citizen scientists) refer to such aberrations as natural variabilities: sort of freak accidents. After all, the lake, as a living organism, is impacted by a variety of factors.

One such factor is our recent infestation of zebra mussels. We, as in the case of virtually all inland lakes in Michigan, have been infested with them in recent years. This exotic species (not native to Michigan but instead brought in on the bottoms of boats) is both a cleaning and corrupting agent. They eat algae on the surface of the lake, thus improving transparency. But they also spit out a green substance –microcystin --akin to pond scum which accumulates once or twice a year on our shores.

A study (which Bills Lake participated in) was conducted by the ML&SA two years ago. Results were presented by Prof. Orlando Sarnelle of Michigan State University at a lake testers workshop held at Houghton Lake on Oct. 21, 2008.  He said  that we in Michigan lead the nation in the number of lakes with zebra mussels. “This organism shouldn’t be in an oligotrophic lake,” he said, “but they are.”

But are they the cause of our increased transparency? Our own Carol Dalebout of Deer Point, who holds a Bachelor’s degree in biology from Hope College and a Master’s degree from Central Michigan University, says that the introduction of zebra mussels, which she calls “invader species,” will increase the clarity of the lake. “With the clarity of the water, there will be more photosynthetic activity which will create many more weedy spots,” she adds. From her experience working in labs and doing lake testing, she calls these “predictable changes.”

However, Prof. Sarnelle stopped short of saying that, in the long term, zebra mussels clean up lakes and improve transparency. He admits to the short term effects but asserts that nobody really knows what the long range result is. After all, there is a concept called “collapse,” the point at which the zebra mussels have done their work, have nothing to feed on, overpopulate, and lose their effect.

This is why we do lake testing year after year. It will be interesting to see if last year’s extraordinary readings were a one-shot natural variability or a trend toward increasing our lake’s clarity. In the meantime, be sure to avoid stepping on the critters with your bare feet, especially if they are dry. And be sure to raise your motors out of the water when docked so that they don’t cling to and clog intakes.