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The Lakeland Times | Minocqua, Wisc.

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October 21, 2017

10/6/2017 7:24:00 AM
the lake where you live
Light in the water
Ted Rulseh
Columnist

Light is an essential life force in lakes. How it behaves in the water depends on various factors, some immutable - pure physics - and others related to the characteristics of each lake's water.

Let's start with one of those immutable behaviors - refraction. Light (and here we mean sunlight) slows down when it strikes the water, and as a result, the direction in which the light waves travel bends upward. So, imagine that you are an archer and see a fish in shallow water. Because of refraction, the fish is actually lower in the water column than it appears. So in order to hit the fish with an arrow, you would have to aim slightly below where the fish's image appears.

Another immutable property of light is reflection. The percentage of the light meeting the water that reflects off the surface depends on the angle at which the light travels. At noon on a clear day, when the sun is almost directly overhead, about 95 percent of sunlight penetrates into the water - just five percent or so is reflected back. But by late afternoon when the sun is lower, about half the light is reflected.

As for the light that penetrates the surface and enters the water, that behaves differently depending on what the lake and its water are like. In general, portions of the light entering the lake are absorbed by plants to make food by photosynthesis, absorbed by the water or the particles in it and converted to heat, scattered back out of the lake by particles, and reflected or absorbed and converted to heat by the bottom sediments.

The character of the lake determines how much of the light behaves in these ways. For example, dark bottom sediments will absorb more light (and heat) than lighter-colored sediments. Water with a high concentration of floating (suspended) particles may absorb and reflect most of the light within the first few feet from the surface, so that plants can't get enough light to grow except in the shallowest areas.

Dissolved substances in the water can affect how efficiently a lake absorbs different wavelengths (colors) of light. For example, waters high in dissolved organic substances absorb all wavelengths quite efficiently. Therefore heat input from the sun concentrates in the upper water layer, and again the growth of plants in deeper water may be limited.

Naturally, snow and ice cover in winter reduce light penetration. A layer of crystal-clear ice transmits light just about as well as clear water. But if the ice is cloudy because of air bubbles or impurities, or stained by organic matter, more light is absorbed and less passes through. Snow increases sunlight reflection off the surface by about 75 percent. When the snow is heavy, very little light penetrates the surface.

What this means, as the sun sinks lower with the approach of winter, as the light strikes the water at a sharper angle, and as ice and snow cover take hold, the lake becomes a progressively darker place.

Ted Rulseh, who lives on Birch Lake in Harshaw, is the author of "The Lake Where You Live," a blog where readers can learn about the lakes they love - the history, geology, biology, chemistry, physics, magic, charm. Visit lakewhereyoulive.blogspot.com. Ted may be reached at trulseh@tjrcommunications.com.





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