Cindy Hagley, Great Lakes Environmental Quality Educator, Univ. of Minnesota Sea Grant, 218-726-8713

Empire Bluff at Sleeping Bear Dunes National Lakeshore, MI. Image credit: Travel Michigan

Are you old enough to remember the old commercial jingle, “From the land of sky blue waters….?” When we think about a beautiful lake scene, we usually picture blue water surrounded by green trees, but how many of us know what determines the color of lake water?

The reasons a lake is a particular color are complex. In fact, lake colors can vary widely, ranging from nearly colorless, to yellowish or reddish or brownish or greenish or bluish! The same lake can even range in colors over different seasons, weather cycles, or types of human activity in the watershed.

So what are some of the factors that determine the color of lake water? Pure water has a pale blue color, caused by selective absorption at the red end of the visible light spectrum, but no “real-live” lake is made up of pure water. The color we perceive when we look at lake water is called apparent color. Apparent color is determined by many factors, including how much light is being reflected or absorbed by materials suspended or dissolved in the water, the color of the lake bottom, the depth of the lake, reflections from the sky or surrounding vegetation, and aquatic plant presence or absence. If you collect a sample of lake water in mid-summer and hold it up to the light, you will probably see things floating in it, including algae, dead plant and animal matter, possibly some clay, and maybe even some little animals, or zooplankton. These are all examples of suspended substances that can influence apparent color of a lake. For example, a nutrient-rich lake with high algal densities might appear greenish when you look into it.

Apparent color is hard to quantify or compare among lakes because of all the factors that can influence it, so scientists prefer to measure true color. To measure true color, scientists filter out all the suspended materials. This leaves them with a water sample containing only dissolved substances. Dissolved substances include metallic ions from rocks and soils (for example, iron and manganese ions) as well as organic acids from decomposing vegetation. Do you ever brew and drink tea? When you soak tea leaves, the organic acids and tannins from the tea leaves stain the water the lovely reddishbrown color we associate with a great cup of tea. The very same process brings much of the true color to lakes.

After filtering the water, the resulting true color is compared to a standard color scale (in the U.S. we usually use the platinum-cobalt color scale). This color scale allows us to compare among lakes or in one lake over time. Platinum-cobalt values of less than 20 indicate clear water, while values from 20 to over 100 indicate a highly colored lake. Lakes that are highly colored have often been “stained” by water flowing in from wetlands or forests.

Why do scientists care about lake color? Again the complete answer is complicated, but probably the most important reason is that the very same dissolved and suspended substances that give lakes their color do so by changing the way light is refracted, reflected, and absorbed. Not only is light essential for the growth of plants, including algae, but light energy and the heat it provides are also critical for plants and animals. Anything that reduces the depth of light penetration has impacts on the biological condition of the lake. Many fish and diving birds, such as loons, rely on water clarity to find their prey. In fact, if other things are equal, a lake with a higher true color value will probably have less biological activity than a similar lake with a lower true color value.

Next time you find yourself humming a song about Minnesota’s sky blue waters, remember – it is not quite that simple!

References

• Florida LAKEWATCH. 2004. A Beginner’s Guide to Water Management – Color. 1st Edition. http://lakewatch.ifas.ufl.edu/LWcirc.html.
• Minnesota Pollution Control Agency. 1999. Minnesota Lake Water Quality Assessment Data: 2000. http:// www.pca.state.mn.us/water/pubs/lwqar.pdf.

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