By David Wartinbee, for the Redoubt Reporter
This past weekend I was flying over Tustumena Lake and I marveled at the beautiful milky color of the water. That cloudiness comes from the finely ground rock particles, called glacial flour, that remain suspended in the lake water.
Tustumena Glacier provides a fair amount of water for the lake, and also creates much of the ultrafine powder as the ice and embedded rocks slowly grind against underlying sedimentary rock surfaces. If you take a tall, clear tube filled with Tustumena Lake water and let it sit motionless for a couple months, the water will become clear and a fine layer of sediments will appear on the bottom of the tube. Shake it up and it turns milky again.
In several places there are streams and impounded waters flowing into the milky-colored lake water. Several of these water sources look frighteningly like dark oil spills polluting the lake. What is so striking is the contrasting color of tea-colored water mixing with the milky-white lake water. What’s going on here?
Many of the natural streams surrounding Tustumena Lake are called brown-water streams by ecologists. They can have brown or reddish water that is transparent but dark. The coloration comes from tannins and lignins dissolved in the water. The tea-colored analogy is appropriate, since much of the color of tea, and even much of the taste, comes from tannins dissolved in the hot water brew.
Tannins and lignins are polyphenol compounds with highly variable chemical formulations. They are formed by plants and are common in bark, leaves and seeds. They are particularly difficult to break down, and only a few bacteria and fungi can actually do it. When plant materials are being decomposed, the very last things to be broken down are the tannins and lignins.
Tannins have been historically used for processing hides into leather; hence the name “tanning.” The town of Tannersville, Pa., where my wife and I lived for almost 20 years, was named because of the tannery that was once there. The surrounding area was heavily vegetated with oaks, white pine and hemlock trees, and their bark was the major source of the needed tannins.
Tannins are a common food constituent that provide particular tastes in a large variety of foods. They are found abundantly in grape skins and provide some of the characteristic tastes and even some coloration in many dark wines. The popular taste of smoked salmon actually comes from tannins that are released during the smoking process. Recently, the tannins found in pomegranates have been recognized as being particularly rich in antioxidants and touted as being a health-promoting food. Tannins found in brown-water streams don’t seem to provide a noticeable taste and are not harmful when consumed. However, it may be somewhat disconcerting to have tea-colored water before adding the tea bag.
Lignins are also a major component in brown-water streams and also result from the breakdown of vegetation. It turns out that they are even more difficult to break down than tannins. And, like tannins, only a few fungi and bacteria produce the enzymes needed to degrade them. Lignins are involved in moving moisture in plants, so they are particularly abundant in the water transport vessels in stems, bark and leaves.
Lignins have more energy stored in them than do cellulose molecules. When burning a log, the majority of the energy comes from burning the lignins, instead of burning the cellulose. Wood is used for making paper and the lignins cause it to yellow with age. Under the heading of interesting but probably useless trivia, lignins make up about 20 percent of the paper this newspaper is printed upon. The rest is cellulose.
It is difficult to remove lignins during the pulping process, but once removed by a process called the “Kraft process,” you have a nonyellowing, more expensive paper. Now you know why the paper you put into your office copier is listed as Kraft paper.
But why are these tannins and lignins in the water in the first place? We have to look way upstream in order to answer that question.
In the headwaters of these streams are boglike or marshy areas. These moist areas encourage the growth of sphagnum moss, which releases hydrogen ions into the surrounding water. The released hydrogen ions cause the water to become somewhat acidic. Only plants like black spruce or Labrador tea that can grow in slightly acidic conditions will flourish, while others disappear.
Most importantly, the acidity inhibits the activities of fungi and bacteria that normally break down dead vegetation. This is why masses of dead sphagnum moss can build up into layers we call peat. These acidic conditions also explain why wooden boats or canoes from hundreds and hundreds of years ago are sometimes found intact in the bottom of a bog pond. The slowed breakdown of vegetation means the last, most difficult process of breaking down tannins and lignins just doesn’t happen. So, these water-soluble compounds end up dissolved in the water and give it the characteristic brown color.
When a stream ecologist hears a stream being described as a brown-water stream, it is immediately known that this stream originates in boggy and somewhat acidic habitats where vegetation is not breaking down completely. Only after the acidity is diluted and the bacteria and fungi start to function again will the tannins and lignins finally be broken down.
David Wartinbee, Ph.D, J.D., is a biology professor at Kenai Peninsula College’s Kenai River Campus. He is writing a series of columns on the ecology of the Kenai River and Cook Inlet watershed.