by Whit Gibbons

February 18, 2018

A great aspect of being a naturalist, which includes anyone who appreciates nature and thrives on new discoveries, is that intriguing environmental mysteries abound. No one can answer all questions arising from outdoor observations. But somewhere out there is an expert who can answer certain ones. When I received the question below, I turned to Gary Mills, former colleague at the University of Georgia’s Savannah River Ecology Laboratory, and currently research professor at the University of South Carolina Aiken. Gary is an organic chemist.

Q: I was hiking in the Smokies, and in Abrams Creek, Tenn., a swiftly flowing stream, were piles of yellowish foam. I assume it is natural but do not know what causes it. So, what does cause it, and is it a problem?

A: Great question, and one I am sure many naturalists have wondered about. Knowing the origin of the foam is especially important in this environmentally unspoiled stream that flows to the Little Tennessee River from Cades Cove. Abrams Creek is enjoyed by many for angling, boating and swimming. It turns out that foam in a stream is a result of natural processes, when dissolved organic carbon results from decaying vegetation upstream. So the yellowish color is a good sign. Human-caused foam resulting from pollution by detergents is usually white. Natural foam is generally tan or brownish, depending on the type of vegetation contributing to the dissolved carbon.

As Gary Mills points out, “These foams can form in all aquatic environments.” He explains that their origin “is from what is called the surface microlayer (SML), a film at the water-air interface.” The SML, or biofilm, may be no thicker than the surface of a soap bubble but can be full of life, supporting “a rich community of microorganisms, including bacteria.” Many people have a negative view of bacteria because of the few that are harmful to us, but most have positive environmental roles and are essential to healthy ecosystems. The activities of bacteria are constant and essential in all natural habitats.

The SML microenvironment is rich in organic substances derived from decaying leaves, wood and algae in the water itself, as well as material that leaches out from humus in the soil of the adjacent forest. In the Smokies, decaying vegetation would be a natural occurrence from conifers and other streamside vegetation. Gary notes that “generally, the SML is not visually evident, but when the concentration of organic matter is high and there is turbulence in the water, a bubbly froth will form at the surface.”

In streams with beaver dams, large rocks or logs, foam will form below the obstruction and be most apparent there; it can also drift downstream. Sometimes foam is skimmed off the water surface and can collect on small limbs or other objects in the stream. The same phenomenon of natural foam can be seen at the beach in coastal areas. “When marine algae are blooming, they release organic matter into the water.” A biofilm like that in a stream will form on the surface, and as waves break on shore “the turbulence will produce foam that will sometimes blow across the beach.” The presence of foam in streams, lakes and the ocean can be a little disconcerting if you’re not sure whether it’s the result of natural processes or is evidence of pollution. Fortunately, because of strict environmental regulations, clean waters are often the norm in many areas these days, so that natural foam is what we get.

I like the hiker’s question for several reasons. First, it shows environmental awareness and intellectual curiosity, a desire to know whether an observed phenomenon is natural or a matter of concern. Second, it emphasizes that complex environmental events are under way in any habitat we explore, including life we cannot see. Finally, it underscores the intricacies of natural environments and reveals how they can be appreciated more fully once they are understood.

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