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April 14th, 2003
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Q. What is the black, iridescent, oil-like compound found on the banks of creeks and rivers in the late fall (I usually only fish creeks and rivers for trout and salmon, so I do not get to investigate whether or not the substance is present in the other seasons). I believe that it is the carbon compounds released by the decaying leaves and other detritus. Is this rich compound detrimental to the ecosystem or is it vital to the health of the ecosystem (if seasonal)?
A.
You're pretty close. Dr. Cliff Dahm helped me on this one. The
sheen you see on the water is usually the seepage of anaerobic
(no oxygen) water into the stream. Under anaerobic conditions,
high concentrations of reduced elements, such as sulfur, manganese,
and iron are found. These reduced compounds are oxidized by microbes
which produces the hydrocarbon compounds that you've asked about.
These hydrocarbons, which chemically are similar to gasoline, can
be further metabolized by heterotrophic bacteria. The reddish
material that you sometimes find along stream margins is the result
of the oxidation of iron by bacteria.
The material you ask about is neither detrimental to the ecosystem,
nor is it vital to the health of the stream. This is due to the fact
that it is not found in high concentrations in streams; it is
rapidly diluted from where you find it in small seepage areas.
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The 'Stream Doctor' is a retired professional stream ecologist and
author, now living in the West and spending way too much time
fly-fishing. You are invited to submit questions relating to
anything stream related directly to him for use in this Q & A Feature
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