What is this Oily Sheen on the Marsh?

The Question: For years I have noticed an oily substance on a stretch of Lake Huron shoreline that leads into a wetland/marsh area. The oily substance has the rainbow pattern that you see when there is a film of oil on water. Is it possible that this is a natural occurrence of some oil deposits, or should I investigate it as an environmental contaminant? There is a road about 30 yards uphill from this spot, and for years it used to be a dirt road that the city sprayed oil on to keep the dust down.

Submitted by: Troy

The Short Answer: Geoff Peach, Coastal Resources Manager at The Lake Huron Centre for Coastal Conservation, in Ontario, Canada,
looked at your pictures. His response:

“This is a common wetland phenomenon. You generally see it in shallow pooled water amongst the vegetation. The oil is produced by microbes and we tend to see it when it becomes concentrated, as it does in these isolated pockets.”

More Information: We often think of the natural world as separated into animals and plants. Or maybe multi-cellular life and single-celled life. Or if you are a biologist, maybe eukaryotes (animals, plants, fungi) and prokaryotes (bacteria, archea). But another interesting way to divide the natural world is into those organisms that live in habitats with free oxygen (aerobes) and those that live in habitats without free oxygen (anaerobes). In habitats with oxygen, aerobic bacteria release carbon dioxide. In habitats without oxygen, anaerobic bacteria release methane. The surface of a marsh is aerobic – it has enough free oxygen for plants, animals, and aerobic bacteria. But because there is so much biological activity by all the oxygen-loving organisms above ground, most of the oxygen in the water gets quickly used up, and the resulting low oxygen water seals off the underlying mud from atmospheric oxygen.

Just a couple of inches down, there is no free oxygen and the anaerobic bacteria rule a world of drowned muck.

Because methane is a small, easily evaporated molecule, most of the methane produced by the anaerobes in their mud world escapes into the atmosphere. But some small percentage of it gets converted into larger hydrocarbons that are less likely to evaporate. They still are lighter than water, however, so they float on the surface. There is little difference between these naturally produced compounds and hydrocarbons like gasoline or oil, so the sheen on the water looks the same as if someone had spilled gas or oil.

Another possible source for oil on the water of a marsh is oil released directly by plants, or oil released when plants and animals die. Because there is so much living and dying of plants and animals in a marsh, there is a fair amount of oil produced.

Having said all that, of course, it is always possible that a gasoline or oil spill nearby caused this sheen. Lakes with heavy boat traffic are always subject to spilled gas and oil. Or it could be oil washed off from the oiling of the road, as you suggest. It’s hard to know for sure, and impossible to tell from pictures alone, of course. You’ll just have to use your judgment as to whether what you are seeing could be the natural marsh production of hydrocarbons, or something too thick and heavy and localized to be natural.

Cite this article as: Pelletier, TC. (September 3, 2010). What is this Oily Sheen on the Marsh? Retrieved from http://askanaturalist.com/what-is-this-oily-sheen-on-the-marsh/ on July 3, 2020.

13 thoughts on “What is this Oily Sheen on the Marsh?”

  1. I received this somewhat delayed response to my inquiry to the Ontario Ministry of Natural Resources:

    “Sometimes, an oily residue appears on stagnant water due to natural breakdown processes. However, you may wish to notify the Ministry of the Environment Spills Line at 1-800-268-6060 in the event this is an unusual occurrence or contamination may be present. Your photos may also assist them if a spill is suspected and they choose to investigate.”

  2. Petroleum “refined” hydrocarbon will swirl on top of water when it is touched with a stick…

    Natural biodegradation hydrocarbon will break apart into polygons when touched.

    In areas will a lot of vegetation it is easy to have free floating hydrocarbon on the surface, as in meanders of creeks with deciduous trees overhead.

  3. William,
    Thanks for that info. Do you know this from personal experience or is there a source I can quote that would back this up? Do you have a picture of the difference in how the hydrocarbons react to disturbance?
    Thanks, Tom

  4. Whether the hydrocarbons are produced from spills or natural sources, most will be broken down by bacteria eventually. The issue is always about how much hydrocarbons are present. In most marshes, the naturally produced hydrocarbons are balanced with organisms that “eat” them. A little bit of man-made hydrocarbon most often will just be added to that system of natural breakdown. A large man-made spill can overwhelm the natural processes, however, leading to the death of many kinds of organisms. As toxicologists like to say, “The dose makes the poison.”

  5. This is very strange and I wish I could attach the photo I took but, this is what I have a question about.
    As I sat at the edge of a tiny lake /pond a metallic oily looking substance came to surface and started “drawing” on the surface. It was about three inches thick and actually “drew” a question mark right there in front of me! The length of the “question mark” was probably about 12 to 15 inches. When it got to the end of this the question mark it just stopped. No sign of any animal that may have made it. Again, I wish I could send you the photo but, the strangest part was watching it “draw” the question mark. It only took about a minute. Any ideas on that? Thanks

  6. Hi Rani, there is no advantage to the oily sheen. It is just a result of the animals living in the marsh eating and eliminating wastes. Tom

  7. I have this in my backyard coming from drainage from a large hill. Very curious what it is. It’s slow moving water.

  8. All oils and other hydrocarbons, whether natural or manmade, have the potential to be harmful if the dose is high enough. Probably the natural oils you see in a marsh are not that bad. If it’s a manmade spill that’s causing the sheen, then it would depend on what the actual spilled substance is.

  9. From Clemson University (https://www.clemson.edu/extension/water/stormwater-ponds/problem-solving/muddy-turbid-water/index.html)

    “Hydrocarbons and oils are usually the other source of surface films in ponds. Oils are produced naturally by the decay of leaves, algae and organic matter, but these oils behave differently from cooking oil or motor oil. To determine if the oil sheen you see on your pond is from a natural organic source, poke it with a stick. If it is from the decay of leaves and other organics, it will most likely crack and shatter like a thin layer of glass and will not reform as the stick is removed. If it reforms or does not shatter, it is likely from automotive or cooking oils that have washed into the pond. “

  10. Courtney, thank you so much for that info. I’ll have to try that. What is it that makes the natural oils behave differently than the hydrocarbons and cooking oils? Tom at AskaNaturalist

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