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Wetland Sheen

Welcome back to Ask a Naturalist: your own personal “Google” for information on all things natural on PEI.  This time of year, I get a lot of questions about the iridescent sheen that appears around the edges of marshes, ponds, or standing water.  While it may look very much like an oil or gas spill, the good news is it has a natural cause.

This sheen in a PEI wetland is not an oil or gas spill, it has a natural cause.

There are several natural phenomena that can resemble pollution in our waters. In a month or so, the culprit will be tree pollen, and you can read more about that here:  Right now, the cause is harmless, natural bacteria and springtime plant decomposition.


You likely know our Island soil is red due to lots of iron, and that networks of openings throughout soil allow air in (aeration).  In places where there is standing water – in and around wetlands, for example – water replaces air in the soil, and it becomes anaerobic (without oxygen). This is ideal habitat for natural anaerobic bacteria.


These bacteria get their energy by consuming iron. By-products of their metabolic processes include hydrogen sulfide and methane (hence the ‘marshy’ smell in these areas), along with a filmy residue that can sometimes be seen on the surface of the water.  


Coupled with this is the decomposition of last year’s vegetation.  Marshy areas are very productive, supporting large volumes of grasses, sedges, and rushes during the growing season. These plants die in fall, but by then the temperature is too low to allow much decomposition. As a result, decomposers have a lot of work to do in spring, and – like the anaerobic bacteria – their natural byproducts can leave a rainbow-like film on the water.


Anaerobic bacteria and plant decomposition sheens can look like petroleum, but there is an easy way to tell them apart: poke it with a stick. Natural wetland films break apart easily and stay that way; gas or oil will reform into the original mass. Petroleum spills also have a distinctive odour, while all you should get from a natural sheen is a marshy or earthy smell. Spills should be reported immediately, but virtually all the oil-like sheens found this time of year have a natural cause.


If you have a question about PEI’s wild side, it’s likely others do too!  So, follow me here or on Facebook, join the conversation, and Ask a Naturalist about PEI untamed!

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