Iridescent sheen

In the past few years and, again, this year an “oily” looking sheen has appeared on the lake. About 2-3 years ago when it was first brought to our attention by some patrollers, Bob Craycraft identified it as a very natural phenomena and called it an “iron stain.” Recently other patrollers have also seen it, along the shoreline or near docks (Dean, Don, Brenda and Thia and me.)

I did take a paddle and whacked at the stains I had found. They broke into tiny fragments, not regrouping.

AI has this explanation and chart which helps distinguish between different sources that can create this sheen (which seem to be either iron bacteria, petroleum, or even pinecones - see Gloria's very recent posting on her find.)

Surface sheen Identification

An iron stain or iridescent oily sheen on a lake's surface usually comes from iron-oxidizing bacteria naturally reacting with dissolved iron in groundwater seeping into the water. [1, 2]

An iron-bacteria sheen is rigid and brittle, whereas both pinecone resins and engine gas create elastic oils that swirl and flow. However, while pinecone oils and engine gas behave similarly on the water's surface, they can be easily separated by color, smell, and localized impact.

Feature	Iron Bacteria Sheen	Pinecone / Organic Sheen	Engine Gas / Fuel Spill
The Stick Test	Shatters like glass into jagged, separate fragments.	Swirls smoothly and stretches without breaking.	Swirls and reforms instantly into a continuous film.
Color & Appearance	Vivid rainbow or silver, often accompanied by fluffy orange rust.	Dull silver-gray or milky film; may form yellow-green pollen lines.	Brilliant, intense rainbow rings that spread very quickly and thinly.
Odor	Earthy, musty, or completely odorless.	Sweet, woody, or no distinct chemical scent.	Strong, sharp chemical odor of gasoline or exhaust.
Behavior	Stays localized where groundwater seeps up; doesn't spread far.	Clumps around debris or floats in stagnant, tea-colored coves.	Spreads aggressively into a micro-thin layer across open water.

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off of Dean's dock. Notice pinecones in the water

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off of Maria's dock. Notice no pinecones nearby nor any evergreens near by