waskington state lake images
WATERLINE - December, 2021

Ask Dr Waterline: Why was this pond pink?

Dear Dr Waterline,

As I was driving by the local swamp near Klahanee a few winters ago, I saw that the water surface was colored bright pink! I was worried someone had dumped paint into the water and thought about calling Washington Department of Ecology to report a spill. However, the next time I drove by there, it was gone, so I never did. Should I have called to report? Here are some pictures I took at the time.


Concerned Citizen

Dear C.C.,

Thanks for being such a good observer! Paint spills can be toxic to wildlife and are always good to report so they can be investigated. However, in this case, what you saw was likely a native water plant bloom rather than a human-caused spill, because King County received and investigated a similar report about the same time you saw the pink swamp.

What they found was a dense population of the tiny, floating water fern called Azolla mexicana or “mosquito fern,” which can be found from western North America through Central America, mostly in quiet bodies of water such as ponds, protected freshwater coves, and wetlands. While normally green — when it may be mistaken for duckweed –it can become bright pink or lipstick red in strong sunlight or when under stress. Sunny, cold winter conditions could definitely cause this to happen, and a dense population might easily look like a paint spill — as you can see from these photos of a wetland near Issaquah a few years back!

Another cool thing about mosquito fern is that, inside its fronds, it shelters a cyanobacterium named Anabaena azollae that fixes nitrogen from the atmosphere, acting as a nutrient for the fern and thus helping out both species.

So, you can enjoy that bright pink if you see it happen again. It would be a welcome sight during our typical dark, gray winters.

Thanks for the question, and keep looking at our waters! 

Dr Waterline