I had bookmarked information on "watermelon snow" seven months ago, but it wasn't until today that I found a proper article with good photos, in Smithsonian Magazine:
And with these unprecedented temperatures [in Antarctica], the algae that normally thrive in freezing water and lie dormant across the continent’s snow and ice are now in full bloom and cover the Antarctic Peninsula with blood-red, flower-like spores...
This red-pigmented algae, also known as Chlamydomonas nivalis, has the potential to jumpstart a feedback loop of warming and melting... “Because of the red-crimson color, the snow reflects less sunlight and melts faster. As a consequence, it produces more and more bright algae.”
"Blood red” snow has been observed many times before. Aristotle noticed this phenomenon in the third century B.C., reports Brandon Specktor of Live Science. In 1818, Captain John Ross found pink snow during his expedition through the Northwest Passage...
But this type of algae is actually a member of the green algae family. It won’t turn red until the weather warms up, the cell’s carotenoids—the same pigment that gives pumpkins and carrots their orange hue—absorb heat and protect the algae from ultraviolet light...
This is often visible on summer snow patches in the Rockies, albeit in less concentrated amounts.
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