‘Gross’ algae on North Carolina lake is actually thousands of molting flies, expert says

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An investigation into something strange growing atop North Carolina’s High Rock Lake proved to a “gross and scummy” collection of insects, according to Yadkin Riverkeeper.

“YRK responded to a possible fish kill on High Rock Lake yesterday,” the environmental nonprofit wrote July 1 on Facebook.

“Turns out it wasn’t a fish kill or harmful algal bloom (HAB), but thousands of molting Mayflies ... shedding their casings. ... It’s gross & scummy, but doesn’t appear to be a threat to water quality.”

Photos shared by Yadkin Riverkeeper show the growth is thick and yellowish green, and it’s “piling up at the end of a cove.” A closeup of the fly casings revealed they resemble decomposing fish, the agency reported.

The North Carolina Division of Water Resources sent experts to investigate and take samples, officials said.

It’s not clear how long the molting will continue.

High Rock Lake is a reservoir that counts as the state’s second-largest lake, covering 15,180 acres with 365 miles of shoreline, according to VisitNC.com. It is the boundary between Davidson and Rowan counties, northeast of Charlotte.

Yadkin Riverkeeper Executive Director and Riverkeeper Edgar Miller told The Winston Salem-Journal mayfly molting is a natural process, “though it is unclear whether it is normal for it to happen at this scale.”

The mayfly spends most of its life in water and their sensitivity to pollution provides “clues about the health of our streams and other waterways,” according to N.C. State University.

“Not much is known about how or when they shed their skins, or molt, in the water, but mayflies are unusual in that they molt a lot more — 14 to 50 times more, depending on the species — than most other insects,” the university reports.

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