It looks like a clump of soiled sheep’s wool, a cottony green or white mass that’s turning up on rocks and river bottoms, snarling waterways. Already a scourge in New Zealand and parts of the American South and West, the aquatic algae called “rock snot” is creeping into New England, where it is turning up in pristine rivers and alarming fishermen and wildlife biologists.

The algae has the potential to bloom into thick masses with long stalks, blanketing the bottoms of some streams, threatening aquatic insect and fish populations by smothering food sources.

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