Almost everything about Southampton is tony. Its residents (Calvin Klein, David Koch, Tory Burch). Its branzino (especially the $53 oven-roasted version from Sant Ambroeus). And especially its real estate, where a 2,000-square-foot cottage on a quarter of an acre in the village is currently listed for $2.4 million.

But there is one rather significant blight on this charmed corner of civilization: cyanobacteria. This blue-green algae is blooming and blooming and blooming some more in the once majestic Agawam Lake, which lies at the center of the village, connecting its downtown area to the southern coastline.