Sargassum is a rust-colored seaweed that floats. In the Atlantic Ocean, the 2 dominant species have now expanded so much, likely due to agricultural runoff from the Amazon or Mississippi, that blooms practically stretch from the Gulf of Mexico to West Africa. The seaweed provides a habitat for 100s of species of fish and hatchling sea turtles, seahorses, crabs, shrimps, snails, and nudibranchs. But excessive amounts of it are washing ashore and blanketing beaches, with significant environmental and economic consequences.

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