Seaweed Blob Arrives

A 15 million-ton, 5,000-mile-wide patch of seaweed known as the Great Atlantic Sargassum Belt is making landfall throughout the southern Atlantic and Caribbean coasts, disrupting vacations and marine activities as it collects and rots along the shore. An annual open-sea phenomenon, this year's crop is the largest on record (see map) and is expected to inundate shorelines for months.

While the macroalgae have been observed in the central Atlantic—an area known as the Sargasso Sea—for centuries, its levels were inconsequential until 2011, when scientists first noted a spike in growth. The yellowish-brown organism is kept afloat by oxygen-filled bladders the size of berries, providing shelter for small marine life.

Its exponential growth, likely due to an uptick in nutrient-rich wastewater flows from the Mississippi, Amazon, and Congo river systems, now presents a nuisance and potential hazard to sensitive beachgoers at a steep cost to cities looking to keep the beaches clear. Watch a breakdown of the algae here.