Sargassum is a floating macroalgae or seaweed primarily found in the Caribbean, Gulf of Mexico, and South Atlantic regions of U.S. waters, where it forms a unique and highly productive floating ecosystem on the surface of the ocean.

Sargassum aggregates as offshore drift lines and patches that become important habitat for wildlife and fisheries resources. Some shrimp, crabs, and fish are specially suited to life in Sargassum. Certain species of eel, fish, and shark spawn there. Each year, humpback whales, tuna, and seabirds migrate across these fruitful waters, taking advantage of the gathering of life that occurs where ocean currents converge.

An abundance of marine life isn't the only other thing that can accumulate with these large patches of Sargassum. Spilled oil, carried by currents, can also end up swirling among the seaweed and affecting this rich ecosystem. The effects can be toxic and deadly.

Sargassum can also affect human-use features, as it did on the island of St. Croix in the U.S. Virgin Islands where, in July 2022, accumulations of Sargassum clogged water intakes for a desalination plant that was the island’s primary source of water. In addition, Sargassum can be mistaken for oil when aerial observers are viewing the ocean surface or shorelines.

The new report describes Sargassum ecology; impacts on and from Sargassum; oil spill response considerations; as well as summaries of three case studies, focusing on different types and scales of incidents that involved Sargassum.

The report was funded by the OR&R Disaster Preparedness Program through a Lagniappe award to the OR&R Emergency Response Division.