Heavy Sargassum Year in Keys

sargassum Boca Chica

This summer, you may have noticed tons of brown organic material washed up on beaches and decaying all over the Atlantic side of the Florida Keys. But what is it, really? And why is it appear­ing on our beaches?

It’s called sargassum, and it is brown algae, or seaweed, that floats in massive mats out on the open ocean. The area sargassum comes from is called the Sargasso Sea, which is not a true sea at all, but an area far offshore in the Atlantic Ocean between the eastern coast of Florida and Bermuda. 

The Sargasso Sea and the mats of sargassum that float within it are defined by the dominant ocean currents – the Gulf Stream on the west, North Atlantic Current to the north, Canary Current on the east, and the North Atlantic Equatorial Current to the south, which creates a swirling gyre that contains the sargassum floating out at sea. Frequently, the sargassum gets caught up in one of the currents and escapes the Sargasso Sea. 

The last time the Florida Keys experienced a heavy sargassum loading year was 2015. The University of South Florida Satellite-based Sargassum Watch System has predicted 2018 will be a major bloom year, which is already impacting the Caribbean.

If you like to fish, you may have heard these floating algae mats called the “weed line,” and they are fantastic spots to sportfish. Sargassum is actually a very important habitat type out in the open ocean as it provides one of the only places for some species of fish, crabs, and juvenile loggerhead sea turtles to forage and hide from predators. A thick clump of sargassum provides a feast for many species, including birds, and also attracts the larger gamefish who prey upon them, like mahi and tuna. NOAA Fisheries considers sargassum essential fish habitat for snapper, grouper, gray triggerfish, and coastal pelagic migratory species like mackerel.

In the Florida Keys, when we experience prevailing winds from the south and southeast, these massive floating mats of sargassum that have broken free from the Sargasso Sea’s circling gyre are washed onto shore with the wind and waves. Tons and tons! Unfortunately, the dead and decaying sargassum can remove oxygen from the water and lead to fish kills, smother seagrasses and turtle nests, and can become packed so thick inside the residential canals it can become difficult to get your boat out. 

Historically, washed-up sargassum is one of the ways beaches were created in the Florida Keys, as the accumulation of seaweed along the shoreline helps to keep the sand from eroding and provides nutrients to help en­rich the soil. But when the sargassum encounters a seawall or a canal instead of the beach there is little benefit for it decays, sinks, and stinks! Unfortunately, this is a major cause for fish kills because the decomposition of organic matter literally removes the oxygen from the water.