Sargassum is it a Weed?

What makes a weed a weed? It grows fast, it takes up resources of other native species thereby reducing diversity, and it causes ecological problems because it is so abundant – sometimes called a “monoculture”. Did you know there are “weeds” in the ocean, too? 

Sargassum horneri, or as it is not-so-fondly known by some California divers, “devil-weed”, is an invasive algae from Asia that has been steadily taking over Catalina since 2006, when it was discovered by divers who began a citizen science project to document its abundance. In some places, including our coves here on the leeward side of the island, it is outcompeting our native algae, including the giant brown kelp that once formed dense forests home to a diverse abundance of invertebrates and fish.

Invasive species traverse the world’s oceans by many means. The most common is through shipping channels. A ship’s hull can hold thousands of species, accumulating them in one port and depositing them thousands of miles away. Ships also hold and empty water called ballast, used to counteract the weight of offloaded cargo to keep the vessel balanced. Sargassum likes shallow, temperate rocky reef habitats, and as of today can be found as far north as and as far South as Baja California, Mexico. and as ocean temperatures rise, it is predicted to continue to expand its range. Sargassum grows ten feet tall and forms “fields” on the ocean floor, and thick carpets on the water’s surface when its holdfasts, root-like structures that keep it anchored to the rocky bottom, break free. While sargassum is displacing kelp populations, it is also increasing fish abundance.