Sargassum Invasion: Threatening Reefs and Reshaping Southern California’s Seas

An invasive seaweed called Sargassum horneri has sprouted near Catalina Island before, but nothing like this.

Two waves of the algae from Asia struck Catalina after it first arrived in Southern California a decade ago, but those were blips compared with the thick carpet of sargassum that now dominates reefs on the mainland side of the island where kelp once grew.

And though sargassum – a thick, weedy algae that grows 10 feet tall and looks, according to one diver, like fields of wheat – is most prevalent at Catalina, its range isn’t limited to the Channel Islands. Sargassum now grows off mainland beaches from Santa Barbara to Baja California.

In Orange County, pockets of sargassum – sometimes called “devil weed” – have been spotted at Little Corona del Mar, Crystal Cove, Los Trancos, Monarch Bay and near Shaw’s Cove in Laguna Beach.

“In some locations, it’s taken over entire reefs. In Catalina, you’d be hard-pressed to find any place that doesn’t have it,” said Lindsay Marks, a grad student at UC Santa Barbara who is studying Sargassum horneri.

Although scientists say it appears that sargassum prefers clearer, calmer, more light-filled waters like those near Catalina, they also say the algae is along the mainland to stay.

When it was first discovered in 2003 and first studied in 2006, divers wanted it removed. Nothing was done. Now scientists agree that reefs along Orange County’s coast and Southern California will look different for years to come.

The only questions are how much sargassum will grow and what impact it will have on other parts of the marine ecosystem.

So far, sargassum already has displaced kelp on reefs. This year, a hot summer and warm fall decimated the giant kelp forests in Southern California, and sargassum sprouted on those barren rocks. But some believe it may help increase fish populations.

PLANT SELF-REPRODUCES

Sargassum’s life cycle is ideally suited for taking over. The algae reproduces annually, and each plant self-fertilizes, producing both male and female parts and making it possible for one individual to colonize an area.

“You’ll see density recruits of up to 1,000 individuals per square meter,” Marks said. “It’s just enormous numbers. It’s likely those are coming from a couple parent plants.”

Marks is one of the few researchers studying sargassum, which has received scant attention from academics and government regulators despite existing in California for more than a decade.

Maybe 10 years ago, it would have been possible to remove it, said Kathy Ann Miller, curator of algae at UC Berkeley’s herbarium, who did some of the first research on sargassum in 2006. But now it’s too late.

“It’s a conspicuous plant. It takes up a lot of room, and that’s important, and it’s very likely we haven’t seen the end of its story in California. It will expand its range,” Miller said. “It must be having an effect. Just what those effects are, positive or negative, is difficult to distinguish.”

Sargassum spreads by drifting along ocean currents or with the aid of man-made technology. The first sargassum that arrived in Long Beach Harbor from Asia likely came by ship, either stuck to the hull or floating in the bilge water. The algae then likely spread to the Channel Islands through boat traffic.

“It really easily gets caught in anchors. People pull up anchor and it ends up on the boat deck and can get washed off when they drop anchor again,” Marks said.

Right now – winter and early spring – are peak reproductive times for sargassum. When the seasons change and summer starts to heat up, sargassum recedes before beginning its growth and reproductive cycle in the fall.

The real danger comes when sargassum’s seasonal reproduction coincides with the reproductive timing of other perennial species such as kelp.

View original article at: ‘Devil weed’ invades O.C. waters