June 27, 2022

Invasive Seaweed Found in California Marine Protected Area

File photo of Sargassum muticum, by Colleen Wisniewski

On May 12, 2022, a team of citizen science divers with Reef Check California observed Sargassum muticum, an invasive species of seaweed, in the State Marine Reserve at Point Lobos State Park. Sargassum muticum, also known as “wireweed,” is native to the temperate coastal waters of Japan.

Reef Check citizen science divers observed the wireweed growing near the boat ramp in Whaler’s Cove. The specimens were observed growing in several clusters of multiple individuals in shallow water, approximately 2-3 ft deep.

Whaler’s Cove is located inside the marine portion of Point Lobos State Park and the Point Lobos State Marine Reserve. This area is one of the oldest marine reserves in the state and is prized by divers for its unparalleled beauty and diversity.

Reef Check has been conducting monitoring at two sites in the Point Lobos Reserve for 17 years. Reef Check’s citizen science divers undergo specialized training to identify and quantify problematic invasive species, along with nearly 80 other species that indicate ecosystem health.

Wireweed was first documented in Washington in the early 20th century, fouling boats and fishing equipment. Wireweed is known to have established itself in locations along the US West Coast including Puget Sound, San Francisco Bay, and at locations in the open ocean in Southern California, and has previously been observed in Elkhorn Slough and along Pebble Beach, Monterey County. This is the first recorded observation in a Marine Protected Area along the open coast of Central California.

Long-term monitoring programs like Reef Check are critical for tracking species range expansions, including for invasive species, and checking kelp forest ecosystem health. Frequent ocean observations by Reef Check’s vast network of citizen science divers, coordinators, and ecologists help inform and advise policymakers, resources managers, and scientists so they can be proactive before challenges like these become a major issue.

Sargassum horneri, also known as hornweed, a separate invasive Sargassum species, has invaded countless miles of kelp forests in Southern California, particularly around Catalina Island. In 2020, Reef Check citizen science divers observed hornweed growing in Monterey at San Carlos beach. Reef Check notified the California Department of Fish and Wildlife of that observation and department divers proceeded to search the area and remove any hornweed plants that they found. Without Reef Check divers being on the lookout for invasive species, they may go unnoticed for an extended period allowing them to proliferate and potentially impact the local environment.

Invasive species can invade new territories by being carried over great distances by ships, in addition to being transported by ocean currents. Invasives can be incredibly harmful to local ecosystems. In many instances, they have no natural predators to control their populations. If left unchecked they can outcompete native species for resources like food and sunlight, and put pressure on the services the ecosystem naturally provides. This ultimately might lead to a collapse of the ecosystem especially if it is already under pressure from climate change and species extinctions.