ENN , 27 July 2006

Scientists Untangle Seaweed's Effect on Other Species

DAVIS, CA An invasive seaweed that is powered by its ability to grow fast, out-compete local species, reproduce within the first year of life, and fertilize itself can still be slowed by hand weeding.

With funding from the University of California Exotics and Invasive Pests and Diseases Research Program, Jeffrey Goddard and Carol Blanchette from UC Santa Barbara's Marine Science Institute studied the ecological impacts of Sargassum muticum on native tidepool plants and animals in two Marine Protected Areas in southern California. Marine Protected Areas are used as management tools to protect, maintain, or restore natural and cultural resources in coastal and marine waters.

Accidentally introduced with oysters from Japan in the 1940s, Sargassum muticum is now one of the most conspicuous and abundant non-native species on the outer Pacific coast of North America, and is especially abundant in tidepools and shallow subtidal zones in southern California.

The nomadic seaweed is large and yellowish-brown with distinctive small, spherical float bladders and small blades. It grows on rocks, shells or other hard objects.

The seaweed forms dense stands and may compete for space and light, increase sedimentation, and reduce nutrients available for native kelp species. The species, sometimes referred to as "wireweed," has also become a nuisance in recreational waters by fouling propellers, nets and fishing lines. The invader becomes detached and forms large floating mats in waterways and marinas.

Once established in a region, the seaweed spreads and invades new areas largely via the drifting of reproductive fragments from attached plants. The seaweed reproduces during the winter and spring in the southern part of its Pacific Coast range. The reproductive bodies, containing both male and female reproductive organs, grow near the outer ends of the branches in the middle part of the plant. A plant 2 m tall can produce up to a billion embryos, and detached, floating fronds can remain reproductive for months.

First found in California in 1963 in Crescent City, the invader has since spread to Humboldt Bay, Tomales Bay, San Francisco Bay, Elkhorn Slough, Monterey, Carmel Bay, Santa Barbara, Los Angeles and Long Beach harbors, Palos Verdes Peninsula, Santa Cruz Island, Anacapa Island, San Nicholas Island, Santa Barbara Island, Santa Catalina Island, San Clemente Island, Orange and San Diego Counties, including Crystal Cove, San Onofre, La Jolla, and Mission and San Diego bays.

Chemical methods using herbicide have been tried but failed due to lack of selectivity and the large doses needed. Whatever method is used, the alga always quickly regrows, and effective methods for its permanent removal have not been found.

Goddard and Blanchette conducted their studies at Coal Oil Point Natural Reserve and Cabrillo National Monument. Coal Oil Point is located adjacent to the UC Santa Barbara campus. The preserve protects a wide variety of coastal and estuarine habitats. Largely undisturbed coastal dunes support a rich assemblage of dune vegetation, while older and more stable backdunes are covered with southern coastal scrub habitat. In the heart of the reserve, Devereux Slough is a seasonally flooded tidal lagoon that dries out in the summer to form salt flats and salt ponds and channels. A variety of intertidal habitats are found along the sandy beach and the large rocky reef at the point.

Cabrillo National Monument is in San Diego on the Point Loma peninsula. The 160-acre stretch of native habitat is one of the best remaining examples of a coastal-strand environment which is characterized by wave-pounded reefs and beaches, shifting sands, relentless winds, and saline soils. The subtidal zone is an underwater forest with large kelp that provides food and shelter for snails, urchins, abalone, sea stars, kelp bass, sheephead, and octopi.

As part of a three-year experiment, Goddard and Blanchette manually removed (by carefully scraping the holdfasts) all Sargassum muticum in replicated plots averaging 6 square meters in area. "By doing this three times annually, before the seaweed could reproduce, we achieved sustained, ten-fold or more reductions in the average abundance of the seaweed in our plots at both locations," says Goddard. "These reductions, in turn, increased the abundance of a large sea anemone, red algal turf, sea hares, and surfgrass. The seaweed can be effectively controlled in limited areas by sustained manual removal just a few times a year."

Larger-scale, ongoing removal programs using interested visitors to the reserves as trained volunteers are proposed. The researchers are drawing up specific recommendations and guidelines for reserve managers for developing community-based programs to control the seaweed.

The UC Exotic/Invasive Pests and Diseases Research Program funded this three-year project. The program targets research on exotic pests and diseases in California. Its aim is not only to improve our knowledge and management of pests that are already here, but also to reduce the potential impact of those pests and diseases that pose a threat to the state. The program is collaboration between the UC Statewide Integrated Pest Management Program and the UC Riverside Center for Invasive Species Research. U.S. Department of Agriculture, Cooperative State Research Education and Extension Service, funds the program.