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.