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Project description

Impacts and control of an invasive seaweed in California marine protected areas. (02XN020)
Program Exotic Pests and Diseases Research Program
Principal
investigators
J.H.R. Goddard, Marine Science Institute,
C.A. Blanchette, Marine Science Institute
Host/habitat Marine Area
Pest Strangleweed Sargassum muticum
Discipline Weed Science
Review
panel
Natural Systems
Start year (duration)  2002 (Three Years)
Objectives Experimentally investigate the ecological impacts of introduced Sargassum muticum on native tidepool biota in two popular intertidal Marine Protected Areas (MPAs) in southern California.

Study the efficacy of reducing its abundance and mitigating its impacts in these reserves.

Draw up specific recommendations and guidelines, for use by reserve managers, for developing community-based programs for controlling S. muticum in appropriate habitats in MPAs along the California coast.

Project
Summary
Accidentally introduced with oysters from Japan in the 1940s, Sargassum muticum is now the most conspicuous and abundant non-indigenous species on the outer Pacific coast of North America. It is especially abundant in tidepools and shallow subtidal zone in southern California. We will use a randomized block design, with fixed, replicated removal and control plots in the field to experimentally investigate the ecological effects of this large, canopy-forming brown alga on native tidepool biota in two Marine Protected Areas in southern California and study the efficacy of reducing its abundance and mitigating its impacts at these sites. We will then propose specific guidelines and recommendations, for use by managers and biologists at reserves in California, for developing and implementing ongoing, community-based programs for controlling S. muticum.
Final report For three years, we manually removed the large, brown seaweed Sargassum muticum from shallow tidepools at Coal Oil Point Reserve in Santa Barbara and Cabrillo National Monument in San Diego. These removals, which were conducted two or three times per year and before the alga could reproduce, resulted in sustained, ten-fold or more reductions in the average abundance of S. muticum in our experimental plots at both sites. This, in turn, resulted in significant increases in the abundance of native benthic organisms, including a large sea anemone (Anthopleura sola), red algal turf, and surfgrass (Phyllospadix torreyi). Removal of S. muticum also had a significant positive effect on the abundance of the large herbivorous sea hare, Aplysia californica, but not on carnivorous or scavenging molluscs.

After two years, summertime benthic community composition had changed significantly in the Sargassum removal plots compared to the control plots at both sites, but this difference was not observed in the wintertime composition. We found that the average maximum length of S. muticum in our removal plots was highly correlated with the length of time since the last removal, but not the average daily water temperature during that same period. By the second year of the study, the average amount of time for one person to manually remove S. muticum from our experimental plots had tapered off at less than one minute per square meter at Cabrillo National Monument and less than 30 seconds per square meter at Coal Oil Point.

We conclude that S. muticum has (1) reduced the abundance of key native species at our study sites, with significant effects on summertime community composition, and (2) can be effectively controlled in limited areas by sustained manual removal just a few times per year. Both of our study sites were on gently sloping sedimentary reefs, with scattered cobbles and boulders. Larger-scale, ongoing removal programs involving trained volunteers should be feasible and significantly reduce the abundance and impacts of S. muticum at similar sites. We are currently preparing a booklet with recommendations and guidelines for managers and biologists for developing volunteer-based programs for controlling S. muticum in appropriate habitats in Marine Protected Areas along the California coast.

Third-year
progress
We have: (1) continued quarterly sampling of our experimental plots at both Cabrillo National Monument in San Diego and Coal Oil Point Reserve in Santa Barbara; (2) completed for the current growing season two of three manual removals of Sargassum muticum from the treatment plots; (3) measured light levels in submerged surfgrass beds with and without S. muticum at Cabrillo National Monument; (4) continued data transcription, entry and analysis; and (5) have begun to prepare a booklet for reserve managers on managing S. muticum. Manual removal of S. muticum two or three times per year, before it has reproduced, has proven effective at suppressing this invasive alga at both sites. At Coal Oil Point, removal of S. muticum has resulted in significant increases in the abundance of native biota, including the large sea anemone Anthopleura sola, and in all native sessile biota combined. There has also been a significant increase in surfgrass in the removal plots that had an initially high abundance of S. muticum. At Cabrillo National Monument, common red algal turf has increased significantly following removal of S. muticum. However, surfgrass that by chance was initially less abundant in the control plots than in the Sargassum removal plots, has unexpectedly increased more in the former than in the latter. We have continued to answer questions and discuss our research with interested visitors to the reserves. Coal Oil Point, and especially Cabrillo National Monument, are visited by large numbers of people, most of whom do not realize that the ubiquitous S. muticum is introduced.

Second-year
progress
We have accomplished our goals for the second year of this project. Specifically we have continued quarterly sampling of our experimental plots at both Cabrillo National Monument in San Diego and Coal Oil Point in Santa Barbara, completed for the current growing season (the second of the project) the manual removals of S. muticum from the treatment plots, collected and identified voucher specimens of algae that we were unable to identify during field sampling, and continued data transcription, entry and preliminary analysis. Manual removal of S. muticum two or three times per year, before it has reproduced, has proven effective at suppressing this invasive alga at both sites. At Coal Oil Point, where we are in the third year of this experiment, removal of S. muticum has resulted in significant increases in the abundance of large sea anemone, Anthopleura sola and in all native sessile biota (excluding feather-boa kelp) combined. There has also been a trend toward an increase in surfgrass in the removal plots, compared to the controls. This is only our second year of removals at Cabrillo National Monument, and we don't expect to see equivalent increases in native biota there until this summer or next. Nonetheless, we are already seeing trends toward increases in a common red alga and in all native alga combined. We have continued to answer questions and discuss our research with interested visitors to the reserves. About 100,000 people per year, most of who do not realize that the ubiquitous S. muticum is introduced, visit the tidepools at Cabrillo National Monument.

First-year
progress
We have accomplished our goals for the initial phase of this project. Specifically, at Cabrillo National Monument we have: (1) Applied for and obtained a permit from the National Park Service to conduct our research. (2) Qualitatively surveyed the site for Sargassum muticum, native algae and potential study plots. (3) Marked ten permanent plots, quantitatively sampled sessile biota in those plots, and started our experiment by removing all S. muticum (a total of 102.9 kg, or 226.8 lbs) from five of the plots and their surrounding buffer zones. (4) Obtained data loggers for measuring temperature and light in our experimental plots later this season. (5) Answered questions and discussed our research project on invasive S. muticum with numerous visitors to the tidepools, including members of the general public as well as other researchers and Park personnel. The tidepools at Cabrillo National Monument are visited by about 100,000 people per year and therefore present extensive opportunities for public outreach on the problems posed by invasive species in general, and marine invaders in particular.

Combined with our research trips to San Diego in March, the PI also attended the third International Conference on Marine Bioinvasions in La Jolla, where he was co-author on one of the papers presented.

At Coal Oil Point Reserve, where our research on Sargassum muticum was already ongoing, we have: (1) continued our quarterly sampling of the five control and ten treatment (removal of S. muticum at two or three times per year) plots; (2) completed for the current growing season the removals of S. muticum from the treatment plots; and (3) continued data entry and preliminary analysis. As at Cabrillo National Monument, we have continued to answer questions and discuss our research with interested visitors to the reserve.

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