Dr. Jayson Smith (California State University, Fullerton) was conducting Multi-Agency Rocky Intertidal Network (MARINe) monitoring surveys at an BOEMRE-funded site, Shaw’s Cove in Laguna Beach, on October 17, 2009, when he discovered the first known specimens of the invasive seaweed Sargassum horneri (=S. filicinum) in the intertidal zone in California. Two 20-30 cm tall plants were found in the low intertidal turf zone between the mussel and boa kelp zones on a wave-exposed shelf. One of the plants was reproductive. Both specimens were collected for documentation and further study.

Previous to this intertidal record, S. horneri, native to Asia, was known only from subtidal depths of 3-19 m in several areas of southern California. First discovered in Long Beach Harbor in 2003, it appeared in Baja California in 2005 and at Santa Catalina Island in 2006 (quickly spreading around the island), with additional records from 2006 to 2009 at San Clemente Island, San Diego, Orange County, and Anacapa Island. This invasive brown seaweed was named S. filicinum until a recent scientific publication (Uwai et al. 2009) determined it to be a variety of S. horneri. It is related to S. muticum, another non-native alga that invaded California in the 1960’s.

Concern over the rapid spread and growing abundance of this new Sargassum weed prompted the formation of a S. horneri Working Group to coordinate efforts to explore control measures, document its distribution, and evaluate its ecological impacts. The Working Group, which met initially on October 19, 2009, includes representatives of MARINe, California Department of Fish and Game, Channel Islands National Marine Sanctuary, Channel Islands National Park, University of California Santa Barbara Marine Science Institute, Santa Barbara Channelkeeper, and Reef Check California.

Surveys conducted by Dr. Smith and his team this spring found a large increase in the abundance of S. horneri, with a total of 34 individual S. horneri plants in the lower portions of the  intertidal zone.  This species is currently quite abundant in the subtidal zone at this site, mixed in with S. muticum, but the high number of individuals in the intertidal zone is startling. Most of these plants were small in stature (<30 cm) as compared to subtidal plants that can reach about 1 m in length. Some of these were also reproductive. Although this species has not been observed at the other monitoring sites, it seems it is quickly spreading. There are likely a number of subtidal populations in Orange County that have not been discovered yet. In the winter, Dr. Smith found that the biomass of seaweed wrack at the sandy beach by the Newport Beach Pier was dominated by S. horneri plants, second in biomass only to Macrocystis.

Relevant Literature:

Miller, K.A., J.M. Engle, S. Uwai, and H. Kawai. 2007. First report of the Asian seaweed Sargassum filicinum Harvey (Fucales) in California, USA. Biological Invasions 9:609-613.

Aguilar-Rosas, L.E., R.  Aguilar-Rosas, H. Kawai, S. Uwai, and E. Valenzuela-Espinoza. 2007. New record of Sargassum filicinum Harvey (Fucales, Phaeophyceae) in the Pacific Coast of Mexico. Algae 22:17-22.

Uwai, S., K. Kogame, G. Yoshida, H. Kawai, and T. Ajisaka.2009. Geographic genetic structure and phylogeography of the Sargassum horneri/filicinum complex in Japan, based on the mitochondrial cox3 haplotype. Marine Biology 156:901-911.

Miller, K.A. and J.M. Engle 2009. The Natural History of Undaria pinnatifida and Sargassum filicinum at the California Channel Islands: Non-native seaweeds with different invasion styles. Proceedings of the Seventh California Islands Symposium (In Press).