Abundance and distribution of non-native seaweed Sargassum muticum in southern California

Sargassum muticum, a brown seaweed native to Japan, was transported to Washington State (USA) via oyster shipments in the mid-1900s.  It has been present in the California rocky intertidal zone since 1970, yet to our knowledge there have been no comprehensive studies documenting the abundance and distribution of Sargassum in southern California. We surveyed eight sites along the southern California coast to evaluate the status of this non-native species. We used parallel transect lines to guide placement of randomly selected survey quadrats (0.25m²). In each quadrat, we recorded whether Sargassum was: 1) present, 2) juvenile or adult, 3) on open rock surfaces or in tide pools, 4) reproductive, and 5) holdfast and/or canopy. Both juveniles and adults were present in tide pools and on open rock surfaces across all 8 sites.  Sargassum was found more frequently in tide pools, but tide pool availability did not seem to affect the distribution of the seaweed. The seaweed’s abundance varied among sites from a low of 7% of surveyed quadrats to a high of 65% of surveyed quadrats. Multiple factors, such as sand scour, could contribute to this variation among sites. We have also collected data on temperature, light levels, and salinity in tide pools across sites and seasons.  In addition to these field surveys, we have ongoing laboratory experiments designed to investigate the effects of changes in temperature and salinity on Sargassum’s performance. Together, these studies will allow us to better understand the range of environmental conditions in which Sargassum is successful.