December 21, 2015

Invasive Seaweed Sargassum horneri Spreading in Southern California

Reef Check divers recording the presence of S. horneri at Catalina Island in the spring of 2015

By Charleen Conlogue, Reef Check California Southern California Volunteer Coordinator

A recent scientific paper in the Journal 'BioInvasion Records' documented the rapid increase and spread of an invasive alga named Sargassum horneri along the coast of Southern California and down into Baja California, Mexico. The seaweed, which is native to the west coast of Japan and South Korea, was first detected in Southern California in 2003 when it was spotted near Long Beach. Reef Check has been tracking this species since 2006 and the data collected by our citizen scientists was used as the basis for this publication. The paper states: “The geographic expansion of S. horneri is characterized by isolated introductions to new islands and locations on the mainland widely separated from existing populations, followed by the steady colonization of surrounding areas.” According to co-author Dr. Jan Freiwald, who is also the Director of Reef Check's California Program, Reef Check's consistent monitoring of the kelp forest ecosystems throughout Southern California allowed us to track changes in the S. horneri distribution.

The rapid spread of S. horneri is affected by both natural dispersal and humans moving individuals unintentionally. For example, divers or boats dislodging algae or fouling of vessels might transport algae to new locations. Based on the water temperature that S. horneri typically prefers, researchers predict further spread of S. horneri along the Baja coast, and if the ocean warms in the future, it may cause S. horneri to continue expanding northward along the California coast.

In response to the rapid spread of this invasive algae, Reef Check California recently revised its monitoring protocol so that our citizen scientists are not only recording the presence or absence of this species at our monitoring sites, but also quantifying the amount of S. horneri found at each site. With 90 monitoring sites statewide and three monitoring regions in Baja, Reef Check California will continue to look for this invasive species and report on its spread. Reef Check's observations of S. horneri can be found on our online database – the Global Reef Tracker.

Citation: Marks, L. M., et al. (2015). Range expansion of a non-native, invasive macroalga Sargassum horneri (Turner) C. Agardh, 1820 in the eastern Pacific. BioInvasions Records, 4(4), 243-248.

Chronology of geographic expansion of S. horneri in the southern California, USA (A) and Baja California, México (B) regions