Thermal Tolerance May Slow, But Not Prevent, the Spread of Sargassum horneri (Phaeophyceae) along the California, USA and Baja California, MEX Coastline

Biological invasions have become increasingly prevalent in marine ecosystems, modifying biodiversity and altering the way ecosystems function. Understanding how variation in environmental factors influences the success of non‐native species, especially their early life stages, can be a crucial step in identifying habitats that are under threat of invasion, and in predicting how rapidly and far these species may spread once they arrive in novel habitats. The invasive marine macroalga Sargassum horneri was first observed in Long Beach Harbor, CA, USA in 2003, and has since spread throughout the Southern California Bight and along the Baja California Peninsula, MEX where it now forms dense stands on subtidal rocky reefs and displaces native habitat‐forming macroalgae. We examined how variation in temperature, nutrients, and irradiance affect survival, growth, and development in S. horneri early life stages over a three‐week period. Our experimental treatments consisted of orthogonally crossed temperatures (10, 15, 20, and 25°C), nutrient concentrations (ambient and nutrient‐enriched seawater), and irradiances (50 and 500 µmol photons · m −2 · s −1 ). Overall, temperature exerted the greatest influence on S. horneri ’s germling and juvenile life stages, with moderate temperatures facilitating their greatest survival, growth, and development. In contrast, fewer germlings developed fully under the lowest or highest temperatures, and juvenile survival and growth were reduced, especially when combined with low irradiances. Together, our data suggest that ocean temperatures of or below 10˚C and of or above 25°C may slow, but likely not stop, S. horneri ’s northward and southward expansion along the California and Baja California coasts.