The Effects of an Invasive Alga on the Trophic Dynamics of Temperate Rocky Reefs

Invasive species can alter habitats dramatically when they thrive in foreign conditions. Sargassum horneri is an invasive alga from the northwestern Pacific that is flourishing in the waters off Santa Catalina Island, California. The effects of this alga’s presence on higher trophic levels, such as fish, is unknown. This study quantified the foraging rates of three common reef fishes (Halichoeres semicinctus, Hypsypops rubicundus, and Semicossyphus pulcher), the abundance of small invertebrate epifauna the fish prey upon associated with understory algae, including S. horneri, and the concentration of total phenolic compounds these algae possess. Foraging observations were made via SCUBA at six sites along the leeward side of Catalina Island. Three of the most abundant native algae and S. horneri were collected at all six sites, rinsed of epifauna, and used for phenol extractions. Fishes foraged at higher rates among the native Sargassum palmeri, than the invasive S. horneri, even at sites with low algal diversity, and epifaunal abundance was greater on the native algae. Total phenol concentrations of S. horneri were greater than S. palmeri but were similar to Dictyopteris undulata and Zonaria farlowii. Our results indicate that the invasive S. horneri negatively impacts the foraging of fishes, perhaps due to the low abundance of epifauna inhabiting this alga, which may be due to the increase in phenol concentrations. These results suggest that the increasing prevalence of S. horneri on rocky reefs in southern California and Baja California may impact higher trophic levels in invaded ecosystems.