California Sea Grant College Program

Rhodoliths are free-living coralline algae (Rhodophyta) that form large beds on the seafloor. Rhodolith beds are globally widespread and biologically diverse shallow marine habitats. Beds are ecologically sensitive, disturbed by humans, and in Europe are protected by law. While rhodolith beds have been found in California waters at Catalina Island, no literature exists regarding their distribution or ecological significance. This study sought to (1) map the distribution and characterize the rhodolith beds around Catalina, (2) determine the seasonal growth rates, and (3) investigate the effect of common sources of disturbance. A systematic search of shallow subtidal (0-40 meters) areas revealed seven beds. These were mapped by divers using SCUBA. Living and dead rhodolilths and rhodalgal sediments covered approximately 22,900 and 42,696 square meters of seafloor, respectively. Percentage cover, and mean size of living rhodoliths varied across beds. Growth rates varied seasonally but were consistent with global averages; branches grew just over a millimeter a year. Vessel mooring chains were a common source of disturbance, and decreased 3-dimensional rhodolith habitat and living rhodolith cover. More benthic fauna were seen in living beds compared to dead, impacted rhodolith sediments or adjacent sandy benthos. Highly disturbed and slow to recover, this novel habitat should be considered a high priority for future protection, monitoring, and restoration efforts.