Living carbonate habitats in temperate California (USA) waters: distribution, growth, and disturbance of Santa Catalina Island rhodoliths

ABSTRACT: Rhodoliths are globally widespread, free-living coralline red algal nodules (Rhodophyta). Living beds of rhodoliths create carbonate-based biogenic benthic habitats, which are sensitive to human disturbance and slow to recover. Despite their ecological importance, few quantitative assessments of the distribution and anthropogenic impacts on rhodoliths exist for the eastern Pacific Ocean. Following historical reports of beds in the Channel Islands off California, USA, we mapped bed distributions, examined rhodolith habitat sensitivity to human disturbance, and determined seasonal growth rates to assess rhodolith recovery potential at Santa Catalina Island. Rhodolith material from 7 beds covering 23000 m2 of live rhodoliths and 43000 m2 of dead rhodolith sediment were mapped. Beds were patchy, dominated by small Lithothamnion australe rhodoliths (mean ± SD: 10.6 ± 0.8 mm in diameter), and 6 beds were disturbed by mooring arrays. Surveys of mooring frequency and experimental manipulation of mooring chains showed that the cover of live rhodoliths, their physical structure, and the diversity and abundance of associated benthic species were negatively related to this prevalent anthropogenic disturbance. Axial growth rates of Santa Catalina Island rhodoliths were relatively slow, seasonally variable, and similar to those reported in other rhodolith studies (annual mean: 1.25 ± 0.62 mm yr-1). Due to their importance as biogenic habitat, sensitivity to disturbance, and slow recovery potential, California rhodolith beds should be considered a priority for monitoring efforts and restoration and should be included within marine protected area planning.