Rhodolith structural loss decreases abundance, diversity, and stability of benthic communities at Santa Catalina Island, CA

Species that increase habitat structural complexity often have a disproportionate influence on their ecosystems. Rhodoliths are bed-forming unattached coralline algae which increase benthic structural complexity and enhance biodiversity in coastal soft-bottomed ecosystems worldwide. Consequently, their degradation due to anthropogenic disturbance, such as crushing from boat mooring chains, may lead to reduced biodiversity and ecosystem functioning. To examine how anthropogenic disturbance impacts rhodolith community dynamics, we used a comprehensive sampling and analytical approach to compare macroalgal, invertebrate (infauna and epifauna), and fish assemblages between rhodolith beds and adjacent mooring-disturbed crushed rhodolith sand. Sampling was conducted during 2 sampling times across 3 sites at Catalina Island, CA, USA. Our results demonstrate that the more heterogeneous structure provided in less disturbed rhodolith beds supported greater community richness and abundances than crushed rhodolith sands. Specifically, disturbance-related rhodolith structural loss was associated with significantly reduced richness of invertebrates and abundance of macroalgae, invertebrates, and fish. In particular, deposit-feeding infaunal tanaids were far more abundant in rhodolith beds and drove much of the difference in invertebrate abundance between habitats. Further, spatiotemporal variation in the infaunal invertebrate assemblages was 54 % lower in the rhodolith beds than crushed rhodolith sand, suggesting that rhodolith beds support more stable communities. Our results suggest that structured rhodolith bed habitats support more abundant, diverse, and stable benthic communities than do disturbed rhodolith sand habitats. Better management of rhodolith ecosystems and the factors that disturb them could help maintain coastal biodiversity and stability.