Assessing the effectiveness of artificial structures as a monitoring tool for benthic diversity in temperate reefs is crucial to determining their relevance in reef conservation and management. In this study, we utilized Autonomous Reef Monitoring Structures (ARMS) to evaluate sessile benthic communities that colonized ARMS units after 12 and 34 months of immersion within distinct habitats (coral-dominated and macroalgae-dominated habitats) in Jeju Island, Korea. We used two methods: image analysis of the ARMS plates and DNA metabarcoding of the ARMS units. We found significant differences in the sessile benthic community between the plate faces, installation periods, and habitats. DNA metabarcoding also revealed differences in sessile benthic diversity among habitats. Additionally, we identified the Lithophyllum genus within the crustose coralline algae community, whose dominance might trigger a transition to coral-dominated habitats in Jeju Island. We recommend integrating ARMS image analysis with DNA metabarcoding to enhance and complement studies focusing on benthic diversity. By utilizing ARMS, this study provides valuable information for understanding sessile benthic communities and biodiversity, contributing to an enhanced understanding of the responses of ecological communities to climate change.

Coastal benthic communities in temperate regions have been influenced by climate change, including increasing sea-surface temperature. Nevertheless, scleractinian coral Alveopora japonica Eguchi, 1968, is thriving in shallow subtidal hard bottoms around Jeju Island, off the southern coast of Korea. The presence of this corals has negatively impacted subtidal kelp populations in Jeju Island. However, there is no study to document how the presence or absence of this coral relates to other benthic communities. This study investigated the benthos in three shallow subtidal sites (Shinheung (SH), Bukchon (BC), and Seongsan (SS)) in northern Jeju using underwater photography. Macro-benthic organisms appearing on a 1 × 20 m line transect installed at depths of 5, 10, and 15 m at each site were analyzed. Results showed that of the three sites investigated, A. japonica colonies were most abundant at BC, accounting for 45.9% and 72.8% of the total transect area at 10 m and 15 m, respectively. At SS, A. japonica occupied 15.3% of the total area at 15 m and less than 1% at 5 m and 10 m. The same at SH accounted for 10% of the total area at 5 m, and less than 1% at 10 m and 15 m. Dead and bleached colonies accounted for 1.2–11.5% and 1.8–5.7%, respectively, at 5, 10, and 15 m at three sites. At SS, canopy-forming brown algae Ecklonia cava and Sargassum spp. accounted for 20.2 and 24.3% of the total transect area, respectively, at 5 m depth. In contrast, the percent cover of E. cava and Sargassum spp. at SH and BC ranged from 0.1 to 1.8%, respectively. Moreover, non-geniculate coralline algae dominated the subtidal substrate at SH, ranging between 60.2 and 69% at 15 and 10 m. The low cover of A. japonica in SS (at 5 m) coincided with a high percent cover of canopy-forming brown algae. However, canopy-forming brown algae were rare at all depths at SH and BC and were dominated instead by coralline algae and the scleractinian corals. This study, by utilizing a non-destructive method, provides a baseline qualitative and quantitative information for understanding the site and depth-dependent distribution of A. japonica and algal populations, which is important to understand climate change related changes in benthic communities in Jeju and elsewhere.

This study was carried out to determine the levels of resistance and resilience of kelp forests to large-scale physical disturbances. Our study site, Seongsan, Jeju Island, was impacted by super typhoon ‘Hinnamnor’. Before the typhoon, Seongsan had shown high ecosystem stability. Our results indicated that the ecological stability of a kelp forest facing a severe typhoon is strongly linked to the prevailing environmental conditions. Although typhoon impact resulted in a significant loss of brown macroalgae canopy, Ecklonia cava remained dominant within the kelp forest community. Resistance and resilience levels strongly depended on water temperature and movement and presence of turf-forming algae. Hence, hydrodynamic and biological factors strongly influence the overall stability of a kelp forest. We also report the first occurrences of a scleractinian coral species (i.e., Montipora millepora) at Seongsan, which became visible after canopy loss following the typhoon. Our findings provide valuable ecological information about the benthic community of kelp-dominated ecosystems and are essential to mitigate the impacts of expected climate change-driven rises in seawater temperature and the frequency of super typhoons.

SUMMARY: Lithophyllum Philippi (Corallinales, Corallinophycidae, Rhodophyta) is a diverse genus of non‐geniculate coralline algae. Lithophyllum neo‐okamurae and L. okamurae are commonly found in the intertidal and subtidal zones of Jeju Island, Korea, where they contribute to local benthic communities. Despite their ecological importance, their morpho‐anatomical and genetic variation remain poorly understood in this region. We investigated these two species through morpho‐anatomical and molecular analyses ( psb A and rbc L) based on specimens collected from 13 sites around Jeju, Korea. Lithophyllum neo‐okamurae exhibited both unattached, free‐living (rhodolith) and attached forms, each showing distinct distributions associated with local hydrodynamic and substrate conditions. Molecular analyses revealed distinct haplotypes corresponding to the two forms, and morpho‐anatomical analyses detected statistically significant differences in tetrasporangial conceptacle chamber dimensions (diameter and height) between them. In contrast, L. okamurae occurred exclusively as unattached rhodoliths and was mainly distributed in deeper subtidal rhodolith beds compared with L. neo‐okamurae . Our findings demonstrate form‐related morpho‐anatomical and genetic differentiation in L. neo‐okamurae and underscore the ecological role of L. neo‐okamurae and L. okamurae as major rhodolith‐forming species in Jeju, Korea.

Lithophyllum is a highly diverse and widely distributed genus of non-geniculate coralline algae. Despite its ecological importance, a comprehensive taxonomic treatment of Lithophyllum species in Korea remains lacking, primarily due to substantial unresolved cryptic diversity within the genus. In this study, we describe Lithophyllum jejuensis based on an integrated taxonomic approach combining molecular phylogenetic analyses with detailed morpho-anatomical observations of specimens from Korea. Phylogenetic analyses of plastid markers ( psb A and rbc L) resolved L. jejuensis as a distinct lineage. Morpho-anatomically, the species exhibits a multilayered, encrusting growth form with successive applanate branches, bistratose margins, and predominantly palisade basal (hypothallial) cells. Lithophyllum jejuensis resembles L. prototypum (originally described from the U.S. Virgin Islands), but differs from the holotype in having widely triangular tetrasporangial conceptacle pore canals that taper toward the thallus surface. Additionally, L . jejuensis can be distinguished from Korean L. neo-okamurae and L. okamurae by an exclusively adherent, encrusting growth form with applanate branches, bistratose margins composed of palisade cells, and larger tetrasporangial conceptacle chamber dimensions. These results reaffirm the importance of integrative taxonomy in elucidating the cryptic diversity of Korean Lithophyllum .