Abstract:
Introduction: Jeju Island, on the southern coast of Korea, has recently
experienced increases in high-latitude scleractinian corals, accompanied
by shifts in hard-bottom benthic communities. However, the local
distribution and condition of the high-latitude corals Montipora millepora
and Alveopora japonica at Seopseom remain poorly resolved despite
ongoing benthic change. We tested the working hypothesis that their
occurrence and condition are structured primarily by depth-related habitat
filtering, expressed through variation in light environment, hydrodynamic
exposure, and available hard-bottom benthic cover, rather than by
demonstrated competitive interaction alone.
Methods: Depth-related patterns in epibenthic cover and the occurrence
of M. millepora and A. japonica were quantified using underwater
photographic transects of 20 m length at 5, 10, and 15 m depth at
Seopseom, southern Jeju Island, in 2016 and 2017. Benthic cover
categories and coral occurrence, condition, and colony size-frequency
patterns were assessed across depths to evaluate habitat-associated
distribution patterns.
Results: Sargassum sp. dominated the shallow 5 m assemblage and
declined with depth, whereas filamentous turf algae and Ecklonia cava
increased with depth. Coralline algae, including geniculate and crustose
coralline algae, showed non monotonic variation across depths.
Montipora millepora occurred at all surveyed depths, whereas A.
japonica was recorded only at 10–15 m. Colony size frequency
distributions of both species were dominated by small colonies,
suggesting recent recruitment at the study site.
Discussion: The observed depth-related patterns support the interpretation that habitat filtering, and substrate context are central to structuring the
distribution of M. millepora and A. japonica at Seopseom. Competition
for hard substrate remains a plausible mechanism, but it was not directly
tested in this study and requires explicit contact-frequency, spatial co-
occurrence, or interaction-based analyses. These results provide a
hypothesis-driven baseline for future monitoring aimed at resolving the
ecological and environmental drivers of ongoing benthic community
change on Jeju Island.
