An uneven rhodolith distribution controlled by sea-bottom conditions near Jeju Island, Korea

Actively growing, dense rhodolith beds are found in the wide channel between the volcanic Jeju and Udo islands in Korea. This shallow area (water depths of up to 25 m) has unique geological features affected by the marine hydraulic regime. Spheroidal and sub-spheroidal, living or dead rhodoliths with the size range from sand to pebble are common (mostly 2–4 cm in diameter), but some of them are actively growing up to over 10 cm. Most sand- to granule-sized sediments are characterized by nonliving fragments, and the pebble-sized sediment beds are a mixture. In order to understand the factors affecting the characteristics (morphology, distribution, the living portion of rhodoliths, etc.) Seabed topography and sedimentary texture associated with hydraulic energy were investigated. We divided the study area into four types based on topographical properties. Among these classifications, the “Flat plain” and “Gentle slope” geomorphic categories have dense rhodolith beds, whereas a low distribution of rhodoliths was identified in areas with uneven topography or extreme high hydraulic energy flux. In addition, rhodolith growth was affected by the size of carbonate grains on the seafloor, and rhodoliths with more spheroidal shapes tend to occur on flatter floors. This study delineates that the substrate with coarse-sized substrate at relatively deeper water depth as well as flatter bottom topography provides favorable conditions for continuous growth of rhodoliths. Our findings will help the understanding of the growth conditions of rhodoliths in other parts of the world.