Influence of submarine topography and associated sedimentary processes on the distribution of live and dead rhodoliths near Udo Island, Korea

The distribution of rhodoliths growing near the Udo islands in the Republic of Korea was identified and the controlling factors influencing their growth and distribution were investigated. Their mineralogical and textural features of growth patterns and the physical oceanographic properties that influence the rhodolith distribution were also examined. The study area is relatively shallow with a water depth of less than 20 m and thus is influenced by seasonal typhoons and winter storms as well as wind-driven waves and strong tidal currents. The study site was divided into four areas based on the distribution of the rhodoliths and the associated physical and biological factors. The bedrock-exposed area partially covered with live rhodoliths (area I) at depths of up to 10 m provides an environment characterized by high wave energy and irregular seabed topography, which precludes continuous rhodolith growth and accumulation. Living rhodoliths were rarely observed and the sediments continuously migrate to adjacent beaches and shallow waters. The depth range of 10–15 m can be divided into the northern ( the active growth area of live rhodoliths , area II) and southern ( the sand dune area with dead rhodoliths , area III) areas. The southern part, where underwater sand dunes appear, is predominantly distributed with sandy sediments and ripple marks, which makes it an adverse environment for continuous growth of rhodoliths. In contrast, the northern part has a bowl-shaped seabed with gentle slopes, providing a stable condition for the constant and active growth of rhodoliths. In areas deeper than 15 m ( the seagrass-covered area with live rhodoliths , area IV), an irregular seabed topography is present and the wave energy is reduced. Consequently, various sizes of rhodoliths with bumpy surfaces are distributed throughout this area. Therefore, it is suggested that the distribution of the rhodoliths is affected by the bottom topography and the associated physical energy, in conjunction with different types of surface sediments. The results of this study will increase the understanding of rhodoliths elsewhere in the world.