At Jeju Island, Republic of Korea, hard coral communities consisting of Alveopora, Montipora, and Psammocora represent some of the northernmost photosymbiotic Scleractinia in the Northwest Pacific. Here, we document taxon-specific, depth-specific bleaching conditions observed during the 2024 marine heatwave associated with the Fourth Global Coral Bleaching Event. Alveopora japonica and Montipora showed pronounced bleaching at shallower depths, whereas Psammocora appeared comparatively less affected. A follow-up survey two months later documented mortality, particularly among A. japonica colonies. These observations highlight heterogeneous bleaching responses among taxa and depths in a high-latitude marginal coral communities near the poleward distributional limit of photosymbiotic scleractinian corals. The depth-related pattern observed in A. japonica raises the hypothesis of possible bathymetric redistribution, although quantitative long-term observations are required to determine whether this represents a persistent shift.

After collecting sediment using a box corer in the northern East China Sea, sub-samples were collected from the sediment chamber and sediment incubation experiments were performed to calculate sediment oxygen consumption rate, hydrogen sulfide flux, methane flux and nutrient flux. Sediment incubation experiments were conducted at a total 5 sites, and the experiments were conducted for approximately 144 hours (6 days). Organic carbon regeneration rate was calculated as the sediment oxygen consumption rate. The sediments at all sites were muddy or muddy sand sediments, and the organic carbon content ranged from 0.25% to 0.94%. The C/N ratio is in the range of 3 to 7, and most of the organic matter in the sediment is assumed to be of marine origin. The sediment oxygen consumption rate ranged from 0.82 to 4.2 mmol/m²/day, a relatively low value compared to the sediment oxygen consumption rate in coastal area. The organic carbon regeneration rate ranged from 0.39 to 1.99 g/m²/year. The fluxes of hydrogen sulfide were 0.86 μmol/m²/day and 0.39 μmol/m²/day at the Z3-A03 site and Z3-D03 site, respectively. The fluxes of methane ranged from 0.022 to 4.838 μmol/m²/day. Methane fluxes showed a good correlation with the sediment oxygen consumption rate. Nitrate concentrations showed tended to little change or increase at sites with low oxygen consumption rates, whereas at sites with high oxygen consumption rates, concentrations tended to sharply decreased during the incubation. Ammonium concentration showed no change during the initial phase of incubation, but increased sharply from the 2 days after. The silicate concentrations continuously increased during the incubation, with the silicate flux being proportional to the oxygen consumption rate.