We investigated the photosynthetic characteristics of the crustose coralline alga Pneophyllum fragile (Corallinales, Rhodophyta) according to elevated water temperature and irradiance on the coast of Jeju in 2018. P. fragile was cultured under different temperature (11 °C, 21 °C, 26 °C, and 31 °C) and irradiance (0–1250 μmol photon m⁻² s⁻¹) conditions. Oxygen (O₂) concentrations at the P. fragile mat–water interface (MWI) were measured using an O₂ microsensor. At the MWI, the diffusive boundary layer thicknesses ranged from 200 to 400 μm. The O₂ concentrations at the mat surface increased in response to increasing irradiance, and reached 344% air saturation. The maximum photosynthesis capacity (P_max) and respiration rate in the dark (R_d) at 31 °C were about 3 times higher than those recorded at 11 °C. The compensation irradiance (E_c) and saturation irradiance (E_k) increased with increasing water temperature. The P_max, R_d, and E_c were statistically correlated with temperature (p < 0.05). The E_k increased up to 833 μmol photon m⁻² s⁻¹ at 31 °C and exhibited a strong dependence on irradiance at high temperatures. The adaptability of P. fragile to high temperatures and strong irradiance was distinct from that observed for coralline algae in other temperate waters.

We measured oxygen (O₂) fluxes in two major shallow subtidal benthic habitats (kelp bed (KB) and bare rock (BR) covered with crustose coralline algae) of Dokdo islet in the East Sea by applying noninvasive in-situ aquatic eddy covariance (AEC). The AEC device allows time series measurements (~24 h) of three-dimensional velocity (u, v, and w components) and high-resolution dissolved O₂. This allows estimation of O₂ exchange flux via benthic habitats. Local flow rates and irradiance levels were found to be major factors controlling O₂ exchange flux in the rocky habitats. Gross primary production rates tended to be significantly higher in KB (163 mmol O₂ m⁻² d⁻¹) than in BR (51 mmol O₂ m⁻² d⁻¹). The net ecosystem metabolisms were assessed as opposite types, with 8 mmol O₂ m⁻² d⁻¹ in KB (autotrophy) and –12 mmol O₂ m⁻² d⁻¹ in BR (heterotrophy). Our results indicate that kelp beds are important for organic carbon cycling in rocky coastal waters and that AEC application to macroalgae habitats is a useful assessment approach.

We characterized the biogeochemical organic carbon (C_org) cycles in the surface sediment layer of the Ulleung Basin (UB) of the East Sea. The total oxygen uptake (TOU) rate and the diffusive oxygen uptake (DOU) rate of the sediment were measured using an autonomous in situ benthic lander equipped with a benthic chamber (KIOST BelcII) and a microprofiler (KIOST BelpII). The TOU rate was in the range of 1.51 to 1.93 mmol O₂ m⁻² d⁻¹, about double the DOU rate. The high TOU/DOU ratio implies that the benthic biological activity in the upper sediment layer is one of the important factors controlling benthic remineralization. The in situ oxygen exposure time was about 20 days, which is comparable to the values of other continental margin sediments. The sedimentary C_org oxidation rates ranged from 6.4 to 6.5 g C m⁻² yr⁻¹, which accounted for ~2% of the primary production in UB. The C_org burial fluxes ranged from 3.14 ± 0.12 to 3.48 ± 0.60 g C m⁻² yr⁻¹, corresponding to more than 30% of the deposited C_org buried into the inactive sediment deep layer.

This study assessed the impact of intensive human activities on organic matter (OM) and heavy metal cycles in Shihwa Lake, South Korea. Sediment oxygen demand (SOD), benthic nutrient flux (BNF), and benthic heavy metal flux were estimated using in situ benthic chambers. The combined analysis of sediment trap and SOD showed that the vertical supply of OM was a major controlling factor for benthic respiration. The BNF accounted for 35–144% and 32–184% of the N and P required, respectively, for primary production (PP) in the water column. The higher SOD may have also accelerated the release of Mn, Fe, Co, and Ni from the sediment. Benthic fluxes of Cr, As, Cd, Pb, Cu, and Zn were highest near the industrial complex, with ranges of 1.3 ± 0.9, 6.4 ± 4.9, 0.2 ± 0.1, 0.5 ± 0.4, 7.7 ± 1.4, and 452 ± 133 μmol m⁻² d⁻¹, respectively. Mn, Fe, Co, As, Pb, Ni, and Cu contributed more than 10% of the sediment to the current standing stock at Shihwa Lake.