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 (Pmax) and respiration rate in the dark (Rd) at 31°C were about 3 times higher than those recorded at 11 °C. The compensation irradiance (Ec) and saturation irradiance (Ek) increased with increasing water temperature. The Pmax, Rd, and Ec were statistically correlated with temperature (p < 0.05). The Ek 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 (Corg) 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 Corg 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 Corg 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 Corg buried into the inactive sediment deep layer.