This study aimed to investigate the dynamics of the nutrient budget of a macroalgal community exposed to water discharged from aquaculture farms. This study was conducted in the coastal area exposed to water discharged from aquaculture farms located on Jeju Island, Korea, in May and October of 2017. Water-column-dissolved inorganic nutrient concentrations were higher in the intertidal zone than in the subtidal zone. High nutrient concentrations in the intertidal zone resulted in the bloom of Ulva spp., consequently leading to a low species number. This indicates the vulnerability of the intertidal macroalgal community to discharge from land-based aquaculture. Ecklonia cava contributed to 29–53% of the total incorporation of carbon (C), nitrogen (N), and phosphorus (P) in the subtidal zone. In the intertidal zone, the highest tissue N content was found in Ulva spp. due to the high and rapid nutrient uptake rate from the effluent with high nutrient concentrations, thereby indicating high total N incorporation. The estimated total C, N, and P incorporation rates by macroalgae were 181.5, 8.6, and 0.95 tons year⁻¹, respectively. These results suggest that the macroalgal community in this area absorbs or removes large amounts of nutrients from the water column and plays an important role in the budgets and cycling of nutrients in the surrounding coastal areas.

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.

Since the 1980s, the number of land-based fish farms on Jeju Island has increased rapidly. With increasing land-based fish farms, a large amount of nutrients from fish farm wastewater is discharged off the coast of Jeju. To understand the characteristics of coastal seawater and the ecological environment on the coast of Jeju, the effect of land-based fish farm effluent on coastal seawater should be evaluated. Temperature, salinity, nutrients, and chlorophyll-a concentration were investigated on the northeastern coast of Jeju during June and July 2023. Nitrate, phosphate, and silicate concentrations in the surface waters were significantly higher in coastal stations than in the outer stations. Unlike the surface waters, nutrient concentrations in the bottom waters are distinctly higher in land-based fish farm effluent stations than in the outer stations. Total organic carbon content in surface sediment was significantly higher in land-based fish farm effluent stations than in the outer stations. This study may provide valuable information for evaluating the impact of land-based fish farm effluent on coastal ecosystems on Jeju Island.

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.