Alveopora japonica is restricted to the Asia-Pacific region, ranging from subtropical to temperate waters. In 2016, a massive bleaching event of an A. japonica population was observed at the south coast of Jeju Island, South Korea, which is within its northernmost limit. After the bleaching event, most of the colonies had recovered by 2017.

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.

Lee, T.; Moon, J.-H.; Jung, S.-K.; Park, G.; Kwon, S.; Min, S.-H., and Son, Y.B., 2020. Changes of nutrient budget due to physiochemical factors in the coastal area of Jeju, Korea. In: Malvárez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 57–61. Coconut Creek (Florida), ISSN 0749-0208.In Jeju coastal area which is UNESCO biosphere reserve, there has recently been eutrophication and serious changes in the coastal ecosystem such as coastal fishery decline and green algal bloom. A large amount of nutrients that inflow to the coastal area through the discharge of land-based aquaculture farms (LAFs) and submarine groundwater discharge (SGD) are transported by the tidal current. We established representative area of the Jeju coastal area and calculated the nutrients budget of the LAFs, SGD, primary production (PP), and sediment in this study area. The advection-diffusion equation applied to the model was calculated through the passive tracer experiments. The water mass transport in and out of the study area was estimated by modeling, and the nutrient fluxes was calculated by water volume and nutrient concentration. The nutrient released from the aquaculture farms are primarily determined by the reversing tidal currents along the coast of the study area, showing a narrow band of coast, indicating little cross-isobath nutrient exchange between the nearshore and offshore water. The amount of total carbon (TC), dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP) and dissolved silicon (DSi) influxes into the study area from the LAFs, SGD and offshore were 2,696,282 kg·day-1, 18,823 kg·day-1, 3,395 kg·day-1 and 65,723 kg·day-1, respectively. The amount of TC, TN, TP and Si outfluxes transported from the study area to the offshore, deposited in the sediment and used for PP were 2,696,274 kg·day-1, 18,821 kg·day-1, 3,394 kg·day-1 and 65,721 kg·day-1, respectively. In the study area nutrients are accumulating due to inflows more than outflows. The accumulation rate of TC, DIN, DIP and DSi are 2.29 mg·m-3·day-1, 0.76 mg·m-3·day-1, 0.18 mg·m-3·day-1, 1.06 mg·m-3·day-1, respectively. The coastal area of Jeju is accumulating nutrients, which could increase coastal pollution such as eutrophication, green algal bloom and whitening.

During the summer season, low-salinity water (LSW) inputs from the Changjiang River are observed as filamentous or lens-like features in the East China Sea. Sea surface salinity (SSS) is an important factor in ocean science, and is used to estimate oceanic carbon fluxes, trace red tides, and calculate other physical processes at the surface. In this study, a proxy was developed using remote sensing reflectance (Rrs) from the Geostationary Ocean Color Imager (GOCI) centered at 490 nm (band 3), 555 nm (band 4), 660 nm (band 5), and 680 nm (band 6), and salinity (data from summer cruises during the period of 2011–2016). It was then validated to map LSW plumes in the East China Sea. The GOCI-derived surface salinity was determined by the empirical relationships between Rrs at the four bands and in situ wave glider SSS data (August 2016), and was validated with synchronous in situ hydrographic SSS data (August 2011, 2012, 2013, and 2016). The GOCI-derived SSS was considered reliable in terms of the validation with the in situ measurement with a high coefficient of determination along with a low RMSE (R² = 0.803, RMSE = 0.914, N = 21), and in comparisons with two previous models that were used to derive SSS in the East China Sea. The GOCI-derived SSS was successfully used to examine time-series variations on diurnal and daily scales, and the effects of a typhoon in terms of marine physical and biological properties in combination with the chlorophyll-a concentration and sea surface temperature.

Since 1995, Margalefidinium polykrikoides blooms have occurred frequently in the waters around the Korean peninsula. In the South Sea of Korea (SSK), large-scale M. polykrikoides blooms form offshore and are often transported to the coast, where they gradually accumulate. The objective of this study was to investigate the synergistic effect of multi-sensor data for identifying M. polykrikoides blooms in the SSK from July 2018 to August 2018. We found that the Spectral Shape values calculated from in situ spectra and M. polykrikoides cell abundances in the SSK were highly correlated. Comparing red tide spectra from near-coincident multi-sensor data, remote-sensing reflectance (R_rs) spectra were similar to the spectra of in situ measurements from blue to green wavelengths. R_rs true-color composite images and Spectral Shape images of each sensor showed a clear pattern of M. polykrikoides patches, although there were some limitations for detecting red tide patches in coastal areas. We confirmed the complementarity of red tide data extracted from each sensor using an integrated red tide map. Statistical assessment showed that the sensitivity of red tide detection increased when multi-sensor data were used rather than single-sensor data. These results provide useful information for the application of multi-sensor for red tide detection.

Kelp forests in Korean waters, mainly consisting of Ecklonia cava, provide ecologically and economically important ecosystems. However, they are severely threatened by increasing sea surface temperature (SST). In 2023, an unusually high SST was observed in the northern East China Sea, where the average SST from August to November 2023 was found to be 1.1 °C higher than the average SST during the same period over the last two decades. Our photo images and videos reveal increasing feeding on E. cava populations by tropical herbivore rabbitfish (Siganus canaliculatus, Siganus fuscescens) associated with the impact of increasing SST. Given the fall reproductive peak of E. cava population, increased herbivory by tropical rabbitfish could have a significant adverse impact on the composition of temperate kelp forests.