Estimating submarine groundwater discharge in Jeju volcanic island (Korea) during a typhoon (Kong-rey) using humic-fluorescent dissolved organic matter-Si mass balance.

We examined the residence time, seepage rate, and submarine groundwater discharge (SGD)-driven dissolved nutrients and organic matter in Hwasun Bay, Jeju Island, Korea during the occurrence of a typhoon, Kong-rey, using a humic fluorescent dissolved organic matter (FDOMH)-Si mass balance model. The study period spanned October 4-10, 2018. One day after the typhoon, the residence time and seepage rate were calculated to be 1 day and 0.51 m day-1, respectively, and the highest SGD-driven fluxes of chemical constituents were estimated (1.7 × 106 mol day-1 for dissolved inorganic nitrogen, 0.1 × 106 mol day-1 for dissolved inorganic phosphorus (DIP), 1.1 × 106 mol day-1 for dissolved silicon, 0.5 × 106 mol day-1 for dissolved organic carbon, 1.6 × 106 mol day-1 for dissolved organic nitrogen, 0.4 × 106 mol day-1 for particulate organic carbon, and 38 × 106 g QS day-1 for FDOMH). SGD-driven fluxes of dissolved nutrient and organic matter were over 90% of the total input fluxes in Hwasun Bay. Our results highlight the potential of using the FDOMH-Si mass balance model to effectively measure SGD within a specific area (i.e., volcanic islands) under specific weather conditions (i.e., typhoon/storm). In oligotrophic oceanic regions, SGD-driven chemical fluxes from highly permeable islands considerably contribute to coastal nutrient budgets and coastal biological production.

Tracing nitrogen sources fueling coastal green tides off a volcanic island using radon and nitrogen isotopic tracers.

The main sources of nutrients fueling coastal green tides off a volcanic island surrounded by an oligotrophic ocean are obscure, although they result in many societal and ecosystem problems. In this study, we attempted to trace the source inputs of nutrients in coastal waters off a volcanic island, Jeju, Korea, where the formation of green tides is perennial, using a radioisotope (222Rn) and stable isotopes (δ15N and δ18O) as tracers. Sampling of groundwater, seawater, fish-farm water, and Ulva spp. was performed during April and July 2015. The contribution of submarine fresh groundwater discharge (SFGD) to the dissolved inorganic nitrogen input was >70%, with additional inputs from aqua-cultural activities and bottom sediments. The δ15N-NO3 and δ18O-NO3 values in the coastal seawater and groundwater indicate that the main source of NO3- is fertilizer, rather than other potential sources, such as aquacultural wastewater, sewage/manure contamination, or precipitation, in this region. The δ15N value (+7.3-+7.7‰) in Ulva spp. also indicates the same source. Thus, our results suggest that the rapid infiltration of land N-fertilizer and subsequent leakage into the coastal ocean through submarine groundwater discharge (SGD) results in green tide massive occurrence in coastal waters off a high-permeability volcanic island.

Significant production of humic fluorescent dissolved organic matter in the continental shelf waters of the northwestern Pacific Ocean.

Fluorescent dissolved organic matter (FDOM) is important for marine organisms and the global carbon cycle contributing to the optical properties of surface seawater and organic carbon budgets. Rivers are known to be the main source of FDOM in coastal oceans and marginal seas. In this study, however, we show that the contribution of FDOM produced from organic sediments of the northwestern Pacific continental shelf is similar to that from the Changjiang River. FDOM showed relatively higher concentrations at stations off the Changjiang River mouth and in the central Yellow Sea. Based on temperature-salinity diagrams, the major source of surface FDOM in summer surface waters was found to be from the Changjiang River while that observed in the winter water column was produced mainly in the continental shelf. A good correlation between 228Ra and the humic-like FDOM (FDOMH) during the winter suggests that the FDOMH is produced mainly from marine sediments and enriched in water over the water residence times. A simple mass balance calculation shows that the excess FDOMH fluxes produced from marine sediments account for 30-40% of the riverine source. This result suggests that the continental shelf is an important hidden source of FDOM in the upper ocean.

Radium tracing nutrient inputs through submarine groundwater discharge in the global ocean.

Riverine and atmospheric inputs are often considered as the main terrestrial sources of dissolved inorganic nitrogen (DIN), phosphorus (DIP), and silicon (DSi) in the ocean. However, the fluxes of nutrients via submarine groundwater discharge (SGD) often exceed riverine inputs in different local and regional scale settings. In this study, we provide a first approximation of global nutrient fluxes to the ocean via total SGD, including pore water fluxes, by combining a global compilation of nutrient concentrations in groundwater and the SGD-derived 228Ra fluxes. In order to avoid overestimations in calculating SGD-derived nutrient fluxes, the endmember value of nutrients in global groundwater was chosen from saline groundwater samples (salinity >10) which showed relatively lower values over all regions. The results show that the total SGD-derived fluxes of DIN, DIP, and DSi could be approximately 1.4-, 1.6-, and 0.7-fold of the river fluxes to the global ocean (Indo-Pacific and Atlantic Oceans), respectively. Although significant portions of these SGD-derived nutrient fluxes are thought to be recycled within sediment-aquifer systems over various timescales, SGD-derived nutrient fluxes should be included in the global ocean budget in order to better understand dynamic interactions at the land-ocean interface.