Biogeochemical consequences of submarine groundwater discharge in the coastal ocean

Full text: The nutrient inputs through the submarine discharge of fresh, brackish, and marine groundwaters may be comparable to the inputs through river discharge in the coastal ocean. However, the biogeochemical influences of submarine groundwater discharge (SGD) in the coastal ocean are poorly understood. We present the biogeochemical and ecosystem changes in the coastal ocean due to the submarine discharge of nutrients in the South Sea of Korea. We estimated SGD in a eutrophic semi-enclosed bay (Yeoja Bay) in the South Sea using radium isotopes (223Ra, 224Ra, and 226Ra). The nitrogen, silicate, and radium concentrations in coastal groundwaters were in general much higher than those in seawaters offshore. Thus, the fluxes of new nutrients through SGD were higher than those through stream flow in this bay. This appears to result in the large input of excess nitrogen and silicate into the coastal ocean. In the offshore of this bay (red-tide areas), good correlations among 224Ra, Si, and DIN (dissolved inorganic nitrogen) were observed, although phosphorus was almost completely depleted. Thus, the outbreak of red tides in this region seems to be due to the limited growth of diatoms under depleted DIP conditions, and the efficient utilization of DIN and DOP (dissolved organic phosphorous) by dinoflagellates. This case study indicates that nitrogen pollution in coastal groundwater and its subsequent discharge into the ocean may cause a significant ecological change that could trigger or facilitate harmful algal blooms. In a volcanic island, Jeju, Korea, seepage rates were in the range 50-300 m/yr, which are much higher than those reported from typical continental shores. On the eastern shore of Jeju, almost all groundwater discharge is attributed to recirculating seawater, while fresh groundwater contributes about 20% of the total SGD on the western shore of Jeju. The measured radionuclides (228Ra, 226Ra, and 222Rn) and nutrients in the groundwater suggest that the discharge of both fresh and recirculated seawater will have a significant influence on the economy of coastal nutrients and other chemical constituents in this region. This large discharge of natural chemical constituents appears to cause natural eutrophication in a semi enclosed bay of this volcanic island. (author)