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.