Factors controlling massive green tide blooms on the coasts of Jeju Island, Korea.

To determine the driving mechanisms between submarine groundwater discharge (SGD) and massive blooms of the green alga Ulva, we examined the magnitude of SGD and SGD-derived nutrient fluxes from November 2019 to July 2020 in Ihotewoo (north) and Bangdu (east) bays, Jeju Island, Korea. The variability of SGD flux at both the sites, where the hydraulic gradient is low, closely followed the daily variability of the tidal range, indicating that SGD flux is primarily driven by tidal pumping. Although the average annual SGD-driven nutrient fluxes were 24-37 % lower in Bangdu Bay than in Ihotewoo Bay, massive Ulva blooms only occurred in Bangdu Bay. A longer residence time (poor water exchange) and continuous SGD input with high dissolved inorganic nitrogen play a significant role in the growth and extentsion of Ulva blooms.