Linking groundwater-borne nutrients and dinoflagellate red-tide outbreaks in the southern sea of Korea using a Ra tracer

We have measured nutrients, photosynthetic pigments, and two radium isotopes (223Ra and 224Ra) in coastal groundwater and seawater during the summer of 2002 and 2003 in the southern sea of Korea, where harmful dinoflagellate blooms (hereafter red tides) have been occurring every year since 1982. In this region, the amount of submarine groundwater discharge (SGD) is known to be two orders of magnitude higher than the surface runoff. Good correlations of dissolved inorganic nitrogen (DIN) and silicate (DSi) with groundwater-borne 224Ra (half life = 3.66 days) in the initial red-tide waters of 2002 confirm that the source of these nutrients is SGD. In addition, the DIN/DIP ratios in 2002 and total dissolved nitrogen/phosphorus (TDN/TDP) ratios in 2003 in red-tide waters ranged from 30 to 300, similar to those observed in groundwater. The spatial and temporal variations of nutrients and photosynthetic pigments showed that red tides broke out when DIN or DIP (dissolved inorganic phosphorus) were almost depleted, limiting the growth of diatoms. Thus, our observations reveal that the emergence of harmful dinoflagellate red tides might be facilitated by inter-specific competition of phytoplankton species when inorganic nutrients are limited and dissolved organic nutrients, converted from groundwater-borne nutrients, are sufficient.