A relationship between submarine groundwater borne nutrients traced by Ra isotopes and the intensity of dinoflagellate red‐tides occurring in the southern sea of Korea

We measured short‐lived radium isotopes ( 223 Ra and 224 Ra), dissolved inorganic and organic nutrients, and photosynthetic pigments during the summers of 2006 and 2007 in the southern sea of Korea, where harmful dinoflagellate blooms occur every year. The Ra tracer measurements reveal that coastal groundwater, rather than other sources previous suggested (i.e., Yangtze River diluted water or Kuroshio currents), is the main source of nutrients that fuel red tides in this region. Although inorganic‐nutrient levels are different for different regions and different years, either dissolved inorganic nitrogen or phosphorus is depleted in the red‐tide region. This depletion is accompanied by highly elevated levels of dissolved organic nutrients, transformed from groundwaterborne dissolved inorganic nutrients either inside Yeoja Bay or in offshore red‐tide areas, thereby creating favorable conditions for the growth of dinoflagellates in competition with diatoms. The intensity of red tides correlates well with the activity of 224 Ra (half life = 3.66 d) in seawater over daily or yearly time scales. Because the chemically conservative 224 Ra can trace groundwater‐borne nutrients, which are utilized by marine biota in this red‐tide region, the intensity of red tides seems to be related to the amount of nutrient‐enriched groundwater supplied to the offshore red‐tide region.