It is a well held concept that the magnitude of red-tide occurrence is dependent on the amount of nutrient supply if the conditions are same for temperature, salinity, light, interspecific competition, etc. However, nutrient sources fueling dinoflagellate red-tides are difficult to identify since red tides usually occur under very low inorganic-nutrient conditions. In this study, we used short-lived Ra isotopes (223Ra and 224Ra) to trace the nutrient sources fueling initiation and spread of Cochlodinium polykrikoides blooms along the coast of Korea during the summers of 2014, 2016, and 2017. Horizontal and vertical distributions of nutrient concentrations correlated well with 224Ra activities in nutrient-source waters. The offshore red-tide areas showed high 224Ra activities and low-inorganic and high-organic nutrient concentrations, which are favorable for blooming C. polykrikoides in competition with diatoms. Based on Ra isotopes, the nutrients fueling red-tide initiation (southern coast of Korea) are found to be transported horizontally from inner-shore waters. However, the nutrients in the spread region (eastern coast of Korea), approximately 200 km from the initiation region, are supplied continuously from the subsurface layer by vertical mixing or upwelling. Our study highlights that short-lived Ra isotopes are excellent tracers of nutrients fueling harmful algal blooms in coastal waters.

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Radon-in-air monitor (RAD7, Durridge Co.) has been widely used to measure 222 Rn in water samples. RAD7 measures 222 Rn via 218 Po + (t 1/2 = 3.1 min), which is electrically attracted to a silicon alpha detector. In this study, a new method was developed for measuring 222 Rn in water samples by modifying a commercially available pulsed ionization chamber (PIC, FT-Lab Co.). The PIC detects and amplifies the electric pulses generated by microspace charges produced by 222 Rn decay. Two passive PICs (volume: 2 × 400 mL) were combined and modified to form an active system (named Rn-SNU) that continuously circulates air (~1 L min −1 ). Rn-SNU is approximately seven to nine times more efficient than RAD7 and does not necessitate a delay of~15 min to reach radioactive equilibrium between 222 Rn and 218 Po + . However, RAD7 is more accurate in discriminating 222 Rn daughters, 218 Po + and 214 Po + . In this study, 222 Rn was successfully measured in coastal seawater samples using Rn-SNU connected to a grab bottle. Our results suggest that 222 Rn measurements in water samples can be conducted more efficiently and rapidly with an Rn-SNU than with other widely used instruments and methods.

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