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.

Vertical profiles of 228Ra (half-life: 5.75 years) in the ocean provide valuable information on water mixing and ages of the upper ocean. However, its application is hampered by extremely low levels of 228Ra in the deep ocean. In this study, we measured high-resolution 228Ra/226Ra ratio profiles (>21 depths) in the East Sea (Japan Sea) by mooring Mn-fiber. Using the measured 228Ra profile from 228Ra/226Ra ratios and 226Ra activities, together with other previously published data, we estimated the vertical eddy diffusivity (8.7–9.6 cm2 s-1) in the permanent thermocline and water ages (10–15 years) in the upper 500–1000 m range. The estimated decomposition rate of organic carbon based on oxygen utilization rates using Ra-ages between 100 and 1000 m was 4.4 ± 0.8 mol C m-2 yr-1. Our results show that ~50% of the upward nutrients through 100 m support export production, and that dissolved organic carbon accounts for ~20% of carbon export. This 228Ra approach provides a holistic understanding of carbon and nutrient cycles in the ocean.

Nutrients in surface waters of the northwest Pacific Ocean are known to be influenced significantly by anthropogenic inputs via the atmosphere. In this study, we evaluated the relative contribution of cross-shelf fluxes in the northwest Pacific Ocean. The shelf fluxes were estimated by multiplying the 228 Ra flux based on inverse modeling by the measured ratios of nutrients to 228 Ra. The ratios were obtained from field observations of nutrients and 228 Ra in the Yellow Sea and the East China Sea in February 2017 when the water column was fully mixed vertically. The cross-shelf fluxes are estimated to be approximately 40% of the atmospheric depositional flux of nitrogen and 2 orders of magnitude greater than that of phosphorus. These results suggest that the cross-shelf fluxes of nutrients are an important, yet previously underappreciated, pathway of nutrients to the North Pacific Ocean, controlling carbon sequestration and biological production.Plain Language Summary Changes in the nutrients and biogeochemical cycles in the surface waters of the northwest Pacific Ocean have been mainly attributed to the atmospheric inputs of anthropogenic nutrients. However, we hypothesized that continental shelf waters, which receive large inputs of nutrients from rivers, groundwater, and the atmosphere, can deliver significant amounts of nutrients to the northwest Pacific Ocean. The relative importance of atmospheric fluxes versus cross-shelf fluxes is particularly important in this region since (1) the Yellow Sea and the East China Sea continental shelf is one of the largest continental shelves in the world, (2) Changjiang River water is highly enriched in anthropogenic nutrients, and (3) the atmospheric deposition of nutrients in the northwest Pacific continental shelves is known to be the largest in the world. Our new approach using 228 Ra as a tracer allows us to estimate cross-shelf fluxes of nutrients which are extremely difficult with direct measurements. The results show that cross-shelf fluxes of nutrients are as important as atmospheric inputs. Key Points:• The cross-shelf fluxes of nutrients are determined by multiplying measured nutrient/ 228 Ra ratios by the model-driven 228 Ra flux • The flux of dissolved N from the shelf water is 30%-40% of its direct atmospheric depositional flux • The flux of dissolved P from the shelf water is 2 orders of magnitude greater than the atmospheric total P flux Supporting Information:• Supporting Information S1• Data Set S1

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