Water mass analysis and origins of bottom water nutrient enrichment in the East China Sea shelf using rare earth elements

To understand the complicated water mass mixing and contributions of nutrients in the East China Sea (ECS), research cruises were conducted in the summer of 2003 and 2004. Water sources are defined by multiple tracers, including salinity and Rare Earth Elements (REE), etc. These sources include mixed shelf water (MSW, highest heavy REE concentration), Kuroshio surface water (KSW, highest temperature), Kuroshio tropical water (KTW, highest salinity), and Kuroshio intermediate water (KIW, highest nutrient content). High-nutrient water (HNW) is identified in the middle shelf (bottom 130 m) and shelf slope. For the first time, REE vertical profiles are reported on the shelf of the ECS, and use to characterize the multiple water mass analysis.

The HNW in the middle shelf is a mixture of MSW, KTW and KIW, while that in the shelf slope is KIW. This is evidence that KIW contributes its high-nutrient water in the middle ECS shelf. The mixing ratios of the three water sources are calculated using six tracers, including five heavy REE, by the least squares method. KIW accounts for 26–55% of the middle shelf bottom water in the northernmost research area, while the proportion of NO3+NO2 from KIW is 55–81% and that of phosphate is 58–90%. The calculated value indicates that the nutrient’s major source of HNW is KIW. This study demonstrates heavy REEs are excellent tracers to discern multiple water mass sources and quantify their mixing ratio. Moreover, our finding that KIW at 500-600 m KIW could reach the central shelf bottom in the ECS, and the nutrients contribution calculation suggests the significance of Kuroshio Water material transport to the ECS.