Tracing surface seawater mixing and nutrient transport by 222Rn on the northern coast of Beibu Gulf, China

The transport and diffusion of substances in seawater are limited by the mixing motion of water bodies, while the main forms of mixing in offshore water bodies are advection and eddy diffusion. The eddy diffusion process of water indicates the possible transport direction of dissolved substances. However, the complex environment in the coastal zone makes it difficult to quantitatively assess the water diffusion process. 222 Rn is a useful tool to trace the diffusion process of water bodies. However, studies on the 222 Rn distribution and its behavior in the Beibu Gulf are scarce. In this study, the activity distribution characteristics of 222 Rn in surface seawater of the Guangxi shelf area of the Beibu Gulf were measured. Based on the one-dimensional, steady-state model, the vorticity diffusion coefficient of 222 Rn in the horizontal direction was calculated as (0.42–2.13) × 10 8 m 2 /d, and the offshore fluxes of 222 Rn under the influence of water mixing were calculated as 2.00 × 10 12 Bq/d. Correspondingly, the horizontal transport fluxes of silicate, phosphate, nitrite and nitrate were 6.28 × 10 −3 mol/(m 2 ·d), 0.10 × 10 −3 mol/(m 2 ·d), 0.20 × 10 −3 mol/(m 2 ·d) and 4.15 × 10 −3 mol/(m 2 ·d), respectively. These results indicate that the study of eddy current diffusion in offshore marine water facilitates a deeper understanding of the water mixing process and nutrient transport and migration.