The Summer Distribution of Dissolved Inorganic Iodine along 18ï°N in the South China Sea

Dissolved inorganic iodine (iodate and iodide) in seawater samples collected in the northern South China Sea (SCS) were investigated using differential pulse polarography and cathodic stripping square wave voltammetry, respectively. High concentration of iodide was observed in the surface layer in agreement with those previously reported for tropical waters. Iodine speciation and macro-nutrients were strongly coupled as observed in other tropical oceans. The vertical distributions of iodine and macro-nutrients can be classified into three biogeochemical regimes: (1) in the upper 50 m of the water column, low iodate was observed as nutrients were depleted and photosynthesis was low in summer. In contrast, substantial amount of iodide produced by high phytoplankton productivity and intense bacterial action in winter and spring was saved till summer; (2) in the water column between 50 m and the Tropical Water at around 150 m, the amount of iodide oxidation varied proportionally with nitrification, leading to rapid increase of nitrate and iodate with depth; (3) below the Tropical Water, iodate and nitrate are regenerated mainly due to re-mineralization of organic matter sinking from the euphotic zone. The distribution of dissolved inorganic iodine and hydrographic parameters (salinity, temperature, dissolved oxygen, and macro-nutrients) were influenced by the downwelling of the mesoscale warm eddy.