Spatial distribution and behavior of dissolved selenium speciation in the South China Sea and Malacca Straits during spring inter-monsoon period

Selenium (Se) has been recognized as a key trace element that is associated with growth of primary producers in oceans. During March and May 2018, surface water (67 samples) was collected and measured by HG-ICP-MS to investigate the distribution and behavior of selenite [Se(IV)], selenate [Se(VI)] and dissolved organic selenides (DOSe) concentrations in the Zhujiang River Estuary (ZRE), South China Sea (SCS) and Malacca Straits (MS). It showed that Se(IV) (0.14–3.44 nmol/L) was the dominant chemical species in the ZRE, related to intensive manufacture in the watershed; while the major species shifted to DOSe (0.05–0.79 nmol/L) in the MS, associated with the wide coverage of peatland and intensive agriculture activities in the Malaysian Peninsula. The SCS was identified as the northern and southern sections (NSCS and SSCS) based on the variations of surface circulation. The insignificant variation of Se(IV) in the NSCS and SSCS was obtained in March, potentially resulting from the high chemical activity and related preferential assimilation by phytoplankton communities. Contrastively, the lower DOSe concentrations in the SSCS likely resulted from higher primary production and utilization during March. During May, the concentration of Se(IV) remained low in the NSCS and SSCS, while DOSe concentrations increased notably in the SSCS, likely due to the impact of terrestrial inputs from surface current reversal and subsequent accumulation. On a global scale, DOSe is the dominant Se species in tropical oceans, while Se(IV) and Se(VI) are major fractions in high-latitude oceans, resulting from changes in predominated phytoplankton and related biological assimilation.