The behavior of dissolved inorganic selenium in the Bohai Sea

Two cruises of “R/V Dong Fang Hong 2” were carried out in September 1998 and May 1999, respectively, to understand the behavior of selenium in the Bohai Sea. Selenium species (dissolved inorganic selenium, selenite) are determined by HG-AFS for 30 grid stations. Selenium concentrations display short-term variability and seasonal change in the Bohai Sea, with higher levels in shallow coastal waters than in the Central Bohai Sea. The influence of riverine discharge on selenium levels can be seen from salinity isopleths, selenium distribution and species ratios. Near-bottom waters have similar selenium concentrations as to the surface waters in the Central Bohai Sea, whereas stratification takes place in the Bohai Strait and North Yellow sea. Based on the data of this study, a preliminary estimate of selenium budgets via riverine input is established. The seasonal variation of Se (IV)/Se (VI) ratio is maximum in Bohai Sea. The predominant species of selenium was selenate in autumn 1998. However, selenite became the dominant species in spring 1999. A simple box model was used to estimate the water-mass balance and selenium budgets for the Bohai Sea. Selenium budgets demonstrated that net sink of selenium can be mainly from the water column into bottom sediments and/or by transformation to other forms of selenium.