Effects of algal blooms on selenium species dynamics: A case study in the Changjiang Estuary, China

The selenium cycle in the marine environment is sensitive to biological activity, but knowledge of dissolved Se species dynamics during coastal algal blooms is limited. Selenium species dynamics during diatom blooms in the Changjiang Estuary were investigated in a survey of dissolved inorganic and organic Se. Dissolved inorganic Se (Se(IV) + Se(VI)) was the predominant species in river-dominanated areas, while dissolved organic selenide (DOSe) was predominant in ocean-dominanated areas. Relationships between DOSe and chromophoric dissolved organic matter involved both humic- and protein-like components, suggesting distinct sources of DOSe in river- and ocean-dominance areas, respectively. A three-endmember-mixing model was used to describe biological processes in ocean-dominanated surface waters. In diatom-bloom areas, the co-occurrence of depletion of Se(IV) and Se(VI) (of ~90% and 30%, respectively) and a 44% increase in production of DOSe indicates that phytoplankton act as vectors for Se species transformation. A Se(IV)*_P indicator was developed to quantify limiting concentrations of Se(IV) in water relative to that of phosphorus. Negative Se(IV)*_P concentrations indicate that Se(IV) is limited due to biological utilization of dissolved inorganic phosphorus by diatoms, resulting in secondary uptake of Se(VI) in the Changjiang Estuary.