Diatom bloom derived summer bottom water hypoxia off the Changjiang Estuary and its variation

Dissolved oxygen (DO), pH, dissolved inorganic carbon (DIC), nutrients data obtained from summer 2009 (with typhoon) and 2013 (without typhoon) are compared and both displayed a close correlation between the bottom hypoxia and the high surface primary productivity off Changjiang Estuary. Based on a mixing model analysis, we identified in-situ primary productivity produced by diatom bloom at the surface as the major source (70%~80%) of the organic carbon that decomposed at the bottom and ultimately led to the bottom hypoxia and acidification. In the plume area, depth integrated net biological production in the water column was 1.8 g C m^-2 d^-1, indicating strong autotrophic production and a net CO₂ sink which led to a high respiration rate of 1.2 g C m^-2 d^-1 in the bottom water. This strong surface-bottom physical and metabolic coupling process was obvious during both summer cruises, but it was strengthened after the passage of typhoon. The accelerated diatom bloom fueled by nutrient supply from bottom-up mixing, and subsequent quicker oxygen consumption of newly formed labile organic matter after the rebuilt of stratification has been identified. The formation of hypoxia can be accomplished within 6 days, which to our knowledge, was the fastest hypoxia development rate ever reported. Lastly we present a schematic framework to illustrate the mechanism of diatom bloom induced summer bottom hypoxia in Changjiang Estuary and costal acidification process in order to stimulate further studies.