The sinking of the phytoplankton community and its contribution to seasonal hypoxia in the Changjiang (Yangtze River) estuary and its adjacent waters

The phytoplankton sinking rates and the annual variations of dissolved oxygen (DO) were studied as part of a multi-disciplinary investigation in the Changjiang (Yangtze River) Estuary (CE) and its adjacent waters from February 2015 to January 2016. The sinking rates of phytoplankton ranged from 0.02 to 3.49 m d(-1), determined with the homogeneous sample method SETCOL. There was clear correlation between sinking rate and the phytoplankton composition. The diatom-dominated community had higher sinking rates than the dinoflagellate-dominated community, and the variation in DO concentration indicated a close relationship with the blooming and senescence of phytoplankton. Low DO (< 3 mg L-1) or hypoxic (< 2 mg L-1) zones were observed in near-bottom waters between July and October. Hypoxia reached its maximum in October, with a low DO zone covering approximately 4500 km(2) of the survey area. The zones of lowest DO in near-bottom water were closely associated with zones of the highest chlorophyll a (Chl a) and the highest phytoplankton sinking rates in the upper layer. The present study presents straightforward evidence on the formation of low DO and hypoxia in near-bottom waters and its correlation with sinking of the phytoplankton community, and provides useful insight to reveal the formation mechanism of seasonal hypoxia in the CE and its adjacent waters.