Impacts of algal blooms on sinking carbon flux and hypoxia off the Changjiang River estuary

Based on 10 multidisciplinary investigations conducted from February 2015 to January 2016, the phytoplankton community and its association with ambient seawater physicochemical parameters in the Changjiang (Yangtze) River estuary (CE) and its adjacent waters were comprehensively examined. In total, 265 taxa were identified, belonging to 5 phyla and 94 genera. Diatoms (63.78%) and dinoflagellates (33.21%) were the dominant groups. The variation of diatom abundance showed a positive relationship with the nutrient concentrations while the dinoflagellate abundance showed a negative relationship. Two algal bloom events occurred during the investigation period. The Changjiang Diluted Water (CDW) induced environmental gradients in the upper layer, favoring the diatom bloom in July. The invasion of the nearshore Kuroshio branch current could affect the formation of a bloom of Prorocentrum donghaiense. With the blooming and senescence of phytoplankton, low dissolved oxygen (DO) and hypoxia occurred in the bottom waters. The bottom DO concentration displayed a significantly negative correlation with phytoplankton carbon flux. The present study provides straightforward evidence for the source of organic matter for oxygen consumption in the CE and its adjacent waters.