Program     Ocean Circulation, Ecosystem and Hypoxia around Hong Kong Waters       Poster Harmful algal blooms (HABs) and their impact-Case study on hypoxia zone in Pearl River Estuary P-SPS4-10 Pengbin Wang* , Key Laboratory of Marine Ecosystem and Biogeochemistry, The Second Institute of Oceanography, State Oceanic Administration (SOA) Douding Lu, Key Laboratory of Marine Ecosystem and Biogeochemistry, The Second Institute of Oceanography, State Oceanic Administration (SOA) Leo Lai Chan, State Key Laboratory in Marine Pollution, City University of Hong Kong Xinfeng Dai, Key Laboratory of Marine Ecosystem and Biogeochemistry, The Second Institute of Oceanography, State Oceanic Administration (SOA) Jiajun Wu* , Presenter Email: jiajunwu@cityu.edu.hk Observations of harmful algal blooms (HABs) are increasing around the world’s coastal ocean, with a growing number of reports indicating anthropogenic influences. These phenomena are caused by blooms of microscopic algae. Excessive microalgae growth in response to nutrient increases and/or shifts in nutrient ratios can result in a HABs of a single or several species that has negative ecosystem impacts. As HABs occur and result in the depletion of oxygen (hypoxia) in the water, fishery disasters were reported by many countries. To study the relationship of HABs and hypoxia, we conducted a study on 34 sites via cruise at the Pearl River Estuary from July 10-21, 2017. The temperature and salinity of surface water ranged from 27.9-30.8 oC and 5.4-33.9psu, respectively. The dissolved oxygen (DO) of surface water was from 6.1 to 11.7, while the DO of bottom water was lowest reach to 1.3 and the highest DO were just 7.4, an obvious hypoxia and low oxygen zone were detected. At the same time a wide range of phytoplankton bloom in surface water were observed over 10 study sites over 5000cells/ml and with maximum 45560 cells/ml at site F201. After the principal component analysis, there showed a positive correlation between phytoplankton abundance of surface water and DO of surface water, and a negative correlation were shown between phytoplankton abundance of surface water and DO at bottom layer. Further, with the analysis of Next Generation Sequencing on bacteria composition of sediment, we have detected several typical bacteria may related HABs and hypoxia, such as family Saprospiraceae and class Clostridia, which give some evidence on HABs and its coupled benthic microbial community may trigger and accelerate hypoxia zone formation at the Pearl River Estuary.