Program     Ocean Circulation, Ecosystem and Hypoxia around Hong Kong Waters       Poster The spatial distribution of mesozooplankton biomass, abundance and size spectra in response to hypoxia in the Pearl River estuary P-SPS4-06-S Zhiyuan Shi* , The Hong Kong University of Science and Technology Mitsuhide Sato, The Hong Kong University of Science and Technology Yanhong LU, The Hong Kong University of Science and Technology Zhimeng XU, The Hong Kong University of Science and Technology Hongbin LIU, Presenter Email: zshiae@connect.ust.hk The Pearl River estuary was investigated to examine the distribution patterns of mesozooplankton (200-2000 um size range) and its relationships with hypoxia (< 2 mg/L in dissolved oxygen). Both plankton net towing and high-capacity pump sampling were used to collect mesozooplankton at different stations and water depths. Mesozooplankton had a higher biomass and abundance in the whole water column where bottom water hypoxia was observed. There were higher mesozooplankton biomass in the bottom hypoxic water during the daytime, which suggests possible tolerance to low oxygen of some migratory species. Copepods were the numerically dominant mesozooplankton group at all sampling stations. Stations with bottom hypoxic waters had lower taxonomic diversity and were most dominated by Acartia spp. and Paracalanus spp. The normalized biovolume size spectrum (NBSS) of mesozooplankton was constructed to analyze the trophic structure of the community. The high intercepts of the NBSS indicated a high productivity in the stations with bottom hypoxic waters, which were consistent with the accompanying high mesozooplankton biomass and total Chl a concentration. The higher mesozooplankton biomass in those stations with hypoxia may be supported by the high phytoplankton biomass and the tolerance of some migratory species. Our results indicate the ecological relevance of mesozooplankton to the estuary hypoxia and provide more information for a further understanding of the estuarine ecosystem.