Program     Carbon Cycle in the South China Sea: Budget, Controls and Global Implications       Poster Assessment of the (de)coupling between phytoplankton growth and microzooplankton grazing in the euphotic layer in the South China Sea P-SPS3-08-S Kailin Liu* , The Hong Kong University of Science and Technology Zhiyuan Shi, The Hong Kong University of Science and Technology Zhimeng Xu, The Hong Kong University of Science and Technology Hongbin Liu, The Hong Kong University of Science and Technology Presenter Email: kliuah@connect.ust.hk Microzooplankton (< 200um) herbivory is a main source of global phytoplankton biomass loss, accounting for about 67% of daily primary production and this proportion varies among marine habitats. However, this dataset is heavily depended on the rates estimated in the upper mixed layers, and the data of the deeper depth in the eutrophic zone is still scarce. At different depths, the phytoplankton growth rates will decrease with the depletion of light availability, while the grazing activities of micro-zooplankton may be less subject to light. As a result, the decoupling between growth and grazing rates may be pronounced with the increase of depths. Therefore, a vertical profile of grazing rates is required to gain insights into the role of micro-zooplankton in marine food-web and carbon cycling. Although significant grazing activities had already been found at the base of euphotic zone in the eastern Pacific Ocean according to Landry et al. (2011), it hasn’t been verified in marginal seas such as the South China Sea. In this study, we used a modified "two-points" dilution technique to estimate taxon-specific phytoplankton growth rates and mortality rates due to microzooplankton grazing activities in several depths of the euphotic zone to investigate the phytoplankton dynamics in South China Sea in summer and winter. Based on the preliminary results, the maximum phytoplankton growth rate occurs in the subsurface layer in summer, but in surface in winter. Nutrient limitation and strong light intensity may be the main reasons for the lower growth rate in the surface water in summer. The daily percentage of primary production consumed by microzooplankton of the euphotic water column ranged in 25%-151% in summer but 7%-35% in winter. At the same time, mesozooplankton biomass and herbivory was also measured, which showed that mesozooplankton only consumed a small proportion of phytoplankton growth (<10%), supporting the notion that microzooplankton is the main consumer of primary production and the most important link in both grazing food chain and microbial loop.