Program     Biogeochemistry of organic matter and associated elements along the river-estuary-ocean continuum       1030 Exchange Process and Biodegradation Mechanisms of Dissolved Organic Carbon Derived by Kuroshio Intrusion in the Northern South China Sea Wednesday 9th @ 1030-1050, Conference Hall Xiaolin Li* , Xiamen University Zhanfei Liu, The university of Texas at Austin Kai Wu, Xiamen University Peng Jiang, Xiamen University Shuai Gu, Xiamen University Bangqin Huang, Xiamen University Rui Zhang, Xiamen University Hongmei Chen, Old Dominion University Minhan Dai, Xiamen University Presenter Email: xlli@xmu.edu.cn Transfer and transformation of dissolved organic carbon (DOC) in marginal sea is a key component of carbon cycling in the ocean. However, we still need better understanding about transformation mechanisms of DOC and its impact to the carbon cycle in the marginal sea. This research focused on the exchange and transformation processes of DOC derived from Kuroshio intrusion in the norther South China Sea (SCS) during the cruise of summer 2014 and winter 2017. Basing on the isopycnal mixing model, we found that DOC distribution in the northern SCS was influenced by the Kuroshio intrusion in both seasons. And significant decomposition of DOC was observed during the mixing (up to 6-7 micro M) which accounting for about 10% of the DOC measured in the surface water of SCS. To better understand the transformation mechanisms of extraneous DOC in the SCS, on-board incubation experiments were carried out under different environmental context (e.g. microbial community, nutrient levels). We found that DOC derived from Kuroshio can be decomposed by the microbes from SCS, and DOC concentration decreased from 80 to 77 micro M within 14 days when the bacterial abundance doubled in the first two days of the experiment. The decomposition was stimulated by increasing the nutrients (nitrate and phosphate) levels to the same levels as what were measured in the 100m depth of SCS. The DOC level decreased from 80 to 73 micro M, and significant higher microbial decomposition rate and bioavailable DOC were observed in the incubation experiment. The decomposition rates and bioavailable DOC were found higher in the experiments when extraneous DOC decomposed by local microbial community than the local DOC decomposed by local microbial community. Our evidences indicated that the context of the microbial transformation, e.g. microbial community structure, nutrients levels, is essential that controlling the bioavailability of DOC. Depending on our estimation, the decomposition of DOC derived from Kuroshio intrusion could contribute 0.36 mmol N/m2/day which could be another important source of nitrogen comparing to nitrogen fixation and aerosol deposition in the SCS.

f7f7f7">