Program     Nitrogen cycling in the ocean: From genes to ecosystems and from the past to the future       1410 Hydro-biological modulation of nitrate and its dual isotopes in the northwest Pacific Ocean Monday 7th @ 1410-1430, Multifunction Hall Xiuli Yan* , Marine Science Institute, Shantou University, Shantou, China, 515063 Yanhua Wu, Shenzhen Marine Environment Monitoring Center Station, State Ocean Administration, Shenzhen, China, 518000 Jin-Yu Terence Yang, State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, China, 361005 Min Nina Xu, State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, China, 361005 Qiao Wu, State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, China, 361005 Huiwang Gao, Key Laboratory of Marine Environmental Science and Ecology, Ministry of Education, Ocean University of China, Qingdao, China, 266100 Shuh-Ji Kao, State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, China, 361005 Presenter Email: yanxl@stu.edu.cn The influences of anthropogenic nitrogen (such as increases in seawater N:P ratios and sedimentary δ15N) has been found to extend far into the northwest Pacific Ocean (NWPO) associated with atmospheric N deposition (AND). However, remains no observational δ15N evidence was reported to witness the cumulative effect of AND on seawater nitrate (NO3-). Coupled with N biogeochemical processes associated with assimilation and recycling, hydrological influence may play important role in modulating the signal of N dynamics in horizontal and vertical scales. The mid-latitude NWPO is composed of multi water masses, such as the Kuroshio Water, Oyashio Water, North Pacific Intermediate Water (NPIW) and North Pacific Deep Water. Under the scenario of increasing anthropogenic N influences and complex physical-biological dynamics, we aimed to explore the NO3- dynamics in the NWPO, especially from the perspective of N isotope. Water samples were collected during the spring of 2015 to investigate the spatial distribution of NO3- and its dual isotopes (δ15NNO3 and δ18ONO3) versus N* (N*=Nobs-16Pobs+2.9). The signal of NPIW (the salinity minima near 26.5-27.0σ) was observed latitudinally in the upper 1000 m varying concomitantly with the depth of nitracline. In the upper layer, NO3- decreased significantly upward while dual isotopic compositions (especially for δ18ONO3) covary becoming heavier due to NO3- assimilation by phytoplankton. Intriguingly, at some stations in the euphotic zone, δ15NNO3 decreased firstly and then increased upward deviating from the upward variability in δ18ONO3. This was contributed to the influences of AND (include N2 fixation) and nitrification according to the judgement of Δ(15,18)<0 and N*>0. Below the water depth of >400 m, both NO3- concentration and its dual isotopic compositions remained relatively a uniform value near the averages of NO3- in global ocean (5.3‰ for δ15NNO3, 2.4‰ for δ18ONO3).

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