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The effect of K uroshio C urrent on nitrate dynamics in the southern E ast C hina S ea revealed by nitrate isotopic composition
Author(s) -
Wang Wentao,
Yu Zhiming,
Song Xiuxian,
Wu Zaixing,
Yuan Yongquan,
Zhou Peng,
Cao Xihua
Publication year - 2016
Publication title -
journal of geophysical research: oceans
Language(s) - English
Resource type - Journals
eISSN - 2169-9291
pISSN - 2169-9275
DOI - 10.1002/2016jc011882
Subject(s) - upwelling , nitrate , oceanography , water mass , isotope fractionation , photic zone , transect , environmental science , geology , fractionation , phytoplankton , chemistry , nutrient , organic chemistry
In spring 2014 (May–June), in the southern East China Sea (ECS) and east of Taiwan, nutrient and isotope samples were collected and analyzed. Also, dissolved oxygen (DO), chlorophyll‐ a , and physical parameters were determined. The Kuroshio subsurface water intruded into the ECS and separated into two branches on the continental shelf: the nearshore Kuroshio branch current (NKBC) and the offshore Kuroshio branch current (OKBC). Nitrate concentration in the Kuroshio subsurface water slightly decreased east of Taiwan by assimilation and was supplied by upwelling currents northeast of Taiwan. The variations of DO, nitrate, and nitrogen isotopes in the bottom water showed that continuous nitrification occurred in the NKBC after intrusion into the ECS. This process might contribute to the hypoxia zone near the coast of Zhejiang Province, China. Our results also indicated that internal nitrogen cycles appeared in NKBC since the isotope fractionation ratio of oxygen and nitrogen ( 18 ɛ: 15 ɛ) inNO 3 −was 1.40. TheNO 3 −flux and budget were calculated in the NKBC via numerical simulations. It demonstrated that at least ∼0.52 kmolNO 3 −·s −1 was produced by nitrification from DH9 to DH5 transect, and ∼0.11 kmolNO 3 −·s −1 was consumed in the DH4 transect. Moreover, according to the Rayleigh model, primary production in most of southern ECS was supported by the intrusion of the Kuroshio subsurface water, causing 5‰ isotope fractionation. In some of nearshore stations which located in the northern investigated area, the assimilated nitrate was contributed from both the NKBC and coastal currents originated from the Changjiang diluted water.