中文摘要：

2014年10月在南海东北部利用15N和14C示踪法实测了28个站位表层固氮速率和初级生产力,结合温度、盐度、营养盐和叶绿素a(Cha)分析了南海东北部表层水体生物固氮及其对初级生产力的贡献。结果表明,受珠江冲淡水影响最明显的站位(A1)肉眼可见大量丝状束毛藻(Trichodesmium),其固氮速率达3475nmolN·m–3·h–1,是其他站位固氮速率的1~3个数量级之多;其他站位表层固氮速率在4~213nmolN·m–3·h–1之间,平均固氮速率为77±54nmolN·m–3·h–1(n=27),固氮速率最低的站位出现在受中国沿岸流影响的台湾浅滩及其邻近海域。近岸(水深小于100m)站位表层固氮速率与温度显著正相关(R=0.68,n=11,p<0.05),相对高温、高磷和低硝酸盐可能是珠江冲淡水影响站位出现高固氮速率的原因,低温低盐则可能是台湾浅滩及其邻近近岸区域出现低固氮速率的原因。初级生产力(14.19~0.17mgC·m–3·h–1)从近岸区域到外海逐渐降低,平均为0.61±0.45mgC·m–3·h–1,最高站位出现在台湾浅滩,最低站位出现在深海区。固氮作用对初级生产所需氮的贡献率(0.08%~9.30%,平均3.90%±3.10%)从近岸到外海逐渐增加,表层水温高于26℃时,固氮对初级生产的氮贡献显著增加。以上结果表明,秋季南海东北部固氮和初级生产受珠江冲淡水和沿岸流等物理过程调控。

英文摘要：

Biological N2 fixation rate and primary productivity in surface seawater were measured using 15N and 14C tracerassays, respectively, at 28 stations in the northeastern South China Sea during October 2014. Combined with variables including temperature and salinity, dissolved nutrients and chlorophyll a, the spatial distribution of N2 fixation rate, and thenitrogen contribution of N2 fixation to primary production were discussed. Our results showed that the highest rate(3475 nmolN·m–3·h–1) was observed at a Pearl River plume-affected station, where substantial filamentous Trichodesmiumcolonies apparent to naked eyes occurred. The highest rate was about two or three orders of magnitude higher than thoseobserved at other stations, where N2 fixation rates ranged from 4 to 213 nmolN·m–3·h–1 with an average of 77±54 nmolN·m–3·h–1(n=27) were detected. The lowest N2 fixation rate was observed in the Taiwan Bank and its adjacent seawaters influenced bythe China Coastal Current. There was a significant negative correlation between N2 fixation rate and surface seawatertemperature in the inshore waters (bottom depth <100 m; R=0.68, p<0.05). Higher seawater temperature, higher phosphateconcentration and lower nitrate plus nitrite concentration may contribute to the higher N2 fixation rate at the stationsinfluenced by the Pearl River plume, while lower seawater temperature and salinity may result in the lower N2 fixation rate inthe Taiwan Bank and its adjacent inshore seawaters influenced by the China Coastal Current. The primary productivity(14.19?0.17 mgC·m–3·h–1) decreased from the inshore area to the open waters. N2 fixation could meet 0.08% to 9.30%(3.90%±3.10% on average, n=28) of the nitrogen demand by primary production and the contribution percentage increasedfrom the inshore area toward the open waters. In addition, the contribution percentage significantly increased when the surfacetemperature was higher than 26℃. Our results indicated that nitrogen fixation and primary production could be