Distribution of reactive aluminum under the influence of mesoscale eddies in the western South China Sea LIU Jiaxing 1, 2, 3 , ZHOU Linbin 1, 2 , TAN Yehui 1, 2 , WANG Qiong 1, 2, 3 , HU Zifeng 1, 2, 3 , LI Jiajun 1, 2, 3 , JIANG Xin 1, 2, 3 , KE Zhixin 1, 2 * 1 Key Laboratory of Tropical Marine Bio-resources and Ecology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China 2 Guangdong Provincial Key Laboratory of Applied Marine Biology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China 3 University of Chinese Academy of Sciences, Beijing 100049, China Received 7 March 2016; accepted 30 June 2016 ©The Chinese Society of Oceanography and Springer-Verlag Berlin Heidelberg 2017 Abstract To understand the distribution of aluminum (Al) under the influence of mesocale eddies in the western South China Sea (SCS), sea level anomaly, geostrophic current, environmental parameters and reactive Al were investigated in the western SCS in August 2013. The highest reactive Al concentration ((180±64) nmol/L) was observed in the surface waters, indicating a substantial atmospheric input. Vertically, the reactive Al decreased from the surface high concentration to the subsurface minima at the depth of chlorophyll a (Chl a) maxima and then increased again with depth at most of the stations. The average concentration of reactive Al in the upper 100 m water column was significantly lower in the cyclonic eddy ((137±6) nmol/L) as compared with that in the non- eddy waters ((180±21) nmol/L). By contrast, the average concentrations of Chl a and silicate in the upper 100 m water column were higher in the cyclonic eddy and lower in the anticyclonic eddy. There was a significant negative correlation between the average concentrations of reactive Al and Chl a in the upper 100 m water column. The vertical distribution of reactive Al and the negative correlation between reactive Al and Chl a both suggest that the reactive Al in the upper water column was significantly influenced by biological removal processes. Our results indicate that mesoscale eddies could regulate the distribution of reactive Al by influencing the primary production and phytoplankton community structure in the western SCS. Key words: reactive aluminum, mesoscale eddy, chlorophyll a, biological removal, phytoplankton, primary production, western South China Sea Citation: Liu Jiaxing, Zhou Linbin, Tan Yehui, Wang Qiong, Hu Zifeng, Li Jiajun, Jiang Xin, Ke Zhixin. 2017. Distribution of reactive aluminum under the influence of mesoscale eddies in the western South China Sea. Acta Oceanologica Sinica, 36(6): 95–103, doi: 10.1007/s13131-017-1046-7 1  Introduction Aluminum (Al) is the most abundant metallic element in the earth’s crust (Taylor, 1964). The distribution of Al in the ocean represents a potent geochemical tracer of terrigenous input to the ocean (Orians and Bruland, 1986; Measures and Vink, 2000; Measures et al., 2005; Giesbrecht et al., 2013). Al distribution in seawater is associated with biological activity (such as biological uptake, and passive adsorption on biogenic particle surface), but to date no established biological function of Al has been found (Dammshäuser et al., 2011). For a better understanding of the roles of Al in the marine biogeochemical cycle, it is important to investigate the distribution of Al in the oceans. Atmospheric input is the main source of Al in the surface of open oceans (Duce et al., 1991; Measures and Vink, 2000; Kramer et al., 2004; Measures et al., 2015). Typically, the scavenging type of vertical distribution of dissolved Al in the Pacific and Atlantic Oceans is characterized with the highest concentration in the surface waters mainly caused by atmospheric dust input, and a mid-depth minimum due to particle scavenging, and an increase by remineralization in bottom waters (Orians and Bruland, 1986; Measures and Edmond, 1990; Hydes et al., 1988; Chou and Wol- last, 1997; Schüßler et al., 2005; Tria et al., 2007). Al in seawater could be removed by either passive or active biological processes (Orians and Bruland, 1986) such as adsorption onto biogenic particles or active incorporation into phytoplankton cell tissues (Stoffyn, 1979; Moran and Moore, 1988; Gehlen et al., 2002; Li et al., 2013; Wang et al., 2013). Significant positive correlation between biological productivity and the removal of Al in the At- lantic Ocean (Dammshäuser et al., 2013), the English Channel (Hydes, 1989) and the southern Yellow Sea (Ren et al., 2011) have been reported. In addition to the biological removal, physical processes (such as mesoscale eddy) and chemical (such as chemical precipitation) processes are also important factors in- fluencing Al distribution in seawater (Stoffyn and Mackenzie, 1982; Brown et al., 2012; Measures et al., 2015). Mesoscale eddies play a critical role in marine biogeochem-   Foundation item: The Strategic Priority Research Program of the Chinese Academy of Sciences under contract No. XDA11020305; the National Basic Research Program (973 program) of China under contract No. 2015CB452903; the Special Fund for Agro-scientific Research in the Public Interest under contract No. 201403008; the National Project of Basic Sciences and Technology under contract No. 2017FY201404; the National Natural Science Foundation of China under contract Nos 41506150 and 41276162. *Corresponding author, E-mail: kzx@scsio.ac.cn   Acta Oceanol. Sin., 2017, Vol. 36, No. 6, P. 95–103 DOI: 10.1007/s13131-017-1046-7 http://www.hyxb.org.cn E-mail: hyxbe@263.net