Differences in planktonic ciliate spatial distribution in spring and autumn in the southern Yellow Sea ZHANG Shan 1, 2 , LI Haibo 1, 2 , CHEN Xue 1, 2, 3 , DONG Yi 1, 2 , ZHANG Fang 1, 2 , XIAO Tian 1, 2 , ZHANG Wuchang 1, 2 , ZHAO Yuan 1, 2 * 1 CAS Key Laboratory of Marine Ecology and Environmental Sciences, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China 2 Laboratory for Marine Ecology and Environmental Science, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266071, China 3 University of Chinese Academy of Sciences, Beijing 100049, China Received 14 June 2016; accepted 2 August 2016 ©The Chinese Society of Oceanography and Springer-Verlag GmbH Germany, part of Springer Nature 2018 Abstract Seasonal variation of marine plankton spatial distribution is important in understanding the biological processes in the ocean. In this study, we studied spatial distribution of planktonic ciliate abundance and biomass in the central deep area (station depth greater than 60 m) and the coastal shallow area (station depth less than 60 m) of the southern Yellow Sea (32°–36.5°N, 121°–125°E) in spring (April) and autumn (October–November) of 2006. Our results showed that both ciliate abundance and biomass in the surface waters were higher in spring ((1 490±2 336) ind./L; (4.11±7.81) μg/L) than in autumn ((972±823) ind./L; (1.11±1.18) μg/L, calculated by carbon). Ciliate abundance and biomass in the surface waters of the coastal shallow area were similar in spring and autumn. However, in the central deep area, those values were much higher in spring ((1 878±2 893) ind./L; (5.99±10.10) μg/L) than in autumn ((738±373) ind./L; (0.74±0.76) μg/L). High values of ciliate abundance and biomass occurred in the central deep area in spring and in the coastal shallow area in autumn. Mixotrophic ciliate Laboea strobila was abundant in the central deep area in spring, when a phytoplankton bloom occurred. However, in autumn, L. strobila was abundant in the coastal shallow area. Boreal tintinnid Ptychocyli obtusa was found in spring. Both L. strobila and P. obtusa were concentrated in the surface waters when their abundance was more than 1 000 ind./L. Peaks of these species were in the subsurface waters when their abundance was less than 400 ind./L. This study showed that both high abundance and biomass of ciliates occurred in different areas in southern Yellow Sea seasonally. Key words: ciliates, abundance, biomass, southern Yellow Sea Citation: Zhang Shan, Li Haibo, Chen Xue, Dong Yi, Zhang Fang, Xiao Tian, Zhang Wuchang, Zhao Yuan. 2018. Differences in planktonic ciliate spatial distribution in spring and autumn in the southern Yellow Sea. Acta Oceanologica Sinica, 37(4): 48–57, doi: 10.1007/s13131- 018-1147-y 1  Instruction Marine planktonic ciliates are a group of single-celled proto- zoan in marine ecosystem. They are divided into loricate (tintin- nids) and aloricate (naked) forms. With length from 5 μm to 200 μm, they are important members of microzooplankton. As key components of the marine microbial food web, they play an im- portant role in plankton ecosystem and act as the primary consum- ers of the pico/nano-producers, major food sources of mesozo- oplanktons and larval fish (Pierce, 1992; Montagnes et al., 2010). Therefore, they are linkages between microbial food web and classical food chain (Edwards and Burkill, 1995). They are also important nutrient regenerators (Laval-Peuto et al., 1986; Pierce and Turner, 1994) in the marine ecosystems. There are seasonal changes of abundance in currents and stratifications in the coastal seas. However, little information was reported about the spatial distribution in different seasons, particularly in coastal seas. The Yellow Sea (average station depth of 44 m) is a coastal sea located on the continental shelf of the western Pacific Ocean. Here a significant seasonal phenomenon is the formation and dissipation of the Yellow Sea Cold Bottom Water (YSCBW), a basin-scale water mass with low temperature under thermocline in the warm period of a year (Ho et al., 1959; Yu et al., 2006). An- other character is the spring phytoplankton bloom (Chl a con- centration greater than 4 μg/L) from April to May in the central Yellow Sea (Tang et al., 2013). Spatial distribution of planktonic ciliates has been studied in different parts and seasons. Horizont- al distribution of planktonic ciliates in the southern Yellow Sea was limited in April, August (Ding and Xu, 2012) and June (Zhang et al., 2002). High abundance area of planktonic ciliates and an- chovy larvae was overlapped in June (Zhang et al., 2002). Jelly- fish grazing on ciliates was ascribed as possible reason for low ciliate abundance in some area of the Yellow Sea (Ding and Xu, 2012). The spatial distributions of large tintinnid were well correl- ated with hydrology (Zhang et al., 2008, 2009). The monthly vari- ation of ciliate abundance in two transects in YSCBW showed that there was low ciliate abundance in the YSCBW while high   Foundation item: The National Natural Science Foundation of China (NSFC) under contract Nos U1606404 and 41576164; the Strategic Priority Research Programs under contract No. XDA11020103.1. *Corresponding author, E-mail: yuanzhao@qdio.ac.cn   Acta Oceanol. Sin., 2018, Vol. 37, No. 4, P. 48–57 DOI: 10.1007/s13131-018-1147-y http://www.hyxb.org.cn E-mail: hyxbe@263.net