Temporal variation of picoplankton in the spring bloom of Yellow Sea, China Yuan Zhao a , Li Zhao a,b , Tian Xiao a,n , Chenggang Liu c , Jun Sun d , Feng Zhou e , Sumei Liu f , Lingfeng Huang g a Key Laboratory of Marine Ecology and Environmental Sciences, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China b Graduate University, Chinese Academy of Sciences, Beijing 10086, China c Key Laboratory of Marine Ecosystem and Biogeochemistry, Second Institute of Oceanography, State Oceanic Administration, Hangzhou 310012, China e State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, State Oceanic Administration, Hangzhou 310012, China f Key Laboratory of Marine Chemistry Theory and Technology Ministry of Education, College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266100, China g Department of Oceanography, Xiamen University, Xiamen 361005, China d College of Marine Science and Engineering, Tianjin University of Science and Technology, Tianjin 300457, China article info Keywords: Synechococcus Picoeukaryote Heterotrophic bacteria Spring bloom Yellow Sea abstract Temporal variation of Synechococcus, picoeukaryote and heterotrophic bacteria abundance and depth integrated biomass during three spring blooms in 2007 and 2009 were investigated in the Yellow Sea, China. Synechococcus and picoeukaryote responded differently to different types and course of spring blooms. During the diatom blooms of 2007 and Bloom B20 in 2009, Synechococcus and picoeukaryote abundances decreased sharply during the bloom period. However, during a mixed dinoflagellate and diatom bloom of Bloom B23 in 2009, Synechococcus and picoeukaryote increased in abundance and biomass along the bloom. During all three spring blooms, heterotrophic bacteria biomass had a similar increasing trend. Ciliate and heterotrophic nanoflagellate grazing could be responsible for Synechococcus and picoeukaryote abundance and biomass decrease during the spring blooms. & 2013 Elsevier Ltd. All rights reserved. 1. Introduction Marine picoplankton refer to a functionally diverse group of organisms which are microscopic in size (≤2 μm in diameter), but very abundant in the marine water column. Picoplankton include cyanobacteria of the genera Synechococcus (Johnson and Sieburth, 1979; Waterbury et al., 1979) and Prochlorococcus (Chisholm et al., 1988), a great diverse assemblage of picoeukaryote and hetero- trophic bacteria which do not carry out oxygenic photosynthesis. Picoplankton have drawn much research attention due to its essential role in oceanic processes such as carbon production, biomass and energy transfer (Stockner, 1988; Li et al., 1983; Azam et al., 1983). Cyanobacteria are very important primary producers among autotrophic picoplankton. Generally, Synechococcus are present in inshore or coastal waters (Jochem, 1988) and can account for 20% of marine total biomass and 60% of total primary productivity (Caron et al., 1991). In some marine environments, picoeukaryote are major contributors in terms of biomass (Worden et al., 2004). Heterotrophic bacteria play an indispensable role in the carbon flux in aquatic ecosystems. They are decomposer of organic material, as well as utilizer of dissolved organic carbon (DOC) in marine ecosystems (Lefèvre et al., 1996; Hansell and Carlson, 1998). Thus heterotrophic bacteria are at the foundation of the microbial loop (Fuhrman and Azam, 1980). Spring phytoplankton bloom is one of the most important seasonal patterns in pelagic food webs (Townsend et al., 1994). The Yellow Sea, located between mainland China and the Korean peninsula, is a typical semi-closed marginal sea of the Pacific Ocean, with depths ranging from 90 m in the central trough to less than 20 m within 50 km off the coast. Several typical hydrological features are present in the Yellow Sea. In spring, with a strong activity of tidal mixing, surface water of the Yellow Sea is rich in nutrients. Together with seasonal warming and the seasonal onset of higher light availability, those conditions would trigger the spring phytoplankton bloom. The picoplankton distribution during spring phytoplankton bloom was investigated in several marine environments and different patterns were observed. On the one hand, an abrupt abundance decrease for Synechococcus and/or picoeukaryote fol- lowed by their abundance increase was observed during phyto- plankton blooms in mesocosm and field investigations (Cunliffe et al., 2009; Larsen et al., 2004; Martinez-Martinez et al., 2006; Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/dsr2 Deep-Sea Research II 0967-0645/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dsr2.2013.05.015 n Corresponding author. Tel./fax: +86 532 82898586. E-mail address: txiao@qdio.ac.cn (T. Xiao). Please cite this article as: Zhao, Y., et al., Temporal variation of picoplankton in the spring bloom of Yellow Sea, China. Deep-Sea Research II, (2013), http://dx.doi.org/10.1016/j.dsr2.2013.05.015 Deep-Sea Research II ∎ (∎∎∎∎) ∎∎∎–∎∎∎