REPORT Spatial distribution patterns of phytoplankton biomass and primary productivity in six coral atolls in the central South China Sea Zhixin Ke 1,2 • Yehui Tan 1,3 • Liangmin Huang 1,3 • Huajian Liu 1 • Jiaxing Liu 1 • Xin Jiang 1 • Junxing Wang 1 Received: 9 November 2017 / Accepted: 14 July 2018 / Published online: 19 July 2018 Ó Springer-Verlag GmbH Germany, part of Springer Nature 2018 Abstract Nutrients’ concentration, phytoplankton bio- mass, and primary productivity were investigated in six coral atolls in the central South China Sea during the southwest monsoon in the early summer. Nutrients’ con- centrations were generally very low in these atolls, and showed no obvious variation among outer reef slope, reef flat, and lagoon. The spatial variation of phytoplankton biomass and primary productivity were significant among the different regions of the atolls, and generally followed the order of outer reef slope \ reef flat \ lagoon. Phyto- plankton biomass might be a more sensitive indicator than nutrients for the habitat differences in coral atoll ecosys- tem. The lagoon of Huangyan Atoll showed the highest phytoplankton biomass and primary productivity. The phytoplankton biomass and primary productivity were mainly contributed by pico-phytoplankton. Moreover, our results suggested that there might have a threshold value of Chl a for pico-phytoplankton abundance. When Chl a was higher than this threshold value (about 0.5 lgL -1 ), the pico-phytoplankton abundance should not linearly increase with the Chl a concentration. Due to the significantly high- phytoplankton biomass and primary productivity, the anthropogenic eutrophication should be paid attention in the lagoon of Huangyan Atoll. Keywords Coral reef Á Phytoplankton Á Chlorophyll a Á Atoll Á Ecosystem health Introduction Coral reefs are considered to be oases with high diversity and productivity in an oceanic desert. Currently, the ecosystem health of coral reefs is threatened worldwide by environmental changes and human activities, mainly including global warming and ocean acidification (Hoegh- Guldberg et al. 2007; Pandolfi et al. 2011), anthropogenic eutrophication (Bell et al. 2014; Cao et al. 2017), physical damage and land reclamation (Mora et al. 2016; Asner et al. 2017), overfishing (Bellwood et al. 2004; Arai 2015), diseases, and outbreaks of coral predators (Uthicket et al. 2015; Vercelloni et al. 2017). Coral reefs have suffered remarkable declines worldwide (Yu 2012; Jackson et al. 2014; Gardner et al. 2003). It has been estimated that over 80% of Southeast Asia’s coral reefs to be under threat (Burke et al. 2002). The decadal degradation rate of coral reefs reached to 16% in the South China Sea basin in 2007 (UNEP 2007). Jackson et al. (2014) reported that the average live coral cover declined from 34.8% in 1970–1983 to 16.3% in 1999–2011 in Caribbean coral reefs. To protect these fragile and unique ecosystems, more studies and restoration efforts are needed in coral-reef regions (Bellwood et al. 2004). In general, nutrient over enrichment is considered to be a major reason for the degradation of coral reefs (Brodie et al. 2011; Slijkerman et al. 2014; Vega Thurber et al. 2014). It is very likely that the addition of even small Topic Editor Dr. Mark R. Patterson & Yehui Tan tanyh@scsio.ac.cn 1 CAS Key Laboratory of Tropical Marine Bio-resources and Ecology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou, China 2 Guangdong Provincial Key Laboratory of Applied Marine Biology, West Xin’gang Road 164#, Guangzhou 510301, China 3 University of Chinese Academy of Sciences, Beijing 100049, China 123 Coral Reefs (2018) 37:919–927 https://doi.org/10.1007/s00338-018-1717-7