Two Production Stages of Coccolithophores in Winter as Revealed by Sediment Traps in the Northern South China Sea.

Coccolithophores, originating in the Late Triassic, are one of the most successful marine calcifying algae living in modern oceans. Coccoliths are the calcareous fossil remnants left in marine sediments after coccolithophores die. These calcite scales record the conditions of the surface ocean (e.g., primary productivity, seawater temperature, and carbonate chemistry) and are expected to be a promising subject in paleoceanographic and paleoclimatic studies. Hence, a comprehensive understanding of the ecology and phenology of living coccolithophores, as well as their interactions with other plankton groups, is needed to develop better constraints on their uses in paleoenvironmental studies. Here we show modern coccolithophore production through the coccolith fluxes from sediment traps at a ~500‐m water depth from 2013 to 2015 in the northern South China Sea. In addition to the expected seasonality of the coccolithophore production that occurs during winter seasons due to the strong water mixing induced by monsoon winds, a two‐stage mode for the coccolithophore production is also recognized in relatively "coastal" waters. The first stage includes the production of Gephyrocapsa oceanica in December, when the macronutrient inventory is built, and the second stage describes the growth of Emiliania huxleyi in late February with the depletion of silicate nutrients. This two‐stage mode originates from subtle differences in the nutrient assimilation ability between the two species. In addition, coccolithophore production is significantly influenced by the decadal oceanic events (i.e., El Niño) in the northern South China Sea. Plain Language Summary: Coccolithophores are a kind of marine unicellular algae. They can secrete calcareous scales named coccoliths around their cells. These scales can record the environmental conditions, for example, temperature and CO2 of surface ocean, when coccolithophores are alive. After death, these scales settle to the seafloor and are stored in sediments for up to millions of years. The scales in old sediments can reflect ancient climates and environments. Thus, understanding coccolithophores' life in modern oceans is important. In this study, we deployed a sediment trap at a 500‐m water depth in the northern South China Sea. These traps collected the scales of coccolithophores and record the annual variations in modern coccolithophore population. Higher numbers of scales are found in winter, thereby indicating that more coccolithophores live during this period because of a higher nutrient availability. The coccolithophore scale records in the traps differ spatially and temporally. In coastal waters, one extreme nutrient‐preference species can live with diatoms (large and completive algae) in early winter, and in later winter, another relatively cosmopolitan species appears. The different phenology of these two species is related to seawater nitrogen and silicon. In addition, we also find that the coccolithophore population declines during El Niño periods. Key Points: Coccolith fluxes increased during winter seasons due to the enhanced water mixing and nutrient supply in the northern South China SeaTwo stages of coccolithophore production of Emiliania huxleyi and Gephyrocapsa oceanica were found in relatively "coastal" watersLower winter coccolithophore production was found during an El Niño event in the northern South China Sea [ABSTRACT FROM AUTHOR]