Seasonal variability of coccolithophore abundance and assemblage in the northern South China Sea

Seasonal variations of coccolithophore abundances and assemblages and their causal factors were studied in the shelf and the basin of the northern South China Sea (SCS). Coccolithophores were counted and identified using scanning electron microscopy on samples from: (A) four cruises between March 2002 and July 2003 to survey the spatial and temporal fluctuations of coccolithophore communities in surface (5 m) water of 12 stations in the SCS shelf and basin; (B) a sediment trap deployed between August 2004 and January 2005 at 515-m depth near the South East Asian Time-series Study (SEATS) station in the SCS basin to estimate downward fluxes of coccolithophores; and (C) three cruises to the SEATS station during the period of sediment trap deployment to study the flux coupling. Abundances of coccolithophores in the surface-water ranged between 1.98 and 127.43×103 coccospheres l−1, and showed clear seasonal and shelf-basin variations. More coccolithophores appeared in cold (winter and spring) months when the northeastern monsoon prevailed than in warm (summer and autumn) months when the southwestern monsoon prevailed. Coccolithophore abundance was greater in the basin than on the shelf during cold months; the reverse was true during warm months. While Emiliania huxleyi, Gephyrocapsa oceanica and Gephyrocapsa ericsonii were predominant in cold months, oligotrophic Umbellosphaera irregularis, Umbellosphaera tenuis and Discosphaera tubifera predominated during warm months. Surface coccolithophore abundances were positively (P<0.05) correlated with surface chlorophyll a concentration and mixed layer depth, and were correlated negatively (P<0.05) to surface-water temperature, water-column stratification and depth of the nitracline. Second-order polynomial relationships established between coccolithophore abundance and surface chlorophyll a concentration or primary production could be best explained by competition for nutrients between coccolithophores and diatoms. The seasonality of downward fluxes of coccolithophore taxa composition and biomass were consistent with those in the overlying surface-water. Downward flux increased from 1.61 to 19.88×106 m−2 d−1 during August–November to 128.83×106 m−2 d−1 at mid-December. Because of their strong seasonal signal, which also may vary with nutrient (surface nitrate) availability, the relationships between the coccolithophore abundance and predominant taxa, as well as the alternating dominancy between coccolithophores and diatoms, could be used as tools to decipher long-term environmental changes relating to nutrient dynamics in the SCS, especially in retrospective studies of paleoenvironments and predictions for future ENSO events.