Seasonal variation, horizontal and vertical distribution, and cell size of nanoflagellates, together with physico-chemical and biological factors, were studied in the northern South China Sea (SCS). It was found that nanoflagellate abundance ranged from 0.157 x 10(3) to 9.193 x 10(3) cells/ml (with a mean of 0.891 x 10(3)) in winter (February, 2004), while it ranged from 0.107 x 10(3) to 5.417 x 10(3) cells/ml (with a mean of 0.599 x 10(3)) in summer (July, 2004). Nanoflagellates were more abundant in winter than summer in offshore regions, showing an unique seasonal pattern in this subtropical marginal sea. The abundance of nanoflagellates decreased from the estuary to the offshore region. Vertical distribution of nanoflagellates coupled well with that of bacteria and Chl a. The small size fraction of less than 5 mu m dominated the nanoflagellate populations. Wind-driven mixing, eddies, availability of nutrients as well as Chl a and abundance of picoplankton seemed to be the major controlling factors for the spatial distribution and seasonal variation of nanoflagellates in the study area
