ECOLOGICAL STUDY OF PICOPLANKTON IN NORTHERN SOUTH CHINA SEA

Abundance, carbon biomass and composition of picoplankton along a transect (from the Pearl River estuary to Dongsha Island) in the northern South China Sea were measured by flow cytom-etry (FCM) ; and the vertical structure, composition variation, relationship between heterotrophic and au-totrophic picoplankton as well as the controlling mechanism of diel variation were studied. Results showed that along the horizontal direction, both Synechococcus (SYN) and picoeukaryote (PEUK) were highest in abundance and dominated autotrophic picoplankton in the Pearl River estuary, but decreased significantly from the estuary to offshore. Prochlorococcus (PRO) dominated autotrophic picoplankton in the offshore water, but its abundance was lower than that of heterotrophic bacteria (BAC) . BAC distribution related closely with autotrophic picoplankton, and was affected significantly by primary production. Four categories, especially PRO and PEUK, were dominant in the subsurface layers, and were usually associated with the subsurface chlorophyll a maximum, which might be mainly due to their ability to live under low light intensity and weak nutrient dynamics. Based on the contribution of carbon biomass, three distinct cases of the studied waters were roughly classified. The first case, observed in the estuary, was dominated by SYN and PEUK; the second case was characterized by oligotrophic status, with the highest contribution of PRO in autotrophic picoplankton; the last case may relate to the cold eddy and was co-dominated by PRO and PEUK. The diel variation indicated that PRO and PEUK abundance peaked late in the night, which could be due to the combined effects of tidal currents, cell division and grazing.