Size-fractionated primary productivity and the phytoplankton-bacteria relationship in the Taiwan Strait

Size-fractionated phytoplankton biomass, primary productivity, photosynthetic dissolved organic carbon (PDOC), vertical distribution of pico-, nano-, and microphytoplankton, and the relationship with bacteria were studied in the Taiwan Strait during 2 cruises conducted in August 1994 and February-March 1995. Nanophytoplankton (3 to 20 mu m) dominated the community in the northern Taiwan Strait (NTS) while picophytoplankton (0.2 to 3 mu m) dominated in the southern Taiwan Strait (STS). Nanophytoplankton accounted for 60 and 57 % of biomass, and 77 and 36 % of productivity in summer and winter in the NTS, respectively. Picophytoplankton contributed to 54 and 63 % of biomass, and 85 and 48 % of productivity in summer and winter in the STS, respectively. The vertical distribution pattern in the Taiwan Strait consisted of microphytoplankton (20 to 200 mu m) mainly at the surface, nanophytoplankton in the middle and picophytoplankton at the bottom of the euphotic zone. Regression analysis indicated that bacterial abundance and biomass were positively significantly correlated with phytoplankton biomass and productivity, respectively, implying that the phytoplankton was an important organic carbon source for supporting bacterial growth. All the results suggested that the microbial loop played an important role in carbon cycling in the study areas, especially in the STS. The factors controlling the size structure, PDOC and vertical distribution of picophytoplankton are discussed.