The Microzooplankton And Its Grazing On Phytoplankton In Typical Waters Of South China Sea

By using the dilution experiment and HPLC-based pigment analysis, the variations of abundance, biomass, composition of microzooplankton, group-specific phytoplankton growth rates and microzooplankton grazing rates were studied in the central and northern South China Sea from 2005 to 2006. Results are as follows:1. Microzooplankton was dominated by ciliates, especially Strombidium spp., Tintinnopsis spp, and Strobilidium sp. in north-east of South China Sea.The abundance and biomass varied with depths and years. The depth-integrated mean abudance of microzooplankton was lower in July 2005(582 ind/L)than that in June 2006(664 ind/L).The depth-integrated mean biomass of microzooplankton was also lower in July 2005(0.98μgC/ L)than that of June 2006(1.18μgC/L). For vertical profiles, microzooplankton was dense at the surface waters for all the transects during summer in 2005, while at the DCM (depth of the maximum Chl a) in 2006.2. The abundance of microzooplankton in winter in central and northern South China Sea, was varied with the different areas.The abundance of three transects ranged from 25 to 475 ind/L, there were higher in the shelf off Zhujiang River estuary than those off Vietnam and northwest Luzon. The ciliates, especially the Strombidium spp., were the dominated species of the microzooplankton. The heterotrophic dinoflagellates and copepod nauplius were only account for less than 10% of microzooplankton. The contributions of them for microzooplankton were different at different layers. The biomass of microzooplankton ranged from 0.04⁰.81μg C/ L, the distribution patterns were similar to that of abundance.3. The growth rates of phytoplankton at Stn. A1 of northern South China Sea ranged from 1.34 d^-1 , the microzooplankton grazing rates ranged from 0.79 d^-1. The ranges of microzooplankton grazing pressure on phytoplankton standing stock and primary production were 55% and 74% at Stn.A1, respectively. The growth rates of phytoplankton at Stn. B1 ranged from 0.55～1.28 d^-1, the microzooplankton grazing rates ranged from 0.27～0.85 d^-1. The ranges of microzooplankton grazing pressure on phytoplankton standing stock and primary production were 24～58% and 35¹14% at Stn.B1, respectively. While in the winter of South China Sea, the growth rates of phytoplankton ranged from 0.83～1.11 d^-1 , and the microzooplankton grazing rates ranged from 0.53～2.64 d^-1. The ranges of microzooplankton grazing pressure on phytoplankton standing stock and primary production were 41～93% and 73¹38%, respectively.4.The grazing pressure of microzooplankton on different groups were different. The growth rates of diatom at Stn. B1 ranged from 0.51～1.17 d^-1, the microzooplankton grazing rates ranged from 0.43～1.27 d^-1. The ranges of microzooplankton grazing pressure on standing stock and primary production were 35～72% and 72¹18% at Stn.B1, respectively. The growth rates of prymnesiophytes at Stn. B1 ranged from 0.64～1.81 d^-1, the microzooplankton grazing rates ranged from 0.23～2.22 d^-1. The ranges of microzooplankton grazing pressure on standing stock and primary production were 21～89% and 36¹24%, respectively. The growth rates of cyanobacteria at Stn. B1 ranged from 0.5～2.36 d^-1, the microzooplankton grazing rates ranged from 0.5～1.62 d^-1. The ranges of microzooplankton grazing pressure on standing stock and primary production were 39～80% and 52¹62%, respectively. While in the winter of South China Sea, the growth rate of dinoflagellates at Stn.A9 was 1.07 d^-1 , and the microzooplankton grazing rate was 0.81 d^-1. The ranges of microzooplankton grazing pressure on standing stock and primary production were 78% and 90%, respectively. The growth rate of diatoms at Stn.A9 was 0.89 d^-1, and the microzooplankton grazing rate was 0.55 d^-1. The ranges of microzooplankton grazing pressure on standing stock and primary production were 31% and 67%, respectively. The growth rate of Cyanobacteria at Stn.A9 was 0.96 d^-1 , and the microzooplankton grazing rate was 1.3 d^-1. The ranges of microzooplankton grazing pressure on standing stock and primary production were 73% and 118%, respectively.5. The results of relative preference index at Stn.B1 showed that there was no preference for a particular phytoplankton groups. In overall, the diatoms, which was always dominated, was avoided grazing or no preference.While the microzooplankton preferentially grazed cyanobacteria and prymnesiophytes. As a whole, microzooplankton avoided diatom, mostly preferentially grazed dinoflagellates and cyanobacteria at Stn. A1 of Zhujiang River estuary in South China Sea in winter of 2006.