Ecology Studies Of Nanoflagellate And Ciliate In The South China Sea

As the main protozoan communities in the ocean,nanoflagellates(NF)and ciliates are one of the most important drivers of material circulation and energy flow in marine ecosystems.Four investigations from March,2017 to October,2018 were carried out to study the horizontal and vertical distribution of protists abundance,biomass,size structure,grazing rates,oral diameters of ciliates,ciliates community and how they coupled with muti-scale bio-chemical-physical processes by means of general investigation and board culture in the Pearl River Estuary and South China Sea.The main results are as follows:1)In summer of the Pearl River Estuary,there are significant differences in NF abundance and size structure between three water masses.The abundance of NF will increase in water mass 1 because of the freshwater input,and this trend is significant in 2-5 ?m NF.The abundance of NF in the Pearl River Estuary has strongly influenced by bottom-up control effect,but the controlling factors of different water masses are different.For HNF,the main control factors in water mass 1 are PNF,PPE and bacteria in water mass 2 and prochlorococcus in water mass 3.For PNF,the main control factors in water mass 1 are NH4+ and NO3-,PPE,synechococcus in water mass 2 and prochlorococcus in water mass 3.The community structure of ciliates in different water masses of the Pear River Estuary varies greatly,which is mainly controlled by salinity,DIP,SiO4-,synechococcus.The community structure of ciliates in water mass 1 is negatively correlated with nutrients,whereas in water mass 2 and 3,the opposite trend is observed;2)There are significantly differences in abundance of NF between spring and summer in the South China Sea(SCS).In summer,the abundance of PNF in surface(5 m,75 m)is higher than that in spring,while abundance of HNF is lower than that in spring.The abundance of HNF in deep layer(500 m,1000 m,1500 m)in summer is higher than that in spring.Deep layer PNF is only found in summer,but no PNF is found in deep sea area in spring.The NF abundance regulation mechanisms in the SCS during summer and spring are different:in spring,the NF abundance correlates well with ciliates but not with bacteria whereas in summer,the NF abundance correlates well with bacteria but not with ciliates,which implies that NF may be top-down controlled in spring and bottom-up controlled in summer.There are great differences in ciliate community structure between spring and summer in the SCS.85 species of ciliates are identified in spring while only 30 species are identified in summer.The main factors leading to the spatial difference of ciliate community structure in spring are nutrients,HNF and bacteria abundance,while dissolved oxygen is the main factor in summer;3)The average grazing percentages of NF in the 5 m and 75 m water layers of the South China Sea are 51%and 42%,and respectively higher grazing percentages in the 1000 m deep sea are 28%.This further indicates that protozoa play an important role in deep sea and have a fatal impact on the biogeochemical cycle by controlling the prokaryotic community.FLB daily cycle culture experiments show that the grazing percentage of NF community in 5 m water layer was higher in daytime than at night which implies that NF may have a circadian grazing rhythm.