Studies On Microzooplankton Grazing In The Northern South China Sea And Its Adjacent Northwestern Pacific Ocean

Microzooplankton plays an important role in marine ecosystems and the study of their grazing activities is particularly important for understanding the impact of environmental changes on the energy flow in the phytoplankton community and food chain.Combining analyses by dilution technique?HPLC?flow cytometry(FCM)and FlowCAM,phytoplankton community composition?taxon-phytoplankton growth rate?microzooplankton grazing rate and their selective grazing were studied in the northern South China Sea(NSCS),July to August 2017 and June to July 2018),northwestern Pacific adjacent region(August to September 2017 and August to September 2018)and Taiwan Strait(March to April 2018),respectively.The main objective is to explore the spatial variaitons of taxon-phytoplankton growth and microzooplankton grazing rates and their selective grazing in the northern South China Sea and northwestern Pacific adjacent region;in addition,I also discussed the grazing ecological characteristics of microzooplankton in different regions of the Taiwan strait under the influence of the spring Zhejiang-Fujian coastal current.The main results are as follows:In the surface layer of the northern South China Sea in summer,Prochlorococcus and Synechococcus occupied an absolute dominant position in the basin area of the northern South China Sea and the location near the Luzon Strait;in the vertical profile,the contribution of Synechococcus to the total phytoplankton biomass decreaseed with the increase of depth;while haptophyta 8 and prasinophyceae increase layer by layer.In the surface layer of the low latitude area of the northwestern Pacific Ocean,Synechococcus occupied an absolute dominant position.Prochlorococcus and Synechococcus contributed more than 80%total phytoplankton biomass.In the surface layer of the Oyashio affected area,the structure of phytoplankton community was significantly different from that in the low latitude area of the northwestern Pacific Ocean.The haptophyta 8 and prasinophyceae were improved.And there was a significant increase in dinoflagellates in the Oyashio afifected area in the DCM layer.Under the influence of coastal currents,diatoms occupied an absolute dominant position in the frontal area in the Taiwan Strait in spring,followed by cryptophyta,prasinophyceae and haptophyta 8,while the dominant position of Prochlorococcus and Synechococcus was more obvious in the outer side of the front.Spatial variations in grazing rates of microzooplankton:in the surface,the average grazing rates of microzooplankton were not significantly different in the northern South China Sea in summer(0.93±0.49 d-1),in the low latitudes of the northwestern Pacific(0.71±0.41 d-1),in the Oyashio affected areas(0.93±0.34 d-1,)and in the Taiwan Strait in spring(0.75±0.66 d-1).In the DCM layer,there was no significant difference in the grazing rates of microzooplankton between the four regions(the northern South China Sea in summer(0.17±0.16 d-1),in the low latitudes of the northwestern Pacific(0.15±0.19 d-1),in the Oyashio affected areas(0.31v0.23 d-1)and in the Taiwan Strait in spring(0.34±0.35 d-1).The average growth rates of surface phytoplankton in the four regions were as follows:the northern South China Sea in summer(0.71±0.66 d-1),in the low latitudes of the northwestern Pacific(0.71±0.64 d-1),in the Oyashio affected areas(1.27±0.47 d-1)and in the Taiwan Strait in spring(0.92±0.51 d-1).The average growth rate of phytoplankton in summer Oyashio affected areas and in spring in the Taiwan Strait was significantly higher than that in the northern South China Sea and the northwestern Pacific Ocean in summer(p<0.01).In DCM,the results of the growth rate of phytoplankton were as follows:the northern South China Sea in summer(0.17±0.16 d-1),in the low latitudes of the northwestern Pacific(0.15±0.19 d-1),in the Oyashio affected areas(0.31±0.23 d-1)and in the Taiwan Strait in spring(0.34±0.35 d-1).The grazing rate and phytoplankton growth rate of microzooplankton in surface layer were significantly higher than those in DCM layer(p<0.01,P<0.01).In summer,the growth rate of phytoplankton and the grazing rate of microzooplankton were positively correlated in surface and vertical profiles in the northern South China Sea and northwestern Pacific adjacent region(p<0.01,P<0.01).Microzooplankton responds rapidly to the changes of food.The growth rate of phytoplankton decreased with the increase of depth,and the grazing rate of microzooplankton decreased with the decrease of growth rate of phytoplankton.In summer,the growth rate and grazing rate of Prochlorococcus and Synechococcus were positively correlated in surface and vertical profiles in the northern South China Sea and the adjacent area of the northwestern Pacific Ocean,while the picophytoeukaryotes did not show this phenomenon in the adjacent area of the northwestern Pacific Ocean.The growth and grazing coupling of five characteristic photosynthetic pigments 19 But,Fuco,19 Hex,Zea and Dv-Chl a were different in those areas and different water layers.At the same time,the growth rates indicated by these five characteristic pigments are not all significantly correlated.In the surface layer,the grazing rates of characteristic pigments 19 But,Fuco and 19 Hex of eukaryotic phytoplankton were positively correlated(p<0.01),while the grazing rates of characteristic pigments Zea and Dv-Chl a of prokaryotic phytoplankton were positively correlated(p<0.01).Based on the historical data of the northern South China Sea(n=94),the average growth rate of phytoplankton in spring was significantly higher than that in summer(p<0.05)and winter(p<0.05),but not significantly different from that in autumn.There was no significant difference in the average grazing rate of microzooplankton in four seasons.In the adjacent area of the northwestern Pacific Ocean,the grazing rates of microzooplankton in Oyashio affected areas were significantly higher in spring than in low latitudes.But there was no significant difference in summer.