The Effect Of Mesoscale Physical Processes On Microzooplankton Group Composition And Grazing On Phytoplankton In Summer In The Typical Waters Of South China Sea

Microscope and dilution technique were applied to study microzooplankton group (heterotrophic dinoflagellates,aloricate ciliates,tintinnids and copepod nauplii) composition, distribution and grazing activities in the typical waters of South China Sea.Study was also conducted to focus on the effect of marine mesoscale physical processes on microzooplankton group composition and grazing on phytoplankton in summer in the typical waters of South China Sea.There were three cruises for our investigations,which were in the northeastern South China Sea(SCS) in July of 2007 and June-July of 2008,and in the western SCS in August-September of 2007,respectively.Main results and conclusions are as following:1.Microzooplankton abundance varied largely from 7 ind/L to 5474 ind/L.Aloricate ciliates dominated microzooplankton in term of abundance with contribution 65±26%. Signifcant variabilites of mean depth-integrated abundance of microzooplankton(65-4043 ind/L) were observed among 3 cruise,with the highest in northeasto'n SCS in summer of 2007(623-4043 ind/L),the middle level in the western SCS in summer of 2007(100-1164 ind/L),and the lowest in northeastern SCS in summer of 2008(65-636 ind/L).In summer 2007 of the northeastern South China Sea,microzooplankton abundance was significantly higher in upwelling zone(2482±1014 ind/L) than which in shelf and slope(1256±652 ind/L), the most abundant was 4043 at Stn A1 in upwelling zone.In summer 2007 of the western South China Sea,microzooplankton was least abundant in reference zone(157±39 ind/L), however,which resembled each other for upwelling,cold eddy and fiver plume influenced zones.In summer 2008 of the western South China Sea,microzooplankton abundance were significantly higher in upwelling(315±171 ind/L) and river plume(320±163 ind/L) than which in shelf and slope.2.Results also showed that mean depth-integrated abundance of microzooplankton was significantly higher in high productive zones with upwelling,cold eddy and river plume areas than that in low productive zones with corresponding reference area(p＜0.05).Moreover, there was significant corelationship between Chl a and microzooplantkon abundance(p＜0.05), indicating that phytoplankton biomass(Chl a) was an important factor influencing abundance and distribution of microzooplantkon.3.Phytoplankton growth rates(k) ranged from 0.17-3.07 d^-1 while microzooplankton grazing rates(g) ranged from 0.48-2.86 d^-1.Higher phytoplankton growth and microzooplankton grazing were observed in upwelling,cold eddy and river plume areas,and significant higher than that in reference area with non-mesoscale physical processes,with k and g were higher than 0.83 d^-1 and 0.78 d^-1,respectively(p＜0.05).Moreover, microzooplankton grazing pressure on phytoplankton(g/k) was higher in the reference areas (1.13-3.71) than that in upwelling,cold eddy and river plume(0.55-1.17),such data indicated that microzooplankton grazing could control on phytoplankton growth effectively in the corresponding reference areas,while less effectively control on phytoplankton in mesoscale physical processes areas.