Study On The Feeding Strategy Of Krill In Amundsen Sea,Antarctica

Zooplankton are an important part of the marine ecosystem,serving as an intermediate link between primary producers and higher trophic level consumers,and playing a top-down role in the marine food web.Zooplankton play an important role in the biological carbon pump,transferring carbon from surface waters to the mid-and deep-sea of the ocean through feeding,respiration,fecal pellet production,and vertical movement.The study of zooplankton community changes is very important for tracking energy pathways in marine food webs and determining nutritional relationships in marine ecosystems.Antarctic krill species,an important part of zooplankton community in the Southern Ocean,plays an important role in the ecosystem.As the main consumers of primary producers and main diet of top predators in the Southern Ocean ecosystem,Antarctic krill can transfer energy from low trophic level to high trophic level.Therefore,studying the feeding strategy of Antarctic krill will help us understand the response of Southern Ocean krill to environmental change under the background of climate change and accurately predict the nutritional dynamic path of Southern Ocean food web.Based on the plankton samples collected using a Norpac net(0.5m~2 net mouth,mesh 330?m)in Amundsen Sea during the 34^th(2017/2018)and 35^th(2018/2019)Chinese National Antarctic Research Expedition,we studied the abundance,community structure and dominant species of zooplankton and phytoplankton in the study area,and discussed the impact of food availability in Amundsen Sea in Southern Ocean summer on the feeding strategy of Antarctic krill.The phytoplankton of the Southern Ocean summer in Amundsen Sea during2017/2018 and 2018/2019 were mainly composed of Diatom,Dinoflagellate,Phaeocystis sp.and other groups.When the abundance of Phaeocystis sp.was abnormally high,it makes a great contribution to the biomass.By cluster analysis,the surface phytoplankton in the study area could be divided into three communities.Community I was located in the northern part of the sampled area with an average abundance of 3.68±17.26 ind.m^-3,and the indicator species were Fragilariopsis spp.,Chaetoceros spp.,Pseudo-nitzschia spp.Community II was located in the southeastern and southern part of the sampled area with an average abundance of 3.74±30.55 ind.m^-3,and the indicator species were Fragilariopsis spp.,A.actinochilus,B.belgicae.Community III was located in the eastern and southern part of the sampled area with an average abundance of 1.15±8.30 ind.m^-3,and the indicator species were G.spirale,P.subpyriforme,O.scolopax.In the 50 m water layer the phytoplankton could be divided into four communities.Community I was located in the northeastern part of the sampled area with an average abundance of 3.22±13.05 ind.m^-3,and the indicator species were D.antarcticus,G.delicatula,Chaetoceros sp.,A.turbo.Community II was located in the western part of the sampled area with an average abundance of 2.29±12.68 ind.m^-3,and the indicator species were C.dichaeta,Cyclotella sp.,F.curta,T.nitzschioides,T.subtilis.Community III was located in the southernmost part of the sampled area with an average abundance of 0.57±3.55 ind.m^-3 and the indicator species were A.actinochilus,C.pennatum,Amphiprora sp.,A.sphenoides.Community?was located in the southeastern part of the sampled area with an average abundance of 0.66±2.73ind.m^-3,and the indicator species were O.scolopax,G.mikimotoi.In the 0-200 m water layer the phytoplankton could be divided into two communities.Community I was located in the northern part of the sampled area with an average abundance of 1.17±5.22ind.m^-3,and the indicator species were Chaetoceros spp.,P.lineola,D.antarcticus.Community II was located in the southeastern part of the sampled area with an average abundance of 0.49±3.01 ind.m^-3,and the indicator species were P.rostrata,O.gladiolus,F.cylindrus,Amphiprora sp.,A.actinochilus.The results indicated that diatoms were important component of phytoplankton abundance and biomass in summer of the Southern Ocean.Factors such as seawater temperature,salinity and mixing layer depth will affect the spatial distribution of phytoplankton.In addition,the predation pressure from predators will also affect the community composition of phytoplankton.Copepods,krill,and hairy jaw taxa as important components of the Southern Ocean summer zooplankton in the Amundsen Sea in 2017/2018 and 2018/2019,and the highest abundance occurs in the middle of the sampled area.Copepod population accounts for more than 90%of the total abundance of zooplankton in most stations of the sampling area,and occupies a dominant position in the zooplankton community.The zooplankton in Amundsen Sea was divided into three communities by cluster analysis.In Community I,four dominant species of copepods(C.acutus,C.propinquus,M.gerlachei,R.gigas)contribute greatly to the abundance of copepods.Small copepods(Oithona similis,Oncaea spp.,Ctenocalanus citer)contribute more to the abundance of copepods in Communities II and III,because different communities are divided largely according to the abundance differences of dominant species of zooplankton in the community(especially the abundance differences of copepods),rather than according to the differences of species composition of zooplankton.Our results indicate that E.superba populations consist mainly of larvae stages and T.macrura populations consist mainly of postlarvae stages.E.superba and T.macrura are dominant krill species of the Southern Ocean and their habitats are usually reported overlapped.Studies of the feeding strategies of these two krill species will help us better understand the coexistence mechanisms and estimate the roles that krill played in the food webs of Southern Ocean.Trophodynamics of E.superba and T.macrura at different ontogenetic stages(Furcilia,Juvenile,Adult)were studied using fatty acid and stable isotope biomarkers based on samples collected in Amundsen Sea during Southern Ocean summer of 2017/2018 and 2018/2019.Diatoms like Fragilariopsis spp.was the most abundant phytoplankton species in the summer of2017/2018,while the abundance of phytoplankton in the summer of 2018/2019 was dominated by Phaeocystis sp.The gradually increasing of the carnivorous index 18:1n-9/18:1n-7 with ontogeny of both species in 2018/2019 indicated more carnivorous feeding of adults compared with juveniles and larvae.Meanwhile,higher 18:1n-9/18:1n-7 and?15N values of T.macrura compared with E.superba were more significant in the juvenile and adult stages during the summer of 2018/2019.Our results indicate that the trophic niche differentiation between the two krill species appeared in postlarval stage and can be influenced by food availability.Compared with E.superba,T.macrura was more prone to feed omnivorously or carnivorously responding to changing food availability.