Structure And Inter-annual Change Of Summer Zooplankton Communities In The Western Arctic Ocean

The Arctic Ocean ecosystem is undergoing dramatic oceanographic change related to decreasing ice coverage and increasing temperature over the last decades.Zooplankton plays a key role in marine ecosystem.It is the linkage between primary producers and secondary consumers.Zooplankton is also a driving force of marine biological pump,playing an important role in the marine biogeochemical cycle.Considering the short life span and weak swimming ability,planktonic sepcies may be more sensitive to environmental changes than other marine organisms,and they have been used as indicators of global changes in many studies.Retreat of sea-ice showed an accelerated trend after 2003 and reached the record minimum of 3.41 million km2 in 2012.To detect the potential response of zooplankton communities to rapid ice decline in this period,its composition,geographical distribution and inter-annual change were investigated with samples collected in epipelagic waters(<200m)in summer of 2003,2008,2010 and 2012.Meanwhile,specific difference in vertical distribution pattern of copepods was investigated in Makarov Basin and Chukchi Abyssal Plain in 2012.Based on species composition and abundance of each station,three geographical communities were identified in the western Arctic Ocean: Coastal Neritic Community(CNC),Slope Transitional Community(STC)and Deep-sea Community(DC).CNC located in shelf waters of the Chukchi Sea and Alaskan costal waters;STC located in the north of Chukchi Slope and the edge of Beaufort Sea;DC include all deep-sea stations of Chukchi Plateau,Chukchi Abyssal Plain and Canadian Basin.Copepods and barnacle larva were two main taxa of CNC(>90%)and DC was dominated only by copepods(>90%),while STC showed both characteristics of CNC and STC.Species composition and abundance of CNC were affected partially by influx of Bering Water,while native species(Calanus glacialis,Pseudocalanus spp.,Barnacle larva)were still the main components of zooplankton.Arctic species(Calanus hyperboreus,Metridia longa,Paraeuchaeta spp.)and widespread species(C.glacialis and Pseudocalanus spp.)were main contributors to total abundance of STC and DC.Total zooplankton abundance of CNC increased from 2003 to 2012(1092.4-1648.7 ind/m3),it indicated that zooplankton community of CNC benefited from prolonged growing season.While it was difficult to compare the inter-annual changes of STC and DC due to the variable station setting and investigate time.Moreover,geographical distribution of different zooplankton communities which result in large-scale investigate areas would cover the inner differences of CNC.Along with shrinking in sea-ice coverage,trophic structure is hypothesized to shift towards planktonic-dominating,which may most likely be tested in the Pacific-influenced Arctic shelf region.In order to evaluate the response of the Chukchi Sea communities to climate changes,we combined zooplankton samples and investigated the inter-annual changes in regions with various physical and biological characters.Three geographically separated communities were identified by hierarchical cluster analysis,including the north Chukchi Sea community(NCS)governed by Arctic cold water,the Bering Sea water influenced central(CCS)and south(SCS)Chukchi Sea community.Zooplankton abundance was low in NCS and dominated by copepods(>90%).It was about ten times higher in CCS and SCS,co-dominated by copepods and barnacle larvae(86-98%).In the NCS community,similar dominant species were recorded in 2003 and 2008,but total abundance increased from 118.9 to 182.5ind/m3.Average zooplankton abundance of all Pacific-influenced stations increased from 2003 to 2012,but lowest average abundance was observed in 2012 and 2010 in the CCS and SCS community,respectively.Barnacle larvae,C.glacialis,Pseudocalanus spp.and Oikopleura vanhoffeni were identified as dominant species of the CCS community,and those for the SCS community included also Eucalanus bungii with Pacific origin,as well as Acartia spp.,O.longissina and other merozooplankton commonly observed in neritic waters.Numerical increase was observed only in C.glacialis.We propose spatial heterogeneity for the Chukchi zooplankton community changes in response to global warming.Numerical increase is expected in Arctic waters with constant structure,and inter-annual variability in composition is also significant in shallow waters receiving Pacific inflows.Despite the fluctuation of average abundance separately in SCS and CCS,average of these two communities increased linearly from 2003 to 2012.Copepods were the most important herbivorous zooplankton group in marine ecosystem.In the Arctic Ocean,copepods have evolved complex life cycle strategies to survive harsh environment,and such adaptation ability can further determine their response to climate changes.Species sensitivity is essential to understanding of climate-induced ecological consequences.To determine the species and regional specific responses of planktonic copepods in the western Arctic Ocean,we compiled field observations during a period of rapid ice retreat,and compared variability in abundance of different ecological groups.Three types of habitats were identified,namely the Chukchi Shelf(CS)characterized by warm water in upper 30 m layer,Canadian Basin(CB)containing stations deeper than 1000 m,and Chukchi Slope/Plateau(CSP)sharing similar temperature with CB.Pacific-origin species presented overwhelmingly in CS and can hardly reach basal area.Coastal Arctic species were sampled from all stations but comparatively high abundance was recorded in CS.Oceanic Arctic species distributed evenly in all habitats in 2003,but was absent from CS in the other years.Opportunistic Arctic species were low in abundance with evident year-to-year difference in distribution patterns and species assemblages.In CB,much lower total abundance was observed in 2010 and 2012(17.6 and 12.5 ind/m3)comparing to 2003 and 2008(56.5 and 82.3 ind/m3).Both coastal and oceanic Arctic species decreased during this period.Highest abundance in CS and CSP present in 2012 as 745.8 and 1049.9 ind/m3,followed by 2003(326.0 and 373.6 ind/m3).Numerical increase in 2012 was contributed mostly by large-sized Calanus glacilis,in which extremely high abundances of 568.8 and 844.4 ind/m3 were recorded.Based on dominance of young copepodites,we propose C.glacialis as the beneficiary of summer sea-ice minimum in 2012,induced by successful local recruitment rather than direct physical transportation.The decrease of copepod abundance in basal area shows consistence with previous opinion that phytoplankton production in Canadian basin does not increase but shift towards smallest algae dominating.In addition,vertical distribution patterns and geographical differences of zooplankton community was also one hotspot of Arctic zooplankton research.Using samples collected in epi-and meso-pelagic layers of the Makarov Basin(MB)and Chukchi Abyssal Plain(CAP)in September 2012,vertical distribution and geographic difference of zooplankton were analyzed.According to species composition and abundance,both diversity and abundance of zooplankton changed with depth.Zooplankton was mainly gathered in 0-200 m and relatively rare in 200-1000 m.Herbivorous species,such as O.similis,C.glacialis and C.hyperboreus,gathered in 0-200 m and dominated in number.Although abundance of omnivorous species such as M.pygmaeus,Oncaea spp.and M.longa aslo decreased with depth,their percentages in total abundance increased to the contrary.Despite similar species assemblages recorded in each layer,epipelagic abundance of large copepod C.hyperboreus was higher and that of small copepods was lower at CAP station than the MB stations,whereas in 200-1000 m layer total abundance was much lower in CAP.Abundance of zooplankton in 500-1000 m layer varied between 22.7 and 92.6 ind/m3 in MB,but was only 1.6 ind/m3 in CAP.Geographical differences of zooplankton abundance in mesopelagic layer were in accord with the heterogeneity in the function of biological pump.It is suggested that low zooplankton abundance in mesopelagic layer of CAP was induced by high density of C.hyperboreus in epipelagic layer,which starts feeding before ice melting in the spring and consequently reduced the downward flux of organic matter formed mainly by ice algae.