Interannual Changes Of Zooplankton Functional Groups In The Yellow Sea

Zooplankton occupies a key position in marine ecosystems.Variations in the zooplankton species composition and abundance will change the structure and function of the whole ecosystem.There are many species in zooplankton communities with complex life cycles.According to the size spectrum and the roles and positions in the ecosystem,the zooplankton can be classified into different functional groups,which can effectively simplify the food webs,and simulate the ecological processes of zooplankton communities comprehensively and accurately.Using zooplankton data collected in many years,we studied the regional and interannual variations of zooplankton functional groups in the Yellow Sea in early summer and winter,interannual changes of zooplankton communities between 2000-2009 and 1959,population dynamics of large copepod Calanus sinicus in August-October 2002,aimed to provide basic data for the study of long-term changes of zooplankton communities.In June between 2000 and 2009,different zooplankton functional groups had their own relatively fixed distribution patterns.Giant crustaceans and large copepods were found to be mainly distributed offshore,small copepods were mainly located along the 50 m isobath and the coastal region,chaetognaths were mainly sampled along the 50 m isobath,and small jellyfish tended to be located in the coastal region shallower than 50 m.The presence of the Yellow Sea Cold Water Mass(YSCWM),resulting in the different hydrological conditions within different domains,affected the distribution of each zooplankton functional group.Sea bottom temperature and salinity were shown to have been major factors affecting the distribution of zooplankton functional groups.Each functional group had different interannual variations.In warm spring years,giant crustaceans and small copepods showed high biomasses,while in cold spring years,the biomass of chaetognaths was high.Large copepods did not show significant interannual differences.In 2007,small jellyfish and salps had high abundances,which may due to the high temperature in the first half year.In addition,the chlorophyll a concentrations in spring could affect the biomass of giant crustaceans.The distribution of zooplankton can affect the distribution of the jellyfish community in the Yellow Sea.Among zooplankton functional groups,small copepods and giant jellyfish Nemopilema nomurai showed similar distribution patterns,suggesting that the abundance of small copepods was feeding that of giant jellyfish.The observed interannual biomass of small copepod was positively related to temperature,and we suggest that this relationship may explain the rarity of giant jellyfish outbreaks in cold years.In January and November 2001,December 2006,and February and December 2009,giant crustaceans,large copepods and chaetognaths showed similar distribution patterns,mainly located offshore north than latitude 34°N.Small copepods were mainly distributed offshore in January and November 2001,while the abundances were relatively high in the entire region north than latitude 34°N.Small jellyfish and salps were distributed inshore with low abundance and occurrence frequency.The average abundances of giant crustaceans and small copepods were highest in January and November 2001,respectively,with low average abundances and little difference during other cruises.The abundances of large copepods and chaetognaths had similar interannual change trends and dramatically fluctuated,both were highest in January 2001 and lowest in December 2006.Both small jellyfish and salps had high abundances in November 2001 and December 2006.The abundance and species number of warm water zooplankton species could effectively indicate the path and range of the Yellow Sea Warm Current(YSWC).From December to February in the next year,the distribution of warm water species gradually moved northward,suggesting that the influence area of the YSWC extended.The influence of the YSWC on the abundance and dominance index of original zooplankton species in the Yellow Sea was small,but the YSWC might affect the spatial distribution of each zooplankton functional group through advancing current or changing the seawater conditions.The abundances of zooplankton functional groups were compared between June 2000-2009 and 1959.Giant crustaceans and chaetognaths had relatively high abundances and exhibited wide ranges,and their abundances were similar between 1959 and 2000-2009.The abundance of large copepods was significantly higher in 2000-2009 than in 1959,and the ranges were 2035-24500 and 912-1330 ind m-2 in the two periods,respectively.Small jellyfish and salps usually presented low abundances,but both increased dramatically in 2007,which were 4.8 and 88.5 times higher than the sub-high mean values in the other years,respectively.Giant crustaceans and chaetognaths ranged widely in the YSCWM region and tidal front region,respectively,while small jellyfish and salps fluctuated widely at west shallow region.The results suggested that the increased copepod abundance related to increased nutrition and over-fishing in the Yellow Sea,while dramatic fluctuations of gelatinous zooplankton were attributed to environmental variation caused by climate change.The small jellyfish community changed significantly in 2006-2007 compared with that in 1959.In the Midwest region of Yellow Sea,the abundance and occurrence frequency of small jellyfish were higher in 2006-2007 than in 1959,which may be due to the increased temperature and nutrition.The species number of small jellyfish was higher in 2006-2007 than in 1959,especially in spring,because the high seawater temperature in spring in 2006-2007 induced some inshore eurythemic species reproduction in advance,resulting in the increase of abundance and species number.The dominant species composition changed between 2006-2007 and 1959.The frequency of becoming dominant species of some species increased,such as Clytia hemisphaerica,Proboscidactyla flavicirrata and Beroe cucumis,while the frequency of Muggiaea atlantica decreased.Several new dominant species appeared in 2006-2007,including Rathkea octopunctata,Euphysora bigelowi,and Eirene ceylonensis,while some previous dominant species in 1959(Sugiura chengshanense,Hydractinia minima,and Eirene menoni)were no longer dominant in 2006-2007.The abundance,stage composition,sex composition,and vertical distribution of the planktonic copepod Calanus sinicus from August to October 2002 in the southern Yellow Sea were studied.The results showed that temperature and food availability were the main factors that influenced the reproduction of C.sinicus.The C.sinicus population could respond rapidly to environmental changes,and once exposed to favorable conditions(moderate temperature and rich food),the population will quickly develop and expand.The population structure of C.sinicus differed among different domains.C.sinicus entered its reproductive period in September-October in the northwest coastal region,and eggs and nauplii dominated the developmental stages.In the central part of the southern Yellow Sea,CV C.sinicus is the dominant developmental stage,and the population reproduction rates remained low from August to October.All developmental stages of C.sinicus had high abundances in August in the southeast region,but they all gradually decreased in abundance as the summer progressed and the chlorophyll a concentrations decreased.The percentage of C.sinicus females was high during the reproductive period,which suggests that the sex composition of adult C.sinicus may reflect whether or not the population is in the reproductive period.Numerous CV C.sinicus aggregated in the YSCWM in a suspended developmental stage,and they served as the main parental individuals during the reproductive period in autumn–winter,so individuals there may be more important for population development.