Food Composition Of Nemopilema Nomurai And Its Trophic Relationship With Zooplankton Community

In the last two decades,jellyfish blooms are reported increasing in abundance and frequency around the world.The mass occurrence of jellyfish causes dramatic problems on food web due to their rapid population growth rate and feeding competitions.Jellyfish bloom also brings negative impacts on the marine economic development and marine ecosystem.The giant jellyfish Nemopilema nomurai widely distributes in Yellow Sea and Bohai Sea,which was frequently reported blooming in China.In this study,we investigated the diet and food composition of N.nomurai and their trophic level in plankton community by using the stable isotopes and fatty acid biomarkers.The aim of the study was to determine the food source of N.nomurai and its role in pelagic ecosystem,and to clarify its impact on the marine ecosystem.First,we collected N.nomurai samples in Yellow Sea and East China Sea in August 2016 to analyze their characteristics of stable isotopes and fatty acids.We further analyzed the diets of N.nomurai,the contribution rate of various plankton,and whether the food compositions various with jellyfish size.The SIAR(Stable isotope in R,based on Bayesian method)results showed that N.nomurai tended to feed on the small organisms(<1000?m),which contributed more than 70%of the total diet.N.nomurai mainly consumes the small particle size food,including small copepods and pelagic organic materials(POM),which was independent of the size of N.nomurai.The results of fatty acids showed that N.nomurai diets contained land-sourced food and detritus.Fatty acid clustering results revealed that the fatty acid composition of N.nomurai was most similar to those of Euphausia pacifica and Lucifer intermedius,which implied that there might be food competition between them.According to our study,N.nomurai bloom will directly cause feeding pressure on small zooplankton.Meanwhile,N.nomurai may compete food with the consumers in low trophic levels,such as krill,and this effect may transfer to the higher trophic level organisms up the food web.The study also found that the food composition of N.nomurai collected in the same month did not differ among different sized individuals.Since the zooplankton community structure differed among months,it meant that the food available to N.nomurai changed with time.Meanwhile,the feeding organs of N.nomurai change during their growth.Considering the above two points,we hypothesis that when the N.nomurai developed from ephyra to medusa stage,its food composition was different in different months.To study that if the food composition changes with time,we collected N.nomurai samples in the areas adjacent the Hongyanhe river in the Bohai Sea during May,June,July and September 2018.The result revealed that N.nomurai's diet,food composition,and trophic level changed with time.The stable isotopes result showed that the trophic level of N.nomurai decreased from May to July.The reduction was significant from June to July.The?¹³C value of POM from June to September was significantly positive correlation with the?¹³C value of N.nomurai.In May,the difference between?¹³C of N.nomurai and POM was significantly higher than that of other months.Combined with the fatty acid results,it indicated that N.nomurai ate more carnivorous food in May than in other months.The results of the SIAR model revealed that the contribution of small size food increased with time,which was independent of the size increasing of N.nomurai.From May to July,the food contribution of copepods decreased,while the contribution of POM increased.From July to September,the contributions of different sized food resource remained unchanged.The values of fatty acids C18:2n-6+C18:3n-3 and C20:4n-6,which represented land-derived food and detritus in N.nomurai,increased over time.In May and June when the abundance of zooplankton is relatively high,the C18:2n-6+C18:3n-3 and C20:4n-6 values in the N.nomurai fatty acid composition were still high.Therefore,land-derived food and detritus may be a potential booster for the outbreak of N.nomurai.Stable isotopes would enrich with the growth of organisms.In some species,the morphological of feeding organ changes at different growth stages.In addition,the zooplankton community structure in different months was different.Therefore,there are dynamic changes in the trophic diversity of zooplankton community.In this study,the community-wide-metrics was used to analysis the dynamic changes of the trophic niche and trophic diversity of zooplankton community in the areas adjacent the Hongyanhe river in 2018.The results showed that the trophic diversity of zooplankton was highest in late spring and early summer,and the community occupied the largest trophic niche space.Then in July when the biomass of N.nomurai reached peak,the zooplankton community occupied the smallest trophic niche space,and its trophic diversity became minimized.It indicates that the zooplankton community could be threatened the most by the outbreaks of N.nomurai in July,leading to the reduction in the zooplankton trophic diversity.To study if the food compositions are different from species to species,we investigated the diets of six jellyfish species in the areas adjacent the Hongyanhe river.Those jellyfish species were also compared with N.nomurai.In the study area,there are four giant jellyfish species including N.nomurai,A.aurita,C.nozakii and Rhopilema esculenta(which is rarely sampled in recent years).Common small jellyfish species include Malagazzia carolinae,Pleurobrachia globosa,Tiaricodon coerules and Clytia hemisphaerica.Results showed that there were differences in the food composition of each jellyfish species.In the giant jellyfishes,the composition of carnivorous food in C.nozakii was higher than that of A.aurita and N.nomurai.Small jellyfishes(M.carolinae,P.globosa,T.coerules,and C.hemisphaerica)were omnivorous and ate more carnivorous food than N.nomurai and A.aurita.Among them,C.hemisphaerica and P.globosa consumed more carnivorous food than C.nozakii.To the food of T.coerules and C.hemisphaerica,POM contributed least,while large and medium-sized copepods contributed highest.The contribution rate of large and medium-size copepods was higher than that of small copepods.The other two small jellyfish M.carolinae and P.globosa shared similar food composition.In giant jellyfishes,large and medium-sized zooplankton contributed the most food for C.nozakii,followed by A.aurita,and finally the N.nomurai.The proportions of carnivorous food in the diets of N.nomurai were lowest among all the seven jellyfish species.The nutrition of N.nomurai was mainly provided by POM.The food contribution rate of POM in the other six jellyfish species was less than 50%.Among them,POM contributed highest to A.aurita with the ratio of 45%.On the view of food composition,the outbreaks of N.nomurai will mainly ingest the organism smaller than 1mm.Their negative impacts on large and medium-size zooplankton and fishes were mainly by killing other organisms with their stinging tentacles and competing for the same resources.Impact of A.aurita on the large and medium-size zooplankton was more by feeding than competition.The effects of T.coerules and C.hemisphaerica on large and medium-size zooplankton were mainly by feeding.The bloom of C.nozakii,P.globosa and M.carolinae would cause feeding pressure on zooplankton of all sizes including large crustaceans.