| Cladocerans is a group of small and low planktonic crustacean.Up to now,only 11 species have been found in the ocean,of which only 8 species are considered as real marine cladocerans,and 5 species have been recorded in China.Although there are few species of marine cladocerans,marine cladocerans is an important functional group in the marine ecosystem.Their abundance can reach 2000-3000 ind.·m-3,accounting for 44-51%of zooplankton biomass,and they become the dominant population in a short time during reproductive seasons.The characteristic of the seasonal dominance of cladocerans has triggered many studies on the mechanisms to their superiority in certain sea in terms of food selection,minimum food concentration and temperature adaptability.The aim of this study is to clarify the dominance of cladocerans in Daya Bay based on ecological stoichiometric niche and feeding characteristics.Zooplankton,protist,and phytoplankton samples were collected for 12 times at a site located at 22°33?48.6?N,114°35?0.6?E in Daya Bay,South China Sea from September,2017 to August,2019.Then,community structures of these groups were analyzed,and stoichiometric elemental compositions of predominant copepods and cladocerans were measured.With the high-throughput sequencing technique,the food spectrums of cladocerans and dinoflagellates were identified;feeding selection was studied with controlled experiments in the lab for Penilia avirostris and Parvocalanus crassirostris.The results were obtained as following:1)Community structures of phytoplankton,protist and mesozooplankton and their stoichiometric traitsDuring the investigation,surface water temperature fluctuated in the range of19.2?32.6?,salinity 29.43?34.00 and dissolved oxygen 5.37?11.24 mg·L-1.Ninety-five species of phytoplankton,52 diatom species,and 38 dinoflagellates were identified in all samples.Species composition showed seasonal variation,but diatom was the advantageous groups in most of the months.During this study,28 species of protozoa and 8 Strombidium,4 Tintinnopsis,2 Euplotes,and 2 Uronema were identified in all samples.Fifty zooplankton species were identified in samples,including 37 copepod and 3 cladoceran species.Cladoceran,with abundance of147?23388 ind.·m-3,accounted for 0.56?51.16%of the total zooplankton abundance.The abundance of marine cladocerans was the highest in spring,with P.avirostris as the majority,whereas Pseudevadne tergestina and Pleopis schmackeri were very few.The highest abundance(22963 ind.·m-3)of P.avirostris occurred in April with overwhelming advantages.Furthermore,there existed a significant negative correlation between their abundance and that of protist?ciliates?.During this study period,atomic C:P ratios of the seston,in a range of142?323,fluctuated with seasons.The ratio exceeded 200,the threshold for P limitation,in spring and summer,2018 and summer and autumn,2019,showing potential P deficiency in this study area.Cladoceran and copepod shows differences in all stoichiometric indices.Averaged mass specific elemental contents of cladoceran were slightly higher than those of copepod and correspondingly,copepod had higher C:P,N:P,and C:N ratios than cladoceran.Copepod had higher high value of the range than cladoceran,expecially in N:P ratio.The seasonal and interannual variation of all element ratios of P.avirostris were present.The C:P and N:P ratios of the dominant copepod,P.crassirostris,varied with season.Cladoceran and copepod had higher mass specific elemental contents,but lower C:P ratios than the seston?0.7?100?m?.The mass specific P content of P.avirostris was positively correlated with that of the seston with Pearson correlation analysis,while C:N ratio of P.crassirostris was negatively correlated with that of the seston.In spring,2019,when the seston had lower C:P ratios,cladoceran reached the highest abundance.2)Food composition of cladocerans in Daya BayIn April,May and July 2019,the food composition of zooplankton was studied by high-throughput sequencing.We selected V4 region of 18S r DNA to detect the situ diet of P.avirostris,P.tergestina and Noctiluca scintillans.206 OTUs were obtained from 6 samples,of which 173 were useful OTUs,belonging to 23 families and 63food species were identified to genus level.The results show that the main food sources of marine cladocerans were metazoa,such as Chaetognatha,Cnidaria and Arthropoda,in addition to dinoflagellate and diatom,protozoa and fungi were also important components of their food.The main food source of N.scintillans was Arthropoda,and dinoflagellate and diatom were also important food.Our study showed that these 3 species are omnivorous species.Because their main food sources were larger organisms,it is speculated that they mainly consume these foods in the form of detritus.The food spectrum of P.avirostris in April and May,2019 was similar,but they differed greatly from that in July,2019.P.avirostris had a wide food spectrum,and its apparent food spectrum depends on the food environment.The food spectrum of P.avirostris and P.tergestina differed greatly.The niche overlap index of P.tergestina and N.scintillans was 0.753,demonstrating a strong competition between them.3)Controlled experiments on the feeding selectivity of cladoceran in Daya BayIn 2019,indoor controlled feeding experiments were carried out on P.avirostris and P.crassirostris.The total ingestion rate of P.avirostris on phytoplankton was 248±69 cells ind.-1h-1in April and 69±33 cells ind.-1h-1in May,whereas the total ingestion rate of P.crassirostris in May was 28±7 cells ind.-1h-1.P.avirostris and P.crassirostris preferred phytoplankton,especially dinoflagellate,most of which was Gyrodinium sp.,but they had negative ingestion rate on protist.The food spectrum overlaps greatly?niche overlap index was 0.994?,suggesting a strong competition between them.The investigation of zooplankton in Daya Bay in 2018 to 2019 show evident seasonal dominance of marine cladoceran in this area.The change of element compositions of marine cladoceran P.avirostris was described preliminarily,and there is a correlation between its phosphorus composition and that of seston.In addition,its highest abundance appeared in spring when the C:P ratio of the seston was low.Therefore,we speculate that marine cladoceran may prefer high phosphorus environment,but it needs to be further confirmed in the future.We measured and analyzed the food spectrum of common or dominant cladoceran,copepod and heterotrophic dinoflagellate by molecular means or traditional feeding experiments,and found that the niche separation of two cladocerans and heterotrophic dinoflagellate was obvious.In addition,we also found that the food spectrum of P.avirostris obtained by the two methods was quite different.It appeared that the subtraction method of food concentration may underestimate feeding of small particles and detritus because of limitation of microscopy in observing small particles.Although the apparent food spectrum of P.avirostris and P.crassirostris in the experiment is very similar,it is difficult to make a conclusion on the actual food spectrum,due to the lack of comparative analysis by molecular technique,so further analysis is needed in the future. |
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