Population Dynamic Of Noctiluca Scintillans And Its Grazing Properties In Jiaozhou Bay

To investigate the mechanism of Noctiluca scintillans blooms in the Jiaozhou Bay,water samples were collected in 12 stations from January to December,2015.Correlation among population dynamics,population structure,and environmental factors were studied.The correlation between N.scintillans aboudance and annual variation of phytoplankton data,and grazing behavior of N.scintillans in both laboratory experiments and field experiments was also analyzed to investigate its affects on phytoplankton communities.The major findings were listed as following:(1)Inter-monthly variation was founded in the abundance of N.scintillans and the ratio of reproductive individuals.The abundance of N.scintillans varied from 0 to 31.37×10~3 ind./m3,and the average monthly abundance presented a bimodal distribution pattern,peaks appeared at Feburary and July.The ratio of binary fission was between 0~31.25 ‰,and the ratio of sexual reproduction was between 0~29.41 ‰.There were overt seasonal and spatial variations in the ratios of sexual reproduction and binary fission,indicating that the two reproduction strategies were affected by environment factors.The ratios of the two reproduction strategy decreased along with decreasing of phytoplankton abundance,the sexual reproduction responded faster than the binary fission when the abundance of phytoplankton risen again.(2)Water temperature,chlorophyll a,nitrite and nitrate were the key factors influencing the abundance and reproduction of N.scintillans.Water temperature was negatively correlated with the abundance of N.scintillans and the ratio of reproductive individuals.Chlorophyll a was positively correlated with the abundance of N.scintillans and the ratio of reproductive individuals,since chlorophyll a indicates the abundance of food for N.scintillans.Nitrite and nitrate were negatively correlated with the abundance of N.scintillans and the ratio of sexual reproduction,indicating that nitrite and nitrate might play an important role in regulating the metabolism pathway of reproduction.(3)Chained diatoms might be the major food source for N.scintillans in Jaozhou Bay.The abundance of N.scintillans was positively correlated with the abundance of chained diatoms,suggested the chained diatoms might be the major food source for N.scintillans.The abundance of N.scintillans were negatively correlated with the Shannon-Wiener index,indicating the decrease in diversity of phytoplankton might increase the risk of N.scintillans bloom.(4)N.scintillans had overt food preference in laboratory trials.The laboratory experiment indicated the grazing rate and the proportion of indvidals containing food vacuoles increased with increasing bait concentration.The grazing rates on Skeletonema costatum and Platymonas helgolandica Kylin var.tsingtaoensis were higher than the others.The grazing rate on Akashiwo sanguinea was the lowest.Grazing behavior was affected by the surface property and mobility of different baits.As to diatoms and Platymonas helgolandica Kylin var.tsingtaoensis,the grazing rate and the proportion of food vacuoles was positively correlated with the cell volume,while the results were in the opposite for dinoflagellates.(5)The grazing of N.scintillans impacted the structure of plankton community.The grazing of N.scintillans in field water caused decreasing in the abundances of both diatoms and dinoflagellates.The decreasing in chained diatoms was higher than the decreasing in unchained diatoms.The abundance of Thalassiosira rotula and Chaetoceros didymus decreased more than 50 %,with higher abundance of dinoflagellates in experiment group than control group.The ratio of dinoflagellate/diatom and unchained/chained diatoms in the experiment group were 43.13 % and 43.34 % higher than the control group.The results indicated the chained diatoms are more likely to be grazed by N.scintillans and the grazing of N.scintillans will change the structure of phytoplankton community in short period.