Ecological Characteristics Of Phytoplankton And Physiological Mechanism Of Interspecies Relationship Between The Key Phytoplankton Species In A Scallop Culture Sea Area Of Bohai Bay

Phytoplankton is the foundamental component of the marine ecosystem and plays significant role for fishery resources.Because phytoplankton is the primary producer and the main energy source for higher consumers,the population dynamics of phytoplankton affect the growth and survival of shellfish and its larvae directly or indirectly.Harmful phytoplankton blooms will cause severe damage to mariculture.More and more researches have showed that the interspecies relationship and nutritional physiology of phytoplankton were important to explain the succession of phytoplankton community during harmful algal blooms.To develop a healthy scallop aquaculture rapidly,the efficient plans of pollution prevention and bloom controlling should be very necessary.Therefore,it is significant to study the ecological and physiological charateristics of phytoplankton in scallop culture sea area and to understand the relationship between phytoplankton and scallop culture.In this thesis,the species composition,community structure and seasonal distribution of phytoplankton in scallop culture sea area were studied based on the 48 samples collected during eight cruises.The relationship between phytoplankton assemblages and the environmental factors and scallop culture was also explored.Moerever,the physiology and formation mechanism of brown tide were studied by exploring the interspecies relationship and nitrogen nutritional characteristics of Aureococcus anophagefferens and some other diatoms.The main results were shown as follows:1.A total of 134 species were identified,including 104 diatoms which belong to 33 genera,27 dinoflagellates which belong to 14 genera,2 chrysophytes belong to 1 genus and 1 euglenophyte belongs to 1 genus.The diatom and dinoflagellate were the main groups of phytoplankton which accounted for 77.61%and 20.15%of the identified taxa.The dominant species of phytoplankton varied seasonally.During spring,the dominant species were Nitzschia lorenziana,Melosira sulcata,Skeletonema costatum and Protoperidinium pellucidum.During summer,two dominant species were Pseudo-nitzschia pungens and Chaetoceros lorenzianus.During autumn,the only one dominant species was Schroederella delicatula f.schrdderi.There were three dominant diatom taxa during winter M.sulcata,Cerataulina pelagica and Ditylum brightwellii.2.The phytoplankton cell abundance ranged between 0.16×103 cells/L?98.7×103 cells/L and the average vaule was 17.15×103 cells/L.The cell abundance was different during four seasons:autumn>summer>spring>winter.3.The seasonal variations of phytoplankton assemblages were primarily influenced by the water temperature and nutrient,and hydrodynamic condition was a behind factor that controlled the environmental variables.Results of canonical correspondence analysis showed that phosphate,silicate,DIN/SiO2 ratio and temperature were the most important environmental factors influencing the variation of phytoplankton community.4.The grazing pressure caused by scallop culture affected the phytoplankton composition significantly.With high scallop density,grazing might exert an effective control on phytoplankton composition by leading to lower phytoplankton biomass and proportion of diatoms.Although the ecological balance maintained by scallop and phytoplankton in the ecosystem might keep the seawater in high water quality,the eutrophication resulted from the overloaded scallop culture would cause a potential red tide risk.5.The growth of A.anophagefferens and Minutocellus polymorphus was allelopathically interfered by the other species,but no out-competement was observed.The interference in the bi-algal culture(The inhibition rate ranged between 46%-74%)was stronger than that in the culture filtrate(The inhibition rate ranged between 6%-41%)prepared from the same microalgae cell density.Growth simulation by using the Lotka-Volterra competition model indicated that A.anophagefferens and M.polymorphus could co-exist in the bi-algal culture.The similar allelopathy existed between A.anophagefferens and M.polymorphus might be an important factor that made these two species co-dominant in nature during brown tides.6.The result that allelopathic effect of M.polymorphus at the stationary growth phase on the growth of A.anophaefferens was stronger than that at the exponential growth phase indicated that the intensity of M.polymorphus allelopathic effects depended on its growth phase.The result that the growth of A.anophagefferens was significantly increased in the filtrates of M.polymorphus pre-treated under different heat conditions(50 ? and 90 ?)compared to the fresh culture filtrate indicated that the allelochemicals might be a mixture with temperature-sensitive components in it.The effect ingredient was dissolved by petroleum ether and dichloromethane.In addition,the results of GC-MS showed that the major potential allelochemicals were phthalates.7.The nitrogen nutritional characteristics of M.polymorphus,Skeletonema marinoi and Thalassiosira pseudonana were compared with that of A.anophagefferens,and the results showed that M.polymorphus had similar nitrogen nutritional characteristics to A.anophagefferens.Therefore,M.polymorphus and A.anophagefferens would be more competitive in the water where DIN was relatively low and DON was relatively high.This might be the reason why M.polymorphus and A.anophagefferens could be co-dominant in nature during brown tides.8.The fact that Fv/Fm value of M.polymorphus,S.marinoi and T.pseudonana was similar in different nitrogen conditions indicated that there were not significant effects of different nitrogens on the photosynthesis of these three microalgae.