| Based on the data about components and quantities of plankton in East China Sea and the results of harmful effects of Prorocentrum donghaiense and Alexandrium catenella on different organisms, analysis was made on the effects of harmful algal bloom(HAB) on structure of pelagic ecosystem in East China Sea. The possible mechanism of the harmful effects caused by different species of Alexandrium was also studied. A simple model of marine food chain, "harmful algal→Artemia salina" was designed, the effects of 12 strains of Alexandrium and three strains of Prorocentrum on survival and growth of Artemia salina, as well as the effects of Prorocentrum donghaiense and Alexandrium catenella on gross conversion efficiency in dry weight of Artemia salina were investigated. On the basis of results above, the impact of HAB on function of pelagic ecosystem in East China Sea was discussed. The results were showed as following: During April to May 2002 in East China Sea, P. donghaiense became the dominant species. The quantity and diversity of diatom decreased, and the dominant species of diatom changed. The index of diversity, evenness and richness of zooplankton were low, which indicated that the community structure of zooplankton in this area was not stable. Moreover, the quantity of zooplankton and Calanus sinicus in Spring decreased gradually since 2002 in studied area, and the species composition of zooplankton changed. A. catenella showed more strong adverse impacts on organisms than P. donghaiense, which affected the survival or reproducing of Artemia salina, Brachionus plicatilus, Moina Mongolica, Neomysis awatschensis, Lateolabrax japonicaus and Argopecten irradiants lamark adversely at the density of 105 cell L-1. P. donghaiense had the detrimental effects on reproducing of zooplankton, but it did not affect Neomysis awatschensis and Lateolabrax japonicaus obviously at the densities of 107-108cell L-1. Comparative toxicity study of four strains of Alexandrium demonstrated that the toxicities and the harmful mechanisms of Alexandrium varied among species and strains, and other non-PSP toxic substances could be produced by the alga. Ten strains of Alexandrium, including A. tamarense (ATHK, AT5-1, AT5-3, ATCI02, ATCI03) , A. catenella (ACDH), A. affine(AC-1, AS-1), A. minutum (AM-1) and A. spp (A-K) had the lethal effects on Artemia salina. Exposing to A. tamarense (AT-6), the survival rate of A. salina decreased and was equivalent to starvation. A. salina could survive and grow when exposed to A. lusitanicum (AL-1), P. micans, P. minitum and P. donghaiense respectively. The gross conversion efficiency(GCE) in dry weight from P. donghaiense and Nitzschia closterium to A. salina both rised with the density increasing at the lower densities, and decreased gradually at the higher densities. When P. donghaiense mixed with N. closterium, A. salina selectively increased grazing for P. donghaiense, and the GCE of A. salina decreased gradually with the density of P. donghaiense increasing. The respective GCE from P. donghaiense, N. closterium and mixture of these two species to A. salina were not different remarkably(P>0.05)when the biomass of alga were same. Exposing to A. catenella culture, the weight of A. salina decreased, material and energy couldn't transfer. When the re-suspended cells and cell-free medium of A. catenella mixed with N. closterium respectively, the re-suspended cells restrained the grazing of A. salina for N. closterium, but they did not influence the GCE from N. closterium to A. salina. The cell-free medium resulted in the decrease of grazing of A. salina and GCE for N. closterium. When the re-suspended cells and cell-free medium of A. catenella mixed with P. donghaiense separately, the grazing of A. salina for P. donghaiense were not influenced remarkably(P>0.05). Although the GCE from P. donghaiense to A. salina tended to decrease, they all had no obvious differences with control(P>0.05). P. donghaiense and A. catenella could influence the transfer efficiency of materials through the pelagic food c...
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