Keywords: 
• N. scintillans /
• harmful algal bloom /
• aquatic environmental factor /
• phytoplankton

Abstract: Based on the data of Noctiluca scintillans bloom happened in the Guanghai Bay and Zhenhai Bay of the Pearl River Estuary in April 2016,the water quality and the change of the community composition of phytoplankton in the sea area under algal bloom were analyzed by using water quality evaluation,hierarchical cluster analysis and principal component analysis in order to establish a correlation between the patch distribution of N.scintillans bloom and the aquatic environmental factors.The results showed that the sea area under investigation was eutrophicated according to the trophic status index,and was slightly or moderately polluted based on the toxic substances of the sea water evaluated by the organic contamination index,the toxic pollution index and the comprehensive pollution index.The abundance of N.scintillans varied from 0 to 29 443.7 ind·L^-1 among the sampling stations,which indicated a notable patch distribution of the algal bloom.The composition of phytoplankton community was changed during the algal bloom.The average abundance of Bacillariophyta and Pyrrophyta increased evidently,while that of Chlorophyta,Cryptophyta and Cyanophyta decreased significantly in the algal bloom area.Moreover,the number of average species of the alga was 18,24 and 13 in the bloom area,the transition area and the non-bloom area,respectively.The dinoflagellate species,second to N.scintillans,were Akashiwo sanguinea and Ceratium furca,while the diatoms mainly included Dactyliosolen fragilissima,Guinardia flaccida（castracane） and Eucampia zoodiacus.Cluster analysis of the water quality factors revealed that N.scintillans bloom was detected in all the sampling stations in the river estuary,harbor area and aquaculture area where N.scintillans was distributed obviously in patches.The principal component analysis of water quality factors indicated that bloom of N.scintillans induced significant changes of the contribution rate and correlation coefficient of water quality factors.According to the rotated component matrix,the first principal component indicated the huge algal growth brought about consumption of nutrient（ammonia,total nitrogen,phosphate and total phosphorus） and increase of SS and CODMn,while the second principal component indicated the selected absorption of metals.Eutrophication of the sea water was the aquatic environmental basis of N.scintillans bloom.The patch distribution of N.scintillans changed obviously the structure of the phytoplankton community and the composition of the physical and chemical factors in the sea water of partial sea area.