The Spatio-Temporal Distribution Of Phaeocystis Globosa Colonies And Prokaryotic Communities And Their Responses To Environmental Factors In The Beibu Gulf,China

Due to the increasingly serious eutrophication in the Beibu Gulf,Phaeocystis globosa blooms occur frequently,and impairs marine ecosystem and social economy in recent years.The spatio-temporal distribution of P.globosa colonies and its relationship with main environmental factors were studied based on the monthly multi-disciplinary investigations from September 2016 to August 2017 and from November 2018 to April 2019.Furthermore,the heterotrophic bacteria dynamics in high-incidence area of P.globosa blooms were analyzed,and the spatio-temporal distribution of particle-attached and free-living prokaryotic communities during P.globosa blooms and the relationship with main environmental factors were discussed as well.The main findings were as follows:（1）The spatio-temporal distribution of P.globosa colonies and related environmental factors in the northern Beibu Gulf During the survey period P.globosa colony abundance ranged from 0 to 18.98×10⁴ col·m^-3.The colony abundance peaked in in winter months（February 2017 and January 2019）.The high-colony-abundance zone was spatially different with time.It first appeared in the Weizhou Island surrounding waters（December 2016 and November 2018）,then appeared in the Qinzhou-Beihai coastal waters（February 2017 and January 2019）,and finally appeared in the Qiongzhou Strait-Weizhou Island waters（March 2017and February 2019）.It is indicated that temperature and salinity are main environmental factors affecting the spatio-temporal distribution of P.globosa colonies,and sufficient nitrate and phosphate facilitate the formation of P.globosa colonies in Qiongzhou Strait-Weizhou Island waters.The P.globosa colony abundance was positively correlated with other-phytoplankton abundance,and its spatio-temporal distribution was influenced by the succession of other-phytoplankton community.（2）The spatio-temporal distribution of heterotrophic bacteria in the northern Beibu Gulf From September 2016 to August 2017,the heterotrophic bacteria abundance ranged in（2.75–56.86）×10⁵ cell·m L^-1,with an average of（11.01±6.31）×10⁵ cell·m L^-1.There were obvious monthly variations,and the heterotrophic bacteria abundance in each season from high to low was:summer（June,August and September）,winter（December,January and February）,spring（March and April）,and autumn（November）.Heterotrophic bacteria were densely distributed in the shallow coastal water,and gradually reduced in the deep offshore water.The vertical distribution of heterotrophic bacteria was evenly distributed in the coastal areas,while the seasonal vertical heterogeneity occurred in the areas deeper than 20m,where the bacteria abundance was higher at surface than at bottom.The spatio-temporal distribution of heterotrophic bacteria was controlled by the environmental conditions,especially the temperature,salinity,chlorophyll a,and dissolved oxygen.Correlations between inorganic nutrients and heterotrophic bacteria abundance were significantly negative in some months,when the phytoplankton indirectly affected them.In addition,the heterotrophic bacteria abundance was also closely related to the P.globosa colony abundance in the period of colony existence.（3）Particle-attached and free-living prokaryotic community composition and structure during the Phaeocystis globosa bloom in the Beibu Gulf From December 2016 to February 2017,the dominant phyla in the northern Beibu Gulf were Proteobacteria,Cyanobacteria,Thaumarchaeota,Verrucomicrobia,Euryarchaeota,Actinobacteria,and Bacteroidetes.The composition and structure of PA and FL prokaryotic communities were significantly different.The PA fraction was more diverse and unstable temporally compared to the FL fraction.The variations in the composition and structure of the prokaryotic community were closely associated with major environmental variables,particularly DO,NH₄⁺,PO₄^3-,and DOC.The FL fraction was more sensitive to P.globosa than the PA fraction.Rhodobacteraceae, Alteromonadales,Porticoccaceae,Vibrio,Flavobacteriales,and Verrucomicrobiae were the key prokaryotic groups during the outbreak and recession of P.globosa blooms.They could play important roles in degrading the algal-derived organic matter.（4）Spatial and diel variations of the prokaryotic community in the Phaeocystis globosa blooms area of Beibu Gulf During the outbreak of P.globosa blooms in January 2019,the distributions of prokaryotic communities varied significantly in the horizontal direction,while slightly in the vertical direction.Both spatial distance and environmental variables,particularly P.globosa colony abundance and Chl a,shaped the horizontal distribution of the prokaryotic community structure.The vertical heterogeneity of the prokaryotic community was disrupted by the strong vertical mixing of the water column in winter.PA prokaryotic communities showed significant diel variations,but the diel variations of FL prokaryotic communities were insignificant.Inorganic nitrogen and P.globosa colony abundance were the main environmental variables impacting the diel variations of prokaryotic communities.In conclusion,this dissertation studied the spatio-temporal distribution of P.globosa colonies and heterotrophic bacteria,and their relationships with main environmental factors were discussed.Furthermore,the differences in the composition and structure of particle-attached and free-living prokaryotic communities during the P.globosa bloom in the Beibu Gulf were studied,and the spatial and diel variations of the prokaryotic community in the P.globosa bloom area of Beibu Gulf were revealed.The results are helpful for better understanding P. globosa blooms in Beibu Gulf,and provide fundamental data and evidence to further elucidate the initiation and termination of P.globosa blooms and to monitor and predict the occurrence of P.globosa blooms in the area,and to further reveal the ecological impact of prokaryotes during P.globosa blooms.