The Key Effect Of Nitrate On Phaeocystis Globosa Booms In The Beibu Gulf

In recent years,under the dual pressures of human activities and climate change,the level of eutrophication in coastal waters has intensified.The frequency of Harmful Algal Blooms(HABs)has increased,the duration of HABs has increased,and the influence scope of HAB s has expanded.HABs have become typical ecological disasters in coastal countries around the world.Phaeocystis globosa blooms in the Beibu Gulf in China has not only seriously affected the local fishery production and ecological security,but also brought serious hidden dangers to the safety of nuclear power sources in the past ten years,which had caused widespread concern.Based on refined voyage surveys,field experiment and isotope tracer experiment,the characteristics of environmental factors during the entire P.globosa blooms were revealed,and the key effect of NO3-on P.globosa blooms was found.The uptake strategy of NO3-by P.globosa was revealed.The biogeochemical process of NO3-in typical sea areas were quantified and its role in the process of blooms was analyzed.The main findings are as follows:(1)Study on environmental factors affecting the initiation and disappearance of P.globosa blooms In order to fully understand the changes in environmental factors during generation and elimination of P.globosa blooms and explore the key factors affecting blooms,9 voyage surveys from September 2016 to August 2017(September 2016,November 2016,December 2016,January 2017,February 2017,March 2017,April 2017,June 2017,August 2017)were conducted in the Beibu Gulf.P.globosa colony appeared in December,and the abundance of colony increasing to March of the following year.The abundance began to decline in April.Only a few colonies were found in little stations in June,and no colony were found in August.The study area was controlled by 4 water masses:Coastal Water,Qiongzhou Strait Shelf Water,South China Sea Bottom Shelf Water,and South China Sea Mixed Shelf Water.Affected by seasonal changes of water masses,the spatial and temporal distribution of different environmental factors had their own characteristics.Based on these characteristics,13 physical and chemical factors during blooms were preliminarily screened using principal component analysis.The results showed that the score coefficients of NO3-,PO43-,and SiO32-in the first principal component were 0.19,0.21,and 0.20,respectively and the score coefficients of other factors are all lower than 0.17.The potential relationship between different environmental factors and blooms was further identified.It was suggested that NO3 and PO43-might be key factors affecting P.globosa blooms.Water mass transport,phytoplankton assimilation and organic matter decomposition were the main factors affecting the spatial and temporal distribution of NO3 and PO43-High-value areas of NO3 and PO43-mainly appeared in the adjacent waters of the Qiongzhou Strait.Overall the distribution of NO3 and PO43-in surface and bottom layers was relatively consistent.At sites with higher colony abundance in February and March,the concentrations of NO3-and PO43-in surface layer were significantly lower than those in bottom layer.The loss of NO3-and PO43-in surface layer may be uptaked by colonies.Based on the NO3-nitrogen and oxygen stable isotope method,the assimilation process of NO3-by P.globosa was verified,and the supplementary effect of nitrification on NO3-during blooms was confirmed.With blooms going on,the enhancement of NO3-assimilation,the weakeness of the internal regeneration process,and the reduction of external input reduced the concentration of NO3-.These processes led to relative nitrogen limitation(DIN:PO43-<10 and SiO32-:DIN>1),which may cause the extinction of blooms.(2)The preferential uptake of nitrate by P.globosa colony In order to clarify the uptake characteristics of different nutrients by P.globosa colony,the colonies were collected at the cold water intake of Fangchenggang Nuclear Power Station in Guangxi during P.globosa blooms in January 2018 to conduct field experiment under different enrichment conditions of nitrogen sources.The nutrient concentrations and bacterial community structure were measured during the experiment.In different experimental groups,the maximum uptake rates of NO3-,NH4+,PO43-,SiO32-,NO2-were 7.38±2.06 ×10-9,2.14±0.25 × 10-9,0.14 ± 0.11 × 10-9,0.09 ± 0.03 × 10-9,0.08±0.04 × 10-9?mol·h-1·cell-1(0-12 h)and the average uptake rate of NO3-was significantly higher than the other four nutrients(P<0.05).Under enrichment conditions,the concentrations of NO3-and NH4+ in the colony increased at 1.5 h and 3 h,respectively.These phenomena indicated that the colony had the characteristics of preferential uptake and faster transmission of NO3-.16S rDNA technology confirmed that there were differences in bacterial community between the inside and outside of colony.Under natural conditions,the concentrations of PO43-,NH4+ and SiO32-were higher in the colony,and NO3-,NO2-have higher concentrations outside the colony.It was believed that the semi-permeability of the colony envelope and the bacterial mineralization process,the gluconeogenesis process in the colony led to differences of nutrient concentrations in and out of colony.Based on the field survey results from 2016 to 2017,the abundance of colony cell was much higher than that of other phytoplankton at stations with colony,and its ability of uptaking NO3-was also stronger than other phytoplankton.It was speculated that the decrease of NO3-concentration cause the extinction of P.globosa blooms.(3)Study on the effects of key nitrate transformation process on the initiation and disappearance of P.globosa blooms Nutrient regeneration process can provide nutrition foundation for the maintenance of blooms.In order to further quantify the effect of the regeneration process of NO3-on different stages of P.globosa blooms,the blooms was divided into initiation stage(November),development stage(January)and disappearance stage(February)based on the changing characteristics of colony abundance from November 2018 to April 2019.Relevant physical and chemical parameters were collected in different stages.ZN1-6 in the strait shelf water and ZN4-3 in the frequent blooms area were selected as the experimental station to conduct isotope tracer experiment and assimilation and nitrification rate of NO 3 were determined.The isotope tracer experiments in different stages showed that the nitration rate of ZN1-6 station ranged from 0.27-0.95 ?mol·L-1·d-1 and NO3-assimilation rate of ZN1-6 station ranged from 0.62-2.00 ?mol·L-1·d-1.The maximum value of two rates appeared in development stage,and the minimum value appeared in disappearance stage.In development stage,the nitrification rate and assimilation rate of ZN4-3 station ranged from 0.72-1.44 ?mol·L-1·d-1 and 1.77-2.34 ?mol·L-1·d-1,respectively.The nitrification and assimilation rate of each layer at ZN4-3 were higher than ZN1-6.The above results indicated that there were stronger biogeochemical processes in the frequent blooms area.It was worth noting that nitrification rate gradually increased from the surface layer to the bottom layer in different sea areas,while assimilation rate gradually decreased from the surface layer to the bottom layer.The analysis showed that the above rules were caused by the effects of temperature,NH4 and NO3-,Chl a respectively.The nitrification process was the main way to produce NO3-in seawater.Compared with many domestic and foreign studies,the nitrification rate in this sea area during blooms was low,which was only 43-51%of the assimilation rate.Therefore,the NO3-produced by nitrification alone cannot satisfy the absorption of phytoplankton.The formation of P.globosa colony and the maintenance of P.globosa blooms require a large amount of NO3-.Qiongzhou Strait Shelf Water is the main water mass that inputs NO3-to the study area,which plays an important role in the occurrence and maintenance of P.globosa blooms.The innovations of this dissertation are as follows:The characteristics of the spatiotemporal changes of the main environmental factors in the Beibu Gulf were carefully described.Based on the comprehensive analysis of hydrological,biological,chemical and other synchronous observation indicators,the key effects of NO3-on P.globosa blooms were discovered and verified;The concentration of nutrients and the bacterial community inside and outside of P.globosa colony were different,and the colony envelope had the characteristics of faster penetration of NO3-,which reveals the mechanism of preferential uptake of NO3-by P.globosa colony;The isotope tracing technique was used to quantitatively explain the conversion process of NO3-in different stages,and it was found that the assimilation rate of NO3-in each stage was higher than the nitrification rate.The important role of high NO3-concentration input of Qiongzhou Strait Shelf Water on the occurrence and maintenance of P.globosa blooms was clarified.In conclusion,this dissertation analyzed the spatiotemporal characteristics of main physicochemical factors in the Beibu Gulf.The preferential uptake of NO3-by colony cells was found and its mechanism was explored.It was considered that NO3-had key effects on the occurrence and the extinction of P.globosa blooms.The key transformation processes of NO3-were quantitatively expounded and the effects of these processes during P.globosa blooms were revealed.It speculated the high transport of NO3-from Qiongzhou Strait Shelf Water was an important factor for the formation and maintenance of P.globosa blooms.Relevant research results laid a foundation for further revealing the formation mechanism of P.globosa blooms in the Beibu Gulf.It also provided a scientific reference for further understanding the key elements cycle and scientific prevention and control of P.globosa blooms in this sea area.