Spatio-temporal Distribution Characteristics Of Cyanobacteria And Bloom Mechanism In Hongze Lake On The East Route Of South-to-North Water Diversion Project

Hongze Lake is an important regulating lake in the east route of the South-to-North Water Diversion Project(SNWDP),and also a key hub for hydraulic regulation in Jiangsu Province.Its water quality status and changes directly affect the water safety along and around the East Route of SNWDP.A series of pollution control and nutrient reduction measures have been carried out in Hongze Lake.It is urgent to clarify the effect of these measures on the water environment characteristics.In addition,the area of Hongze Lake is large,and each region have different ecological functions.It is unknown whether there are differences in the water environment and the spatial distribution of phytoplankton,especially cyanobacteria.The study on the dynamic changes of phytoplankton community,especially cyanobacteria community in Hongze Lake can find out the temporal and spatial distribution characteristics of cyanobacteria,provide data support for evaluating the risk of cyanobacteria bloom,provide scientific basis for water resources protection and water ecological management.From March 2011 to November 2020,the water quality and phytoplankton community distribution characteristics of Hongze Lake were analyzed.Eleven sampling points in E region,W region and N region were used for comparing the seasonal and region differences of environmental factors by one-way ANOVA.The interannual variation trend of environmental factors and phytoplankton groups was clarified by using Mann-Kendall test.The dominant species composition,the dynamic change of cyanobacteria cimmmunity and the characteristics of cyanobacteria bloom in each region were analyzed and compared.Using redundancy analysis(RDA)and variation partitioning analysis(VPA),the correlation between dominant groups and environmental factors was established.By analyzing the occurrence characteristics of cyanobacteria bloom in various regions,the key driving factors of cyanobacteria bloom in various regions were found out.On these basis,the key points of prevention and control of cyanobacteria bloom in different regions were put forward by using the species response curve fitted by generalized additive models(GAMs).Chlorophyta,Bacillariophyta and Cryptophyta showed downward trends:the average annual abundance of Chlorophyta decreased from 1011.8×104 cells/L to 536.9×104 cells/L,Bacillariophyta decreased from 247.8×104 cells/L to 80.2×104 cells/L,and Cryptophyta decreased from 304.8×104 cells/L to 123.5×104 cells/L.For Cyanobacteria,non-filamentous cyanobacteria showed a downward trend(the average annual abundance decreased from 628.8×104 cells/L to 75.8×104 cells/L).while the density of nitrogen-fixing filamentous cyanobacteria showed a significant increase trend(the average annual abundance increased from 290.8×104 cells/L to 393.9×104 cells/L),and it became the dominant Cyanobacteria group after 2015.Cynobacteria species grew better in summer and autumn,while Bacillariophyta species grew better in winter and spring.In winter and spring,the growth of nitrogen-fixing filamentous cyanobacteria was superior to other Cyanobacteria groups,and the main species were Pseudanabaena limnetica and Dolichospermum affine.In summer and autumn,nitrogen-fixing filamentous cyanobacteria and non-filamentous cyanobacteria alternately dominated.There is no consistent spatial distribution pattern of different nitrogen-fixing filamentous cyanobacteria species,but the abundance and proportion of the main non-filamentous cyanobacteria species,Microcystis wesenbergii,Microcystis aeruginosa and Chroococcus minor were in the order of N>E>W.There were strong positive correlations between Chlorophyta,Bacillariophyta,Cryptophyta and NO3-N.High turbidity will inhibit the growth of Chlorophyta,Bacillariophyta.Cryptophyta through weakening their photosynthesis.The growth of non-filamentous cyanobacteria was mainly affected by water temperature,nitrogen nutrients,turbidity and precipitation.The response mechanism of non-filamentous cyanobacteria varied by different precipitation conditions:precipitation can promote the growth of non-filamentous cyanobacteria through increasing the input of nitrogen nutrients carried by surface runoff;high temperature weather combined with less precipitation will also provide favorable environmental conditions for the aggregation and abnormal proliferation of non-filamentous cyanobacteria,especially Microcystis spp.Nitrogen-fixing filamentous cyanobacteria can adapt to the increase of inflow,turbidity,water temperature and total phosphorus,the decrease of nitrate nitrogen.The main reasons are as follows:the increase of water temperature in spring can provide favorable conditions for the early growth of nitrogen-fixing filamentous cyanobacteria;nitrogen-fixing filamentous cyanobacteria can gain growth advantage when the nitrogen concentration is low by fixing nitrogen in the air;the distribution of pigments containing phycoerythrin or darker pigments enables nitrogen-fixing filamentous cyanobacteria to maintain good growth in high turbidity environment;one explanation for the success of nitrogen-fixing filamentous cyanobacteria is a better surface to volume ratios with higher surface areas(nutrient uptake potential)than colonies,especially mucilage contained colonies.Microcystis belongs to thermophilic organisms,and its growth needs sufficient nitrogen and phosphorus nutrients.In order to resist the drastic changes of environmental factors,Microcystis often exists in the coating of colloidal sheaths in the form of groups.Environmental conditions suitable for the physiological and ecological characteristics of Microcystis(water temperature>25℃.total phosphorus>0.07 mg/L,turbidity<50 NTU,water depth<2 m)will stimulate the rapid propagation of Microcystis,make it dominant in the phytoplankton community,coupled with hydrological conditions with little disturbance(inflow of E region<2000 m3/s,water depth increment<2.5 m/month;400 m3/s<inflow of W region<1100 m3/s,0.5 m/month<water depth increment<1.1 m/month;100 mm<precipitation in N region<300 mm,0.1 m/month<water depth increment<0.3 m/month)is more beneficial for Microcystis to aggregate and float to the surface of water,inhibiting the growth of other organisms and eventually forming bloom.When the total phosphorus concentration>0.07 mg/L,turbidity<50 NTU and water depth<2 m,we should be alert to the proliferation of Microcystis under suitable hydrological conditions(less water inflow in E region and W region and less precipitation in N region),and we can suppress the abnormal proliferation of Microcystis by increasing the inflow and controlling the total phosphorus concentration.Merismopedia can grow well in the environment with low water temperature and certain hydraulic disturbance,but it is difficult to adapt to the water with high turbidity,and its growth requires high nutrient concentration.When the environmental conditions suitable for merismopedia’s physiological and ecological characteristics appear(water depth<1.5 m,turbidity<150 NTU,total nitrogen concentration>1.0 mg/L),Merismopedia spp.will proliferate in large quantities,and form bloom under suitable hydraulic disturbance conditions(inflow of W aregiv n>900 m3/s,water depth increment>1.1 m/month).When the water depth is low(<1.5 m),turbidity is low(<150 NTU)and total nitrogen is high(>1.0 mg/L)in E region or W region,we should pay close attention to the growth of Merismopedia,and we can control the concentration of total nitrogen to inhibit the proliferation of Merismopedia.Nitrogen-fixing filamentous cyanobacteria can maintain good growth under nitrogen deficiency condition through nitrogen fixation,but their demand for phosphorus nutrients is high.In addition,the morphological characteristics and pigment composition of nitrogen-fixing filamentous cyanobacteria make it suitable for large hydraulic disturbance and with high turbidity.When there are low nitrogen and high phosphorus conditions suitable for the growth of nitrogen-fixing filamentous cyanobacteria(total nitrogen<2.0 mg/L and total phosphorus>0.07 mg/L),nitrogenfixing filamentous cyanobacteria can gain certain growth advantages through nitrogen fixation,and gain greater growth advantages,proliferate and form blooms under strong hydraulic disturbance(inflow of E regioon is>2000 m3/s,water depth increment>2.5 m/L;inflow of W region>900 m3/s,water depth increment>1.1 m/month;precipitation in N region>200 mm,water depth increment is>0.2 m/month).When inflow of E region and W region is large,or when the precipitation in N region is relatively large,we should be alert to the promotion effect of increased turbidity and deeper water depth on the growth of nitrogen-fixing filamentous cyanobacteria,and monitor the changes of nitrogen and phosphorus nutrients at the same time,and be alert to the conditions of low nitrogen(total nitrogen concentration<2.0 mg/L)and high phosphorus(total phosphorus concentration>0.07 mg/L).In addition,controlling the total phosphorus concentration is an effective means to inhibit the bloom of nitrogenfixing filamentous cyanobacteria.