Effects Of Cladophora On The Growth Of Submerged Macrophytes And Cyanobacteria,and Nutrient Migrations Of Sediments

In the process of restoration of submerged macrophytes in eutrophic lakes,the overgrowth of filamentous green algae（FGA）such as Cladophora is often observed.The excessive growth of Cladophora causes poor growth or even decline of submerged macrophytes.Aiming at the ecological problems of the proliferation of Cladophora in the process of submerged macrophytes restoration,this study used Cladophora oligoclora as a research object to construct a C.oligoclora-submerged macrophytes cocultivation system,and to simulate the competition for nutrient between C.oligoclora and restoration pioneer submerged macrophytes（Vallisneria natans,.Ceratophyllum demersum）,and the kinetic equation and interspecific competition model were used to analyze and verify the nutritional competition effect of C.oligoclora and submerged macrophytes.The decay rule of C.oligoclora under different conditions and the influence on the growth of cyanobacteria Microcystis aeruginosa were studied,and GC/MS was used to identify the main organic substances released into the water by the C.oligoclora.The physiological and biochemical responses of different pioneering species of submerged macrophytes pioneers（turion germination,seedling growth,and fragment regeneration）to decomposing C.oligoclora were studied by means of dynamic analysis of rapid chlorophyll fluorescence induction.Stable isotope labeling and high-throughput sequencing methods were used to study the effects of decomposition process of C.oligoclora on the nitrogen and phosphorus nutrient salt structure and microbial community composition at the sediment-overlaying water interface.Through the research,the following results were obtained:（1）In the co-culture system,the dynamic changes and characteristics of nitrogen and phosphorus uptake by V.natans,C.demersum and C.oligoclona were investigated,and the results showed that C.oligoclona showed high affinity for nitrogen,the content of TN in the co-cultured C.oligoclona was up to 5.75 %.The high affinity showed high phosphorus uptake and assimilation ability for C.demersum（Km =0.34 mg·L（-1））.The results of interspecific competition model verified that C.oligoclona and submerged macrophytes were not stable co-existence in the same system.（2）Study on the effect of simulated decomposing process on the water environment revealed that shading and hypoxia treatment significantly reduced the DO and p H value of the culture solution and significantly increased the total organic carbon（TOC）and conductivity（Cond）.This indicated that C.oligoclona,which is deposited at the bottom of natural water of no-light and hypoxia,has the greatest influence on the water environment.The cell density of M.aeruginosa was higher than that of the control group under the treatment of low concentration（10 % of original decomposing liquid）,and the cell density and photosynthetic activity decreased significantly under the treatment of higher concentration of decomposing liquid.Fatty acid and phenolic acid were identified as the main algal inhibiting active substances.The identification and analysis of lipophilic components in decomposed residue and decomposing liquid showed that p-methyl phenol and indole compounds were the main active organic compounds that are difficult to degrade during the decomposing process of C.oligoclona.（3）Studies on the effects of different concentrations of decomposing liquid respectively on the turion germination and seedling growth of Hydrilla verticillata,the rooting and germination of apical fragment of Myriophyllum spicatum showed that,after processed by decomposing liquid produced significant changes in the quality of the water,the p H,DO reduced,and the increase of organic compounds leads to the increase of Cond.The high concentrations of C.oligoclona decomposing liquid（40 %of original decomposing liquid）significantly inhibited the germination vigor of H.verticillata turion,and the germination rate decreased to 84%.The content of chlorophyll a decreases by 43.53 % compared with the control group,and the photosynthetic system suffers certain damage,leading to the inactivation of PSII reaction center and the blockage of electron transfer.Soluble sugar,Ca²⁺/Mg²⁺-ATP and PAL activity increased by 172.46 %,271.19 % and 26.43 %,respectively.The normal growth of seedlings was under stress.Inhibition in rooting and germination of apical fragment of M.spicatum indicated that the regeneration ability was limited.The RDA sorting found that high Cond in culture medium environment was the most important environmental factor affecting the changes of physiological and biochemical characteristics of apical fragment.Under the treatment of 40 % of original decomposing liquid,the total soluble sugar content in the apical fragment tissue was increased to115.26 %,and the Ca²⁺/Mg²⁺-ATP and secondary metabolase-related PAL activity were increased by 490.63 % and 28.13 %,respectively,and the defense response was enhanced.（4）The results showed that the decomposition process of C.oligoclona is in line with the general rule of aquatic plants.The changes of sediments δ¹³C and δ¹⁵N indicated that some ¹⁵N migrated to the sediment during the decomposing process of C.oligoclona,which may affect the nutrient structure of the sediment.The decomposition of C.oligoclona increased the degree of eutrophication of overlying water in a short period of time.TN and NH⁺₄-N rapidly increased between 0-10 days,rising to 78.21 %of TN at the 40th day of decomposition,resulting in serious pollution of ammonia salt in overlying water.The influence on the nitrogen structure of sediments is mainly reflected in the increased risk of ammonia nitrogen release.The decomposition of C.oligoclona also caused a large amount of phosphate to be injected into the overlying water.On the 40th day,the TP and IP incresed to the highest concentrations of6.68±0.64,6.59±0.79 mg·L^-1,respectively.Moreover,the decomposition process of C.oligoclona resulted in the migration of phosphate from the overlaying water to the sediments,and the phosphorus content in the sediments increased.The microbial community changes in the overlying water and sediment have different trends over time.The correlation analysis results showed that the phosphorus has a good correlation with the microbial community in the overlying water in the middle and late stages of decomposition,while the microbial community in the sediment has a high correlation with various forms of nitrogen.