Study On The Mechanism Of Benthic Algae Growth And Submerged Macrophytes Response In Eutrophic Water

Submerged macrophytes are the core members of aquatic ecosystem.In eutrophic water,benthic algae are the companions of submerged macrophytes.A large number of them attach to or entangle on submerged macrophytes,resulting in the growth inhibition or even death of submerged macrophytes.It is a difficult problem to solve in the process of using submerged macrophytes for water restoration.Previous studies have preliminarily shown that nutrition and light are the key factors to control the growth of benthic algae,but there is no systematic conclusion.In this paper,it first studied the effects of sediment nutrient release on the growth characteristics and phosphorus migration and transformation of different types of benthic algae,and selected the Leptolyngbya sp.biofilm as the research object to explore the growth dynamics and phosphorus（P）migration mechanism of Leptolyngbya sp.under the simulated eutrophic lake nutrient conditions（sediment and overlying water nutrient load）.Secondly,the water remediation system was simulated by“the overlying water+Leptolyngbya sp.biofilm+Vallisneria natans（V.Natans）+sediment”,under different nitrogen（N）and phosphorus concentration and light conditions in the overlying water.The biomass of Leptolyngbya sp.biofilm and morphological characteristics,biomass accumulation,distribution pattern of V.Natans were quantitatively evaluated.The range of nutrient and light loads affecting the growth and morphological response of submerged macrophytes were determined,to determine the driving factors for the growth and rapid transformation of benthic-plankton ecotypes of the dominant Leptolyngbya sp.biofilm.Finally,the structure characteristics of benthic diatom community and its relationship with environmental factors under the above different environmental conditions were compared and analyzed,and then the mechanisms affecting benthic algae growth and submerged macrophyte morphological response were systematically elucidated to provide scientific basis for eutrophic water restoration.The results were as follows:1.The growth of benthic algae biofilm was significantly related to water pH,dissolved oxygen（DO）,oxygen reduction potential（ORP）and other environmental factors,which a certain degree inhibition the release of nitrogen and phosphorus in sediment.The phosphorus content and form of benthic algae biofilm were related to the species of benthic algae.The content and forms of phosphorus in the benthic green algae biofilm dominated by Rhizoclonium sp.biofilm were significantly different from those of benthic cyanobacteria dominated by Leptolyngbya sp.biofilm.The total phosphorus（TP）content of the former was 8.2627 mg,mainly Fe/Al-P,accounting for52.05%of TP,the latter of phosphorus retention ability was stronger,the TP amounts reached 26.7095 mg,in which NH₄Cl-P（39.75%）and Fe/Al-P（46.04%）were dominant forms.2.In eutrophic overlying water(TN:3.71 mg·L^-1,TP:0.228 mg·L^-1)-Leptolyngbya sp.biofilm-sediments culture systems,the phosphorus form and the ability of phosphorus retention of Leptolyngbya sp.biofilm were significantly different from those of the control.After 60 days,in the oligotrophic water,the main phosphorus forms of Leptolyngbya sp.were NH₄Cl-P（42.76%）and Fe/Al-P（48.28%）,and its total phosphorus concentration was 3.75 mg·g^-1.In the control,the main phosphorus form of Leptolyngbya sp.was Fe/Al-P（56.91%）,and its total phosphorus concentration was1.24 mg·g^-1.The results showed that the nutrient status of overlying water affected the phosphorus forms and the ability of phosphorus retention of Leptolyngbya sp.biofilm,with no obvious effect on the phosphorus form of the sediment,but it indirectly affected the release of phosphorus from sediment.3.Under the condition of adding different concentrations of phosphorus,the pH,DO and electrical conductivity（EC）of water increased with the increase of TP concentration.The growth of Leptolyngbya sp.biofilm and V.Natans could effectively slow down the increase of nitrogen content in water.The ability of Leptolyngbya sp.biofilm and V.Natans to remove water phosphorus was affected by the concentration of water phosphorus.When the TP≤5 mg·L^-1,the phosphorus of the overlying water could be effectively removed by the Leptolyngbya sp.biofilm and V.Natans.When the TP>5 mg·L^-1,the remove effect of the Leptolyngbya sp.biofilm and V.Natans was significantly reduced.There were differences in the response of P between the Leptolyngbya sp.biofilm and V.Natans.When the TP≤5 mg·L^-1,The biomass of the Leptolyngbya sp.biofilm increased with the increase of phosphorus concentration,and the time to peak biomass was shorter,about 8 days（control:36 days）.The number of ramets and leaf length of V.Natans increased with the increase of phosphorus concentration.When the TP>5 mg·L^-1,the growth of V.Natans was inhibited.The addition of phosphorus significantly increased the TP content in the sediments,and mainly in the form of NH₄Cl-P and Fe/Al-P.4.Under the condition of adding different concentrations of nitrogen in the water,the physical and chemical factors of each simulation system were different.When total nitrogen（TN）≤4 mg·L^-1,the growth of the Leptolyngbya sp.biofilm and V.Natans promoted the increase of DO and ORP,but the EC remained at a low level;When TN>4mg·L^-1,the DO and ORP decreased,but the EC increased.There was a significant difference in the content of the soluble active phosphorus（SRP）and the particulate phosphorus（PP）in different groups of water（P<0.05）.The content of the SRP and the PP in the water decreased with the increase of the TN concentration.When the TN≤8mg·L^-1,The biomass of Leptolyngbya sp.biofilm increased with the increase of nitrogen concentration;When the TN>8 mg·L^-1,the growth of Leptolyngbya sp.biofilm was inhibited.The response of V.Natans to the change of nitrogen concentration in water showed that the leaf length of V.Natans in low nitrogen treatment group was significantly increased under higher nitrogen condition,but the number of ramets of V.Natans increased with the increase of nitrogen concentration.Under different nitrogen concentrations,there were differences in phosphorus content and form composition of V.Natans.When TN≤4 mg·L^-1,NH₄Cl-P retention by V.Natans was enhanced,and the amount of NH₄Cl-P retained increased from 25.85%to 37.97%.Ca-P retention by V.Natans increased with higher nitrogen concentrations at 4-12mg·L^-1,but the main form of phosphorus was Fe/Al-P（70.12%）in TN was 16 mg·L^-1.The addition of nitrogen had no significant effect on the phosphorus form composition and content of sediment.5.The pH,DO and ORP were significantly correlated with the N and P concentrations in the water column under different light intensities（P<0.05）.The optimum light intensities for the growth of Leptolyngbya sp.and V.natans was different.The biomass of Leptolyngbya sp.biofilm was higher at 9-18μmol·m^-2·s^-1,but the biomass of V.natans were positively correlated with light intensities(2-54μmol·m^-2·s^-1)（P<0.05）.The potential effect of light intensity on the growth and physiological activities of V.natans was more important than that of the existence of Leptolyngbya sp.biofilm.The Leptolyngbya sp.biofilm and V.natans could accelerate the release of phosphorus and reduce nitrogen from sediment significantly.The TP amounts retained by V.natans were significantly higher than that of Leptolyngbya sp.biofilm in all treatments,and the potential for phosphorus retention of V.natans increased with the increase of light intensity.The retention of phosphorus amounts in the Leptolyngbya sp.biofilm was mainly Fe/Al-P,accounting for 47.8%,but NH₄Cl-P（57%）was the main retention phosphorus in the V.natans.6.The species composition and the relative abundance of benthic diatom community were influenced by the environmental factors.In a certain range of phosphorus concentrations(TP≤5 mg·L^-1),benthic diatom communities have few dominant species and high relative abundances,but the species composition numbers of benthic diatom community were more than those of other environmental conditions.The composition of benthic diatoms community has a succession from Achnantaceae in low concentration phosphorus to Nitzschia in high concentration phosphorus;Achnanthes minutissima and Nitzschia paleaeformis were the species with absolute advantage in the two simulation systems.Nitrogen nutrition had little effect on the species composition of benthic diatom community,while the species abundance of benthic diatom community decreased with the increase of light intensity.PCA and CCA results showed that DO,EC and phosphorus nutrients were important factors affecting the species composition of benthic diatom community.