The Transport And Transformation Of Nitrogen And Phosphorus During The Decomposition Of Algae Bloom In Lakes:the Process And Mechanism Study

Ecological deterioration induced by algae blooms in the aquatic ecosystem has been a global concern.Nitrogen?N?and phosphorus?P?are essential nutrients for living organisms,increasing input of N and P in waters can promote excessive algae growth,and then leading to harmful algal blooms?HABs?.During the following periods of algae decomposition,algae can precipitate and settle on the lake floo r where abundant particulate/soluble/colloidal nutrients can be released into the surrounding regions.Besides,the aggregation and decomposition of abundant algae are potential to significantly change the physical and biological features of the benthic environment,which can strongly modify the nutrients cycling.Accordingly,the algae decomposition may have a profound effect on the migration and transformation of N and P between sediment and water phase.Currently,the impact of HABs decomposition on nutrient dynamics was studied extensively while the response of N and P during consecutive HABs degradation and subsequent reoccurrence were rarely understood.Thus,microcosm scale study was conducted to evaluate how the decline process of cyanobacterial dominated HABs,under varied initial cyanobacterial density,influence the N and P behavior in the sediment-water interface,as well as the potential mechanism underlying it.The main achievements of this article are summarized as follows:?1?When the density of algal cells was less than 10⁸ cells/L,the HABs decomposition could promote the release of NH₄⁺-N from the sediments.When the density of algal cells was more than 10⁸ cells/L,the NH₄⁺-N in the overlying water mainly came from the algae decomposition,and the process inhibited the further release of NH₄⁺-N from the sediment.At the later stage of HABs decomposition,atmospheric reoxygenation under the low algae density treatment enhanced the nitrification in the sediment,making the sediment as the main site for NH₄⁺-N transformation.Photosynthesis of cyanobacterial cell populations resuscitated under the high algae density treatment significantly increased the nitrification in the sediment and also make the sediment become sinks of NH₄⁺-N.The recovery of the algal cell population promoted the transformation of NH₄⁺-N to NO₃-N in the system to some extent.?2?The sedimentation and subsequent decomposition of decayed algae lead to a rapid decrease of TP concentration and promoted the increase of TDP and SRP concentrations in the overlying water.Among them,the rapid increase of SRP concentration in the aqueous phase may also be caused by the release of SRP in the sediment.The increase of total phosphorus content in sediment during HABs decomposition mainly came from organic phosphorus.The HABs decomposition with different initial algae densities showed significant differences in the release of inorganic phosphorus from sediments.With the decomposition of cyanobacterial cells,the low-density HABs were affected by pH,ORP and DO,the inorganic phosphorus in sediment fluctuated significantly when the total inorganic phosphorus content was stable.With the decomposition of cyanobacterial cells,the high-density HABs released more and more inorganic phosphorus from sediment,of which Fe-P could release up to73.5%.?3?At the end of the HABs decomposition,as the initial algae density raised,the penetration depth of oxygen in sediments was 2.5mm,1.6mm and 0.7mm respectively.The higher algae density led to the higher algal bloom rate,accelerated oxygen consumption,further exacerbated the release of reduced mater ials in the sediment,and the released reductive material continued to consume oxygen,making the system rapidly anaerobic.?4?Significantly positive correlations?p<0.001?existed between DGT-SRP and DGT-Fe???in the control system,clearly verifying the primary mechanism of Fe redox-controlled P release in sediments.The decomposition of high-density HABs significantly altered the P and Fe dynamics at the SWI.The algae acted as a major nutrient reservoir during its decomposition,and the amounts of SRP was directly released into the sediment and the overlying water,respectively.The dynamics of P at the SWI were thus controlled directly by the decomposition of algae.The anaerobic conditions caused by the decomposition of HABs further promoted the release of Fe-P in sediments and the accumulation of SRP in interstitial water.Moreover,the release of SRP and Fe???in sediments mainly occurred below SWI 0⁴cm,which also reflected that below SWI 0⁴cm was the active layer of SRP.Mn???and Zn???had no significant contribution to the release of phosphorus from sediments during the decomposition of HABs,but can indirectly promote the consumption of inorganic phosphorus in overlying water.?5?The bacteria in the sediment are rich.The dominant phytochemicals belonging to the genus Phosphorus were Thiobacterium,Alcaligenes,Geobacter,Azospirillum,Flavobacterium and Janthinobacterium.The decomposition of HABs resulted in significant variation in the diversity,richness,and distribution of phosphate-solubilizing bacteria in sediments compared with the initial sediments.The decomposition of high-density HABs resulted in sediments under conditions of hypoxia and low Fe???concentrations,resulted in a decrease in the number of bacteria that have phosphorus uptake function in the genus Thiobacterium and Geobacter,those promote the release of P in sediments.In addition,the presence and decomposition of cyanobacteria had resulted in more Alcaligenes,Azospirillum,Flavobacterium,and Janthinobacterium species in surface sediments,and their phosphate-releasing effect released the sediment P.In the surface sediments treated with low algae density,phosphorous genus such as Thiobacterium,Alcaligenes,Geobacter and Flavobacterium were mainly found in sediments,and it made the release of the sediment P weak.The HABs decomposition lead to the change of environmental conditions and the characteristics of the microorganisms themselves,which affects the release of phosphorus from sediments together.The dissertation studied the migration and transformation regulation of N and P,as well as their underlying mechanisms during the process of the HABs decomposition.It was found that the regulation of nitrogen flux and migration altered during consecutive HABs degradation and subsequent reoccurrence process,and detailed data information of P,Fe,etc.and related environmental factors at fine-scale were provided,which helped to reassess the cycling of N and P during algae decomposition.And the results would be beneficial to clarifying the contributions of algae bloom decomposition to nutrient cycling in eutrophic lakes.