Sources,Characteristics And Transformation Mechanism Of Natural Organic Matter In Lake Environment

This study was examined the source,composition and transformation of dissolved organic matter（DOM）in different water courses of Pingzhai reservoir（upstream reservoir）and Hongfeng reservoir（downstream reservoir）in Sancha River Basin by using three-dimensional excitation emission matrix（3-D EEM）spectroscopy coupled with parallel factor（PARAFAC）modelling.Simultaneously,dam barrier effects on the transformation of DOM was extensively discussed in the inflowing-reservoir（surface and deeper）-outflowing continuum waters of these two reservoirs.To compare sources,characteristics and transformation of DOM in reservoirs,we have also collected diurnal and seasonal water samples from three closed lakes（Jingye lake,aiwan lake and Qingnian Lake）,which are located in Tianjin University campus.This study has been provided comprehensive transformation mechanisms of DOM with providing a four-phase model and then provided two basic laws based on the results.1.Source,characteristics and transformation mechanism of FDOM in two reservoirs of Sancha river basin,Guizhou province1.1.Source,characteristics and transformation mechanism of FDOM in Pingzhai reservoirThe results demonstrated that several fluorescent DOM（FDOM）components were identified in the inflowing,surface layer（0～5 m）,deeper layer（10～60 m）and outflowing waters of the Pingzhai reservoir and they could be denoted as terrestrial humic-like substances,autochthonous humic-like substances,protein-like substances,tryptophan-like substances,tyrosine-like substances.The fluorescence intensity of terrestrial humic-like substances in comparison with inflowing waters gradually decreases in the outflowing waters,suggesting that the terrestrial humic-like substances are relatively recalcitrant to its complete degradation during transportation from inflowing to reservoir and then outflowing water.Conversely,the fluorescence intensity of the autochthonous humic-like substance was higher in the reservoir than in the inflowing and outflowing waters,suggesting that autochthonous humic-like substances are simultaneously produced from autochthonous organic matter and then degraded under the influences of photochemical,microbial and dam-barrier physical processes.Similar results were also observed for other fluorescent substances.Temperature（T）,p H,DO and CDOM were showed the similarly changed with the seasonal changed.These results suggest that the influences of photochemical and microbial processes along with dam-barrier physical processes are responsible for space-time transformation dynamics of FDOM components in the inflowing,reservoir area surface water,reservoir area deeper water and outflowing waters of the Pingzhai reservoir.1.2.Source,characteristics and transformation mechanism of FDOM in Hongfeng reservoirCombined with physicochemical parameters and absorption spectra,the effects of damming on the characteristics and dynamic transformation of FDOM in the continuous water area of Hongfeng Reservoir were investigated.The EEM-PARAFAC results demonstrated that inflowing water was characterized with three FDOM,including terrestrial humic-like substances（C type）,degraded terrestrial protein-like substances（PLS）and tryptophan-like substances of terrestrial soil origin.Along with these three FDOM,reservoir profile water was further identified autochthonous FDOM,including autochthonous humic-like substances（M type）,newly released PLS,PLS and tryptophan-like substances,which were originated from phytoplankton or peripheral biomass that was estimated by chlorophyll a（Chl a）.Outflowing water was identified with three FDOM,including terrestrial humic-like substances（C type）,autochthonous humic-like substances（M type）and protein-like substances.The fluorescence intensity of terrestrial humic-like substances（C type）was,on average,decreased,approximately24-54%in reservoir profile waters than in inflowing waters,implying the transformation of terrestrial humic-like substances（C type）by the cumulative effects of both sunlight-induced photo and microbial degradation in the reservoir.Subsequently such fluorescence was further decreased～14-47%in outflowing waters than in reservoir water,which caused by strong physical mixing processes that was generated from dam barrier.Terrestrial humic-like substances（C type）in the inflowing-reservoir-outflowing water transportation was,in average,transformed or degraded,approximately 53-69%in three seasons whilst terrestrial PLS did not identify i.e.entirely degraded in outflowing waters.Other FDOM components were significantly varied seasonally along with variation of water temperature,p H,dissolved oxygen and Chl a.These FDOM changes were also supported by the chromophoric DOM（CDOM）estimated by absorption coefficients(a₃₅₀ and a₂₈₀).These results therefore imply that dam barrier is the key factor to influence on sequential degradation of terrestrial and autochthonous FDOM by photochemical-microbial-physical processes in the reservoir system.Such space-time transformation dynamics of FDOM components from inflowing to outflowing waters of Pingzhai reservoir and Hongfeng reservoir might have substantial significances for better understanding to design future sustainable environmental strategies to mitigate any dam barrier implications.Based on the theoretical results of large lakes and reservoirs,the source,characteristics and transformation mechanism of FDOM in small closed Lakes（ponds）were studied.Compared with the two large reservoirs in Guizhou,no terrestrial humic-like substances（C type）were detected in closed lakes,only the autochthonous FDOM components were detected.In addition,affected by dam construction,the reservoirs show thermal stratification in summer and vertical mixing in winter in the reservoir area.Influenced by the complex hydrological conditions of the water environment system of the reservoir,more kinds of autochthonous protein-like substances were produced than those of closed lakes.In order to further explore the photochemical transformation mechanism of FDOM driven by light,the diurnal change experiment of FDOM in three closed lakes,Jingye lake,Aiwan lake and Qingnian lake,Tianjin University,was carried out.A four-phase model of FDOM transformation was proposed and two basic laws of organic matter transformation were revealed.2.Diurnal sunlight-mediated high-temperature can accelerate diurnal production-degradation transformations of lake fluorescent DOM:new insights into mechanismsProduction of fluorescent DOM（FDOM）from phytoplankton and its subsequent degradation impact many biogeochemical processes and functions in aquatic environments.Hitherto,the effects of diurnal sunlight-mediated high-temperature or global warming（GW）are unknown.Here,we show that various FDOM components in diurnal lake water samples are simultaneously produced and degraded under conditions of diurnal sunlight-and dark-induced processes.Three-dimensional fluorescence（excitation-emission matrix,EEM）spectroscopy coupled with parallel factor（PARAFAC）model was applied to identify FDOM components,which includes extracellular polymeric substances（EPS）,autochthonous humic-like substances（C type）,autochthonous humic-like substances（M type）,newly-released protein-like substances（PLS）,protein-like substances（PLS）,tryptophan-like substances（TLS）,tyrosine-like substances,a combined form of humic-like（C type）and humic-like substances（M type）,a combined form of tyrosine-like and phenylalanine-like substances as well as their degradation products.Interestingly,such production of FDOM components and their subsequent degradations are vastly dependent on sunlight-mediated temperature.The results on July diurnal samples reveal that production of EPS（a very early-stage DOM）from phytoplankton at early morning（6:00-9:00 am）diurnal sub-samples and then its disintegrate into various FDOM components[e.g.autochthonous humic-like（C type）,autochthonous humic-like（M type）,newly-released PLS and TLS]over noon time（10:00-15:00）,then partly alter those four FDOM components on next diurnal timescale（16:00-20:00）,which then remained only two FDOM[autochthonous humic-like（C type）and TLS]along with complete degradation of two FDOM,which finally on last diurnal sub-samples degraded all FDOM and freshly derive EPS from phytoplankton.The fluorescence intensity of all FDOM components further supports such production-degradation scenarios at every diurnal sub-interval timescale.Such complete production-degradation cycle is occurred under conditions of high solar intensity,high water temperature and air temperature,suggesting the critically endangered on rapid FDOM mineralization under the future impacts of the global warming.But such complete mineralization cycle of FDOM do not observe on October diurnal samples,but observe partial production-degradation scenarios which follow relatively low solar intensity along with low WT and AT.Similar partial production-degradation transformation of FDOM is taken place on May and June samples,which are collected only early morning（6:00）and noon（14:00）to represent respective nighttime microbial-and daytime sunlight-induced degradation processes.Diurnal production-degradation transformation of FDOM are simultaneously linked with alterations of nutrients(NH₄⁺,NO₃^-,NO₂^-,PO₄^3-and dissolved Si),dissolved organic carbon and dissolved organic nitrogen.Four model-phases could be developed to ascertain biogeochemical production-degradation transformation mechanisms of FDOM under conditions of GW or high-temperature.Phase 1:Production of FDOM from phytoplankton and their sequential entire degradation in 24-h diurnal scale under high-temperature.Such complete mineralization of FDOM are the outcome of gradual increase in sunlight intensity along with highest WT and AT.It therefore indicates that high solar intensity-mediated high WT and AT could accelerate complete transformation of autochthonous FDOM into LMW DOM and other mineralization end-products on every diurnal 24-h cycle,which could exaggeratedly impact by the influences of the future GW or climate changes.Phase 2:Sequential constant production of FDOM and their subsequent transformation in a diurnal cycle.These results therefore indicate that the production-degradation of FDOM is a constant transformation process that continuously occur at unit timescale in the aquatic environments.Phase 3:Sequential transformation of EPS into a combined form of HLS（C type and M type）.Phase 4:Sequential transformation of EPS into newly-released PLS and then aromatic amino acids via a combined state of TYLS plus PALS.These transformation mechanisms could give us a broader understanding of many biogeochemical processes and functions in the aquatic environments.Based on the four-phase model and reservoirs,we can propose two basic laws on transformation of autochthonous organic matter or primary producers or biota（e.g.phytoplankton）as well as terrestrial humic substances.Production law:Primary producers/biota constantly produce the extracellular polymeric substances that rapidly disintegrate into two individual classes of fluorescent DOM（humic-like substances and proteinous）depending on ambient sunlight-mediated temperature in surface water.Transformation law-1:Autochthonous fluorescent DOM constantly undergo sequential photochemical and microbial degradation processes that convert into mineralization end-products,nutrients and low molecular-weight substances depending on ambient sunlight-mediated temperature.Transformation law-2:Terrestrial humic-like substances（C type）sequentially undergo photo-microbial degradation that did not degrade entirely whilst terrestrial humic-like substances（M type）is completely degraded in surface water.