Distribution Characters,Biodegradation Processes And Potential As Water Mass Tracers Of Dissolved Organic Matter In The Kuroshio-oyashio Confluence Region Of The Northwest Pacific Ocean

Dissolved organic matter（DOM）is one of the largest active carbon pools on Earth,and marine DOM is transformed by biogeochemical processes such as marine autogenesis,terrestrial input,air-sea exchange,and particulate organic matter degradation.It is derived from autochthonous marine product,allochthonous terrestrial input and conversion of particulate organic matter.Thus,the concentration,composition,and distribution of marine DOM are closely related to biogeochemical processes and environmental factors.Marine DOM,like terrestrial DOM,is complex mixture comprising thousands of compounds,making it difficult for chemical characterization at the molecular level.Optical spectral methods such as ultraviolet-visible（UV-Vis）absorbance and fluorescence spectral analysis have been increasingly used for investigation of Chromophoric dissolved organic carbon（CDOM）and its subfraction,fluorescent DOM（FDOM）.The composition,molecular structure and reactivity of DOM are the main controlling factors affecting the biogeochemical processes.Compared to offshore-estuarine areas,the pelagic ocean is a less disturbed area by human activities.Changes in environmental parameters（e.g.,temperature,salinity,etc.）will directly lead to the dynamic change processes of DOM.Therefore,the understanding of spatial and temporal changes based on surface-deep,warm-cold,and other spatial and temporal changes in the ocean will help us to systematically study the global carbon cycle.The northwestern（NW）Pacific is a special oceanic region where carbon cycle and biogeochemical processes are greatly influenced by the Kuroshio and Oyashio currents.This study measured concentrations and compositions of DOM in waters（0-1000 m）from the K/O region.Our goal is twofold:（1）The parameters of DOM,CDOM,and FDOM in this region and the spatial distribution characteristics for tracing the water masses;2)The influence of environmental parameters on interannual DOM and spectral parameters;3)Control of the effect of temperature on microbial availability of DOM.This study examined the concentration,composition,and distribution of DOM in waters（depth 0-1000 m）from the Kuroshio-Oyashio confluence region（NW Pacific）in2019.Dissolved organic carbon（DOC）concentration varied between 0.37 mg L^-1 and0.94 mg L^-1 with a higher level in surface waters.Two humic-like components（C1,C3）and one protein-like component（C2）were identified by fluorescence excitation emission matrices（EEMs）-parallel factor analysis（PARAFAC）.The C1 and C3 intensity generally increased with depth,and positively correlated with apparent oxygen utilization（AOU）（p<0.01）,whereas the C2 intensity decreased downward,and negatively correlated with AOU（p<0.01）.These characters suggested in-situ production of bio-refractory C1 and C3 during microbial transformation of labile DOM（like C2）.Large amplitude variation in optical parameters is attributed to the transport and mixing of different water masses.The application of three-endmember mixing model based on the C1 intensity and salinity revealed variable contributions of Kuroshio,Oyashio,and North Pacific Intermediate Waters that is consistent with the predication from water temperature.Our study suggests that the optical property of DOM has a potential to distinguish different water masses in the Kuroshio-Oyashio confluence water region,and provides exact water cluster end element values for in situ fluorescence detection in the Kuroshio-Oyashio confluence region.This study analyzed the distributional characteristics of DOC,CDOM,and FDOM in water samples from three cruises.There is a weak interannual variability in the spectroscopic parameters SUVA₂₅₄ and the fluorescence-like humic components（C1 and C3）.This indicates that the magnitude of interannual variability of water masses is relatively stable in this region.For the water samples from the 2020 cruise,surface waters that are generally dominated by Kuroshio water mass,has significantly higher SUVA254(0.5±0.2 L mg C^-1 m^-1)and fluorescence intensity of C1(8.2±2.69*10^-3 R.U.)and C3(9.22±4.39*10^-3 R.U.).The controlling factors for interannual variation were temperature and water depth.With increasing temperature,the intensity of the protein-like component C2 increased（r=0.33,p<0.01）,whereas the intensity of humic-like component C1 and C3 decreased（r=-0.55,-0.72;p<0.01）.The relatively higher intensity of fluorescence in waters from the 2019 cruise may be related to higher temperature that promotes the production of more labile DOM in the surface layer.Calculation of inter-annual trends in water mass variability between three cruises based on the three-endmember mixing model using C1 intensity and salinity.In the 2019 cruise,the contribution of Kuroshio Water（KW）was 57.70±16.80%,while the contribution of Oyashio Water（OW）and North Pacific Intermediate Water（NPIW）were 20.04±9.72%.For the 2020 cruise,the KW contribution was 60.8±18.2%,the OW contribution was18.2±9.5%and the NPIW contribution was 21.0±12.2%.The statistical analysis suggested no significant variability in different years.These results suggest that in the open ocean,water mass mixing is an important controlling factor influencing the dynamics of water mass parameters.Temperature incubated experiments were carried out on water samples,collected from different depths in this region.The DOC in surface seawater and deep seawater undergoes a significant decreasing trend in the initial stage.According to the model calculations,the surface seawater degradation constant has a maximum value(15°C,0.72±0.10 d^-1)and the surface water system also has the largest DOC_R（25°C,58.15%）.The temperature at which the maximum constant for degradation was 19.55°C in the surface layer and the deep layer was 22.41°C;suggesting that warming has a relatively large effect on deep organic matter.The fluctuation with the parameters of CDOM in the incubation period,and it is possible that the degradation is a dynamic process accompanied by the production and consumption of CDOM.The active substances and degradable molecules are preferentially utilized at the initial stage.The accumulation of highly aromatic substances and decreased in molecular weight.At the later stage of incubation microorganisms utilize more small molecules and highly aromatic substances.Two protein-like components（C1,C3）and one humic-like component（C2）were identified by fluorescence excitation emission matrices（EEMs）-parallel factor analysis（PARAFAC）.The intensity of fluorescence showed a certain increase during the incubation for all components.The bioactivity index（BIX）and the humification index（HIX）,have obvious temperature gradient effects.The above results indicate that a suitable increasing temperature gradient has a significant promotion effect on biological activity and DOC degradation.