Distribution Characters And Degradation Processes Of Dissolved Organic Matter In Summer In The Northern South China Sea And Luzon Strait

The production and transformation of dissolved organic matter(DOM)in the ocean play a central role in marine biogeochemical cycling and significantly impact climate change,since the oceanic DOM stock in terms of carbon is comparable to atmospheric carbon dioxide.The majority of oceanic DOM is contained in the deep ocean and resists bacterial degradation.Deep-ocean DOM can be transported by ocean overturning circulation to sunlit surface waters wherein the Chromophoric DOM(CDOM)is subject to photochemical and/or coupled photochemical-bacterial transformations,thereby potentially affecting marine biogeochemical cycles.As the world's largest low-latitude marginal sea,the South China Sea(SCS)is an important geographic area for investigating the interactions between ocean biogeochemical processes and climate change.The objectives of this study are to 1)elucidate the dynamics of DOM in the northern South China Sea(NSCS),2)assess the bacterial,photochemical,and coupled photochemical-bacterial transformations of DOM from the NSCS and its adjacent Philippine Sea(PS),and 3)evaluate the implications of photochemical and coupled photochemical-bacterial transformations of deep DOM from the NSCS and Philippine Sea.The results of the present study will help to well understand the global oceanic DOM cycling.Depth-profile samples were collected from the NSCS,Luzon Straight(LS),and the PS in the summers of 2016 and 2017 and analyzed for obtaining dissolved organic carbon(DOC)concentrations,CDOM absorption spectra,and 3-D fluorescence spectra of fluorescent DOM(FDOM).The DOC concentration in surface water ranged from 60.7 to 102.6?mol L^-1(mean:77.2±8.1?mol L^-1),decreasing from the shelf to the basin and from the PS to the NSCS.DOC concentrations in the intermediate and deep water in the PS were similar to those in the NSCS.The DOC concentration gradually decreased with depth in the upper 1000 m and was relatively constant(43.4±4.6?mol L^-1)below 1000 m.In contrast,the concentration of CDOM,expressed as the absorption coefficient at 330 nm(a _CDOM(330))gradually increased with depth throughout the water column.The concentration of CDOM in the PS was slightly lower than that in the NSCS at each sampling depth.Based on PARAFAC modeling,five distinct fluorescent components were identified in NSCS and PS,including two humic-like components(C_h)and three protein-like components(C_p).The intensity of C_p was higher than that of C_h in the whole water column.The vertical distribution of humic-like components was similar to that of CDOM,while no consistent vertical pattern was found for the protein-like components.Both C_p and C_hwere higher in the NSCS than those in the PS at the corresponding depths.Water samples,collected from different depths in the shelf,and basin areas of the NSCS(Sta.SEATS)and in the PS(Sta.F2),were filtration-sterilized,inoculated with surface-water bacteria,and incubated onboard in dark immediately after collection to assess the depth-dependent bacterial lability of DOM.No significant DOC removal was observed except for surface-water samples(5-m depth)in which 1.0-6.0%of DOC was lost after the incubation.In contrast,both CDOM and FDOM increased significantly in all samples regardless of sampling depths.These results suggest that bacteria could transform a portion of the transparent DOM into CDOM and FDOM.The filtration-sterilized water samples from Sta.SEATS and F2 were also incubated under solar-simulated irradiation for six days to determine the depth-dependence of the photoreactivity of CDOM.The photochemical losses of DOC at Sta.SEATS and F2 were 0.4-5.2%and 0.5-3.5%,respectively,with the lowest loss rates observed in the oxygen-minimum-zone samples(800-m depth).Excluding the value for the 800-m sample,the percent DOC loss at Sta.SEATS was approximately constant in samples collected from depths>500 m,with a mean value of 4.4±0.6%.This value was about twice that for samples from the upper layer(i.e.<500 m)(2.2±0.7%).For Sta.F2,the mean percent DOC loss in samples from depths<1000 m(2.0±0.4%)was higher than that in the deeper samples(0.9-1.6%).The percent photochemical losses of CDOM and humic-like FDOM were higher in deep samples(>1000 m)than in shallower water samples.The percent loss was in the order of C_h>CDOM>DOC for samples from depths>1000 m.No consistent vertical pattern was observed for the photochemical loss of protein-like FDOM.To evaluate the effect of light-exposure history of CDOM on its bacterial transformation,a deep-water sample(3200-m depth)from Sta.F2 was filtration-sterilized,irradiated under solar-simulated radiation for 22 days,and inoculated with surface-water bacteria,and incubated in the dark for 50 days.The bacterial transformation in the irradiated sample led to 10.4%loss in[DOC]but169.4%,292.0%,and 255.3%increases in a _CDOM(330),C_h and C_p,respectively.These values are far higher than those obtained from the control incubation with the unirradiated water.Prior-irradiation thus enhanced bacterial DOC removal but boosted CDOM production.These results imply that coupled photochemical-bacterial processes can not only remove part of the bio-refractory deep DOM but also regenerate part of it during ocean overturning circulation.