【作者】 吴凯；

【导师】 戴民汉； 刘占飞；

【作者基本信息】 厦门大学 ， 海洋化学， 2017， 博士

【摘要】 海洋中溶解有机碳(Dissolved Organic Carbon,DOC)的储量巨大,与大气中CO2的碳储量相当。因此,海洋中DOC的储量发生细微的变化都可能影响海洋与大气之间C02的平衡,进而影响气候变化。因此,认识海洋中溶解有机物(DOM)的生产、消耗和输运过程对海洋碳循的研究具有重要意义。近海处于陆海交互带,既受陆地影响,又受大洋调控,其中的碳循环过程受多种物理、生物和化学过程所调控,相当复杂。本论文针对该前沿领域,围绕DOM活性、生产、消耗和运输等一系列关键科学问题,选择美国德克萨斯州南部河流,探讨河流输入河口溶解有机物的生物可利用性;选择南海北部陆架研究物理-生物地球化学共同调控下的DOM生产与运输问题;最后论文选择南海北部海盆区域探究边缘海与大洋界面DOM交换和特征。通过上述案例研究,本论文旨在提升从河流-河口-陆架-大洋这一复杂连续体中DOM的生物地球化学认知,从定性及定量角度探究其生产、活性、迁移转化、运输及其最后的归宿。首先,河流是海洋DOM的重要来源,其的生物可利用性对近海区微生物的新陈代谢过程以及近海生态环境均具有重要影响。本研究首次对美国德克萨斯州南部不同特征河流DOM的生物可利用性及其季节变化进行研究,以探讨影响河流DOM生物可利用性的主要过程。2016年1月(冬季)和7月(夏季),通过培养实验,我们评估了美国德克萨斯州南部5条河流DOM进入河口后的生物可利用性。冬、夏季,德州南部5条河流DOC和DON的浓度范围分别为221.7～778.8μmol/L 和 18.4-40.8 μmol/L,以及 208.5～567.6 μmol/L 和 26.9～44.9 μmol/L。无机营养盐的浓度在受污染河流要远高于较为原始、受污染较小的河流。培养实验结果显示河流DOM生物可利用性呈现季节变化特征,BDOC%和BDON%(Bioavailable DOC/DON)的值(BDOC/N 在 DOC/N 中所占的比例)在冬季(4.5%～11.1%和14.6%～38%)均要高于夏季(0%和8.6%～15%),说明冬季河流DOM的生物活性明显要高于夏季,并且相对于DOC,冬、夏季河流DON均具有更高的生物活性。在受人为活动影响较小的河流,相比DIN,DON是河口微生物更为重要的氮源。结合河流DOM中溶解态氨基酸浓度和组分的分析,我们发现冬、夏季河流中DOM的活性和生物降解程度存在明显不同。冬季河流中的溶解态氨基酸对河流DOM生物活性具有重要贡献。然而,氨基酸的主成分分析显示夏季河流DOM的微生物降解程度要高于冬季,说明夏季微生物的降解过程对河流DOM生物活性的影响。因此,河流DOM的来源和降解程度可能是影响其输入河口区后生物活性的主要过程之一。最后,溶解态氮质量平衡的估算表明,冬季河流BDON主要被微生物转化为DIN,而夏季河流BDON主要被微生物转化为PON,这反映了在夏季河流DOM影响下河口区微生物可能具有更高的生长效率。因此,除了河流DOM生物可利用性的季节变化外,德州南部河流DOM输入对河口区微生物的新陈代谢过程同样具有不同的潜在影响,进而可能会影响河口区碳、氮循环过程。其次,受到河流和上升流等过程输入大量营养盐的影响,陆架区往往是全球海洋生产力较高的区域之一。然而,在复杂的物理-生物地球化学过程共同调控下,如何甄别、定量分析DOC的生产仍是尚未解决的科学问题。基于2008年夏季的航次,本研究对南海北部陆架受珠江冲淡水和沿岸上升流影响的区域进行了调查采样,以了解南海北部陆架区DOC的生产及动态变化特征。DOC的分布特征主要受冲淡水和上升流的共同影响,在冲淡水影响区域主要呈现出高的DOC浓度,而在上升流影响区域主要呈现低的DOC浓度。在航次中我们观测到明显的DOC生物生产,在冲淡水影响区域主要呈现DOC的净生产,而在上升流区域同时出现了 DOC的净生产和净消耗。然而,运用三端元混合模型我们计算得冲淡水区域(7.1±7.0 mmol C m-2d-1)与上升流区域(11.5±6.9 mmol C m-2d-1)具有相近的DOC生产速率。冲淡水和上升流区域DOC的净生产与DIC及DIN的消耗呈现显著的正相关关系,说明了 DOC的生产与净群落生产力之间耦合关系。在冲淡水和上升流区域中,相似比例的净群落生产力(19-27%和24-26%)被转化为净生产的DOC。同样,POC浓度与净群落生产力也存在正相关关系。然而,上升流区域POC的生产要高于冲淡水区域,这可能与上升流区域浮游植物群落中硅藻占主导有关。通过质量平衡估算,冲淡水和上升流区域中,大部分的净群落生产力主要通过POC的沉降输出。最后,边缘海与开阔大洋的交换是全球海洋碳循环中重要的一环,准确估算两者之间的交换通量对于研究全球海洋碳收支平衡具有重要意义。此外,加上两者之间营养盐、DOC浓度及其组分可能存在不同,期间的交换可能会改变DOM的生物可利用性。在2009-2011年,本研究对南海北部陆坡和海盆区在春、秋和冬季进行了大面积的调查采样分析,以了解DOC的分布特征及其季节变化,以及南海通过吕宋海峡与西菲律宾海的DOC交换情况。吴凯(2013)对南海与西菲律宾海的DOC浓度进行了比较,并结合等密度面混合模型定性地分析了黑潮水入侵是影响南海北部海盆区上层DOC分布的主要原因。本论文中,我们进一步地定量地分析了黑潮水入侵的影响及DOC的生物生产量。此外,已知南海与西菲律宾海通过吕宋海峡的水交换呈现“三明治”结构,根据已经报道的水交换通量资料,我们首次估算了吕宋海峡的DOC净交换通量。在上层和深层,西菲律宾海净输入南海的通量分别为-107.1±54.6和-16.4±13.1Tg/yr;在中层,南海净输出的通量为54.7±15.0 Tg/yr。我们看到DOC通量的估算具有较大误差,这主要是由吕宋海峡水交换通量较大时空变异性造成的。然而,考虑到南海生产的DOC较为新鲜,而西菲律宾海主要呈现开阔大洋的特征即DOC的降解程度较高,它们之间的DOC交换对开阔大洋的微生物以及南海的碳循环均具有重要意义。此外,在2014年11月份,我们对南海北部海盆及西菲律宾海区域进行了采样,通过培养实验对黑潮水DOC输入南海后的生物可利用性进行了研究。航次中,我们再次观测到了黑潮水DOC的浓度(～81μmol/L)要显著高于南海(70μmol/L)。实验结果表明,南海细菌降解黑潮水DOC的速率要高于黑潮水细菌,说明黑潮DOC进入南海后,其生物可利用性增加了。添加营养盐后,黑潮水DOC的降解速率得到明显的提升,说明南海水较高的营养盐可能是加快黑潮水DOC降解的因素之一。同时,其它因素的影响有待于进一步的证明,例如微生物群落结构等。综上所述,我们对河流-河口-陆架-开阔大洋这一连续体中DOM的生物可利用性、生产、消耗和输运等过程进行了研究。结合培养实验和DOM组分分析,我们评估了美国德州南部河流DOM进入河口区后的生物可利用性,发现除了DOM的来源,河流中细菌的降解过程也是影响其输入河口区后生物可利用性的主要过程。其次,在物理和生物地球化学过程复杂的南海北部陆架区,利用三端元混合模型,我们定量地估算了该区域中DOC的生产情况。最后,南海北部海盆区通过吕宋海峡与西北太平洋发生水体交换,运用等密度面混合模型并结合历史水交换通量数据,我们首次估算了南海与西菲律宾海的TOC交换通量。此外,通过培养实验,我们发现黑潮水DOM的生物可利用性在输入南海后得到提高,说明了近海区较高的营养盐浓度以及不同的微生物群落将可能提高开阔大洋输入DOM的生物可利用性。更多还原

【Abstract】 Dissolved organic carbon(DOC)is one of the largest carbon pools in the ocean,equivalent to the CO2 stock in the atmosphere.Thus,subtle changes in the marine DOC inventory would potentially affect the balance between oceanic and atmospheric CO2,thereby affecting the global climate change.A better understanding of DOC production,consumption and transport,is vitally important to studying global carbon cycle.Coastal ocean is known to be the region that connects the land and ocean,in which the biogeochemical processes of DOM are very dynamic due to the the high degree of interplay among physical,biological and chemical processes.In this dissertation,we aimed to examine the bioavailability,production,degradation and transport of DOM in the coastal ocean.First,we chose the South Texas rivers,U.S.A.that entered the estuaries of the Gulf of Mexico to investigate the bioavailability of riverine DOM.Second,we chose the northern South China Sea shelf to investigate the dynamics and production of DOM on the continental shelf under the influences of complicated physical and biogeochemical processeses.At last,we chose the northern South China Sea basin to investigate the exchange of DOM between the marginal seas and the open ocean.Through these case studies,we aimed to better understand the biogeochemical processes of DOM along the river-estuary-coastal ocean continuum,including the characterization and quantification of the bioavailability,production,transformation and transport of DOM in the coastal ocean.River input is one of most important sources of DOM in the coastal ocean.Moreover,the bioavailability of riverine DOM is always related to the metabolism of bacterial and is considered to contribute to the hypoxia and eutrophication in the coastal ocean.We examined the bioavailability of DOM in the five South Texas rivers entering the estuaries of the Gulf of Mexico based on sampling in Jan.and Jul.2016.DOC and DON concentrations differed in the rivers,ranging 221.7～778.8 μmol/L and 18.4～40.8 μmol/L,208.5～567.6 μmol/L and 26.9～44.9 μmol/L in winter and summer,respectively.Nutrients concentrations were much higher in the polluted rivers than in the pristine rivers.Our incubation experiments indicated that 4.5～11.1%and 14.6～38%of DOC and DON were bioavailable during winter,both of which were higher than those(0%and 8.6%-15%)in summer,suggesting that DOM in the South Texas rivers contained the relatively high fraction of bioavailable DOM in winter.In contrast to the polluted river(San Antonio river),DON was the dominant input of bioavailable nitrogen in the pristine river(Mission river).Based on the analysis of total dissolved amino acids(TDAA)and its components in the rivers,we revealed that the contribution of TDAA to the bioavailable fraction of riverine DOM during winter,while riverine DOM was diagenetically altered during summer.This is consistent with our incubation results,indicating the influence of diagenetic processes in the rivers on the bioavailability of riverine DOM.The mass balance of dissolved nitrogen suggested that riverine bioavailable DON would be transferred to the forms of DIN and PON by estuarine bacteria.Our results indicated that most of the riverine bioavailable DON was decomposed to DIN during winter,while transferred to PON during summer,suggesting higher growth efficiency of bacteria in summer than in winter.Thus,our study also indicated the potentially different effects of riverine DOM bioavailability in the South Texas rivers on the metabolism of bacteria in the coastal ocean..Owing to abundant nutrients carried by the rivers and upwelling,continental shelf is always one of the most productive regions in the global ocean.However,the quantification of DOM production on the continental shelf remains difficult,due to the high degree interplay among physical,biological and chemical processes.Based on a cruise in summer 2008,we examined the dynamics and production of dissolved organic carbon(DOC)on a large continental shelf in the northern South China Sea,which is largely shaped by a river plume and coastal upwelling.The plume water extended from the mouth of the Pearl River Estuary to the middle shelf and was characterized by high DOC concentrations,while the upwelled water occupying the nearshore area featured low DOC concentrations.Biological production of DOC was observed in both the river plume and the coastal upwelling zones with different behavior between regions.The system appeared to be autotrophic in terms of DOC throughout the plume,while in the upwelling circulation,the metabolism of DOC was mixed trophic.Nevertheless,the integrated net DOC production rate of 11.5±6.9 mmol C m-2d-1 in the upwelling zone was comparable to that in the plume(7.1±7.0 mmol C m-2d-1).The net DOC production correlated strongly with net consumption of dissolved inorganic carbon(DIC)and inorganic nutrients,suggesting that the net DOC production was highly coupled to net community production(NCP)in both the plume and upwelling zones.Both regimes had similar DOC/NCP partitioning,with 19-27%of NCP in the plume and 24-26%of NCP in the upwelling zones converted to DOC.A positive correlation was also found between POC and net DIC consumption,with higher POC production in the upwelling zones where large phytoplankton prevailed.Most NCP removal occurred through POC sinking and/or the diffusion and horizontal transport of DOC.The materials exchange between the marginal seas and the open ocean is very critically important for the constraint of the global carbon budget.More importantly,the open ocean is often characterized by the low nutrients and DOM bioavailability,while the marginal sea is characterized by relatively high nutrients and DOM bioavailability.Thus,such exchanges between the marginal seas and open ocean may potentially alter the DOM biavailability,and then influence the metabolism of bacteria therein.Based on a large and high quality dataset of total organic carbon(TOC,an approximation of dissolved organic carbon)collected from three cruises in spring,fall and winter in 2009-2011,we examined the distribution of TOC and its seasonality in the oligotrophic regime of the northern South China Sea(NSCS)as well as its exchanges with the West Philippine Sea(WPS)in the Northwest Pacific Ocean through the Luzon Strait,the only deep channel linking the South China Sea(SCS)and the Pacific Ocean.Wu Kai(2013)for the first time compared the profiles of TOC between the West Philippine Sea(WPS)and the South China Sea(SCS),and concluded that the surface distribution of TOC in the SCS was mainly influenced by the Kuroshio intrusion.In this dissertation,we further quantified the impact of Kuroshio intrusion on the inventory of TOC and the biological production of TOC in the upper 100 m of the northern SCS.Moreover,following the sandwich structure of water exchange through the Luzon Strait,we conducted a first order estimation of the TOC transport fluxes based on the reported cross strait volume transport.The TOC transport flux was-107.1±54.6,54.7±15.0 and-16.4±13.1 Tg C yr-1 at the upper,intermediate and deep layer,respectively.Note that the positive sign means that the flux was from the SCS to the WPS.The SCS is featured by higher DOC production,the exchange of these fluxes with the open ocean interior where DOC would have experienced more degradation would have important implications for both the microbial community in the ocean interior and overall carbon cycle in the SCS.We examined the bioavailability of Kuroshio-derived DOM in the SCS based on the cruise in Nov.2014.Our data confirmed that TOC concentration in the Kuroshio is significantly higher than that in the SCS.The incubation results indicated that the degradation rate of Kuroshio DOC was faster in the experiment groups innoculated with the SCS bacterial than those innoculated with the Kuroshio bacterial,suggesting that the Kuroshio DOC bioavailability increased in the SCS.Meanwhile,our incubation experiments also found that the biodegradtion of Kuroshio DOC was significantly enhanced under the conditions of high level nutrients,suggesting that relatively high nutrients in the SCS might be one of the factors caucing the quick degradation of Kuroshio DOC.The other factors such as the bacterial community structure are necessary to be considered.Therefore,the enhanced biodegradation of Kuroshio DOC would have many implications for the carbon and nutrients cylces in the SCS.In summary,through the case studies in the South Texas rivers,USA and the northern South China Sea,we aimed to improve our understanding on the biogeochemistry of DOM along the river-estuary-coastal ocean continuum.First,based on the incubation experiments and DOM component analysis,we indicated that the seasonal variation of riverine DOM bioavailability in the South Texas rivers,which was associated with the phytoplankton production and bacterial degradation processes in the rivers.Second,although the distribution of DOC on the northern SCS was mainly modulated by the mixing of water masses sourced from the Pearl river plume,upwelled subsurface water and offshore surface water,we semiquantitatively estimated the biological production of DOC on the shelf using the three end-member mixing model.According to the "sandwich" structure of water exchange between the SCS and the WPS,we for the first time estimated the net exchange flux of DOC between the SCS and the WPS using the historical and published water flux through the Luzon Strait and the isopycnal mixing model.More importantly,based on the incubation experiments,we found that the bioavailability of Kuroshio-derived DOM was increased in the SCS,which may influence the carbon and nutrients cycle in the SCS.更多还原