Characterization of chromophoric dissolved organic matter (CDOM) in the Baltic Sea by excitation emission matrix fluorescence spectroscopy

Chromophoric dissolved organic matter (CDOM) is the major light absorber in the Baltic Sea. In this study, excitation emission matrix (EEM) fluorescence spectra and UV–visible absorption spectra of CDOM are reported as a function of salinity. Samples from different locations and over different seasons were collected during four cruises in 2002 and 2003 in the Baltic Sea in both Pomeranian Bay and the Gulf of Gdansk. Absorption by CDOM decreased with increased distance from the riverine source and reached a relatively stable absorption background in the open sea. Regression analysis showed that fluorescence intensity was linearly related to absorption by CDOM at 375 nm and a_CDOM(375) absorption coefficients were inversely related to salinity. Analysis of CDOM-EEM spectra indicated that a change in composition of CDOM occurred along the salinity gradient in the Baltic Sea. Analysis of percent contribution of respective fluorophore groups to the total intensity of EEM spectra indicated that the fluorescence peaks associated with terrestrial humic components of the CDOM and total integrated fluorescence decreased with decreasing CDOM absorption. In contrast, the protein-like fraction of CDOM decreased to a lesser degree than the others. Analysis of the percent contribution of fluorescence peak intensities to the total fluorescence along the salinity gradient showed that the contribution of protein-like fluorophores increased from 2.6% to 5.1% in the high-salinity region of the transect. Fluorescence and absorption changes observed in the Baltic Sea were similar to those observed in similar transects that have been sampled elsewhere, e.g. in European estuaries, Gulf of Mexico, Mid-Atlantic Bight and the Cape Fear River plume in the South Atlantic Bight, although the changes in the Baltic Sea occurred over a much smaller salinity gradient.     

厦门湾有色溶解有机物的光吸收特性研究

研究了厦门湾九龙江河口区、西海域、同安湾及东侧水道海水中有色溶解有机物(CDOM)的光吸收特性,分析了CDOM的河口行为,并讨论了CDOM光吸收特性与其荧光性质之间的关系。结果表明,厦门湾表层海水CDOM光吸收系数a(355)的水平分布表现为河口区最高、东侧水道最低、西海域和同安湾介于两者之间,底层水a(355)的分布与表层基本相似,表明陆源河流输入是厦门湾CDOM的主要来源;a(355)的垂直分布为表层高于底层,主要受水文和生物因素控制。厦门湾表层水CDOM光谱斜率S的平均值介于0.014—0.018nm-1,但河口低盐度区S值较小,反映陆源腐殖质的影响。a(355)在河口混合中呈保守行为,表明CDOM具有良好的保守性质。CDOM的吸收系数a(355)与其荧光强度之间表现为较好的相关关系,指示可以用灵敏度更高的荧光方法来研究CDOM的分布和行为。     

黄河入海径流对河口海域盐度影响研究

为研究黄河入海径流变化条件下河口附近海域盐度扩散特征,以更好地保护河口海域生物资源多样性,本文以黄河下游利津水文站的长序列实测径流数据资料为基础,利用近海水动力模型FVCOM,分析径流变化对黄河口海域盐度的影响规律。结果表明：黄河口与莱州湾之间存在顺时针的环流系统,在余流作用以及涨落潮方向的影响下,黄河冲淡水长期向莱州湾扩散;丰水期黄河冲淡水几乎影响了整个莱州湾,27盐度锋可以到达莱州湾中部,27等盐线的表层包络面积为2 665.61 km²,占莱州湾的1/4左右,枯水期低盐度水只有向南扩散的趋势,27以下的低盐度水集中分布在黄河口门附近,27等盐线的表层包络面积只有199.65 km²;5月份,随着入海径流量增加,27等盐线扩散的范围、距离、方向都会发生明显变化。在对近海生物资源有迫切保护需求的情景下,适当减少其他用水户供水量以增加入海生态径流量,可以有效改善黄河口海域附近的盐度情况,为生物资源的生长繁殖创造良好条件。     

Characterization of dissolved organic matter fluorescence in the South Atlantic Bight with use of PARAFAC model: Interannual variability

Systematic water sampling for characterization of chromophoric dissolved organic matter (CDOM) in the coastal South Atlantic Bight, was conducted as part of the long term Coastal Ocean Research and Monitoring Program (CORMP). Water samples were collected during a 3.5 year period, from October 2001 until March 2005, in the vicinity of the Cape Fear River (CFR) outlet and in adjacent Onslow Bay (OB). During this study there were two divergent hydrological and meteorological conditions in the CFR drainage area: a severe drought in 2002, followed by the very wet year of 2003. CDOM was characterized optically by the absorption coefficient at 350 nm, the spectral slope coefficient (S), and by Excitation Emission Matrix (EEM) fluorescence. Parallel Factor Analysis (PARAFAC) was used to assess CDOM composition from EEM spectra and six components were identified: three terrestrial humic-like components, one marine humic-like component and two protein-like components. Terrestrial humic-like components contributed most to dissolved organic matter (DOM) fluorescence in the low salinity plume of the CFR. The contribution of terrestrial humic-like components to DOM fluorescence in OB was much smaller than in the CFR plume area. Protein-like components contributed significantly to DOM fluorescence in the coastal ocean of OB and they dominated DOM fluorescence in the Gulf Stream waters. Hydrological conditions during the observation period significantly impacted both concentration and composition of CDOM found in the estuary and coastal ocean. In the CFR plume, there was an order of magnitude difference in CDOM absorption and fluorescence intensity between samples collected during the drought compared to the wet period. During the drought, CDOM in the CFR plume was composed of equal proportions of terrestrial humic-like components (ca. 60% of the total fluorescence intensity) with a significant contribution of proteinaceous substances (ca. 20% of the total fluorescence). During high river flow, CDOM was composed mostly of humic substances (nearly 75% of total fluorescence) with minor contributions by proteinaceous substances. The impact of changes in fresh water discharge patterns on CDOM concentration and composition was also observed in OB, though to a lesser degree.     

舟山海域陆源溶解有机质变化及其对近岸羽状流的响应

2007年夏季在东海舟山海域河口锋区开展了陆源溶解有机质的调查研究。测定了有色溶解有机质(CDOM)在激发波长370 nm/发射波长460 nm处的荧光强度和在λ=355 nm处的吸收系数,用于代表陆源CDOM浓度,并测定了荧光指数以指示CDOM来源。结果表明,CDOM的荧光值和紫外吸收系数之间呈显著正相关性,陆源CDOM浓度大体有向海方向降低的趋势,但是纵向上存在一些"突跃"现象。在舟山海域东北角不时观测到表层水体含有高浓度的CDOM,但变异性很大,推测可能该海区受到长江口羽状流的影响。在连续观测站发现陆源CDOM浓度在低平潮时往往比高平潮时要高。河海水在混合过程中CDOM浓度与盐度呈显著的线性负相关关系。在低盐度的悬沙锋区(S<24)CDOM浓度明显低于理论稀释值,而在较高盐度的羽状锋区,CDOM浓度接近于理论稀释值。在盐度为24~31范围内,大部分水样的荧光指数在1.50上下波动,表明其中CDOM来源以陆地来源为主;在较低盐度(S<24)的水样中荧光指数在1.70至1.90以上,表明CDOM以海洋来源为主,这与其陆源组分在高浊度的低盐度区存在显著的去除过程有关。研究表明,舟山海域水质存在着显著的变异性,与近岸羽状流密切相关,陆源溶解有机质的分布特征对此有较好的响应。     

Spatial and temporal distribution of chromophoric dissolved organic matter (CDOM) fluorescence and its contribution to light attenuation in UK waterbodies

Vertical attenuation of light through the water column (K_d) is attributable to the optically active components of phytoplankton, suspended particulate material (SPM) and chromophoric dissolved organic matter (CDOM). Of these, CDOM is not routinely monitored and was the main focus of this study. Concentrations and spatio-temporal patterns of CDOM fluorescence were investigated between August 2004 and March 2006, to quantify the correlation coefficient between CDOM and salinity and to better characterise the contribution of CDOM to K_d. Sampling was conducted at a broad range of UK and Republic of Ireland locations; these included more than 15 estuaries, 30 coastal and 70 offshore sites in the southern North Sea, Irish Sea, Liverpool Bay, Western Approaches and the English Channel.An instrument package was used; a logger with multi-sensor array was deployed vertically through the water column and concurrent water samples were taken to determine salinity, CDOM fluorescence and SPM. Surface CDOM fluorescence values ranged between 0.05 and 16.80 S.Fl.U. (standardised fluorescence units). A strong, negative correlation coefficient of CDOM to salinity (r² = 0.81) was found. CDOM absorption (aCDOM_λ) was derived from fluorescence measurements and was in the range 0.02–2.2 m⁻¹ with mean 0.15 m⁻¹. These results were comparable with direct measurements of aCDOM_λ in the same geographic regions, as published by other workers.Spatial differences in CDOM fluorescence were generally explicable by variation in salinity, in local conditions or catchment areas; e.g. CDOM at the freshwater end was 3.54–11.30 S.Fl.U., reflecting the variety of rivers sampled and their different catchments. Temporal changes in CDOM fluorescence were related to salinity. A significant and positive correlation was found between CDOM and K_d, and although CDOM was found to be less influential than SPM on K_d, it was still of significance particularly in coastal and offshore waters of lower turbidity.     

珠江口磨刀门溶解有机物CDOM 三维荧光光谱特征

采用三维荧光对珠江口磨刀门夏秋季有色溶解有机物(CDOM)时空变化进行研究,分析其组成及荧光强度。结果表明, CDOM 三维荧光峰谱包括 UV 类腐殖质 A、陆源 Vis 类腐殖质 C 和海源 Vis类腐殖质M,以及类蛋白质T。在入海过程中,其组成未发生变化,但其荧光强度随盐度增加逐渐减小,反映了CDOM主要来源是陆源,并且主要受海水物理稀释控制,是一种典型的保守混合行为。在定点站位涨落潮周期中, CDOM的荧光强度不仅受到海水稀释的作用,表层水体CDOM受到紫外线的光降解作用,同时中层水体CDOM受到浮游植物的影响,反映了盐度、紫外线强度、生物活动对CDOM具有的共同影响。     

西太平洋冬季上层水体有色溶解有机物的分布和转化特征

为深入解析西太平洋溶解有机碳的生物地球化学过程,本研究于2015年12月至2016年1月,开展了西太平洋上层水体有色溶解有机物（CDOM）吸收光谱和荧光光谱特征研究。研究结果表明,西太平洋上层水体CDOM吸收系数a（320）变化范围为0.01~1.07 m-1,平均值为0.18 m-1;其较高值位于100~200 m水层,表层的海水相对含量较低,主要以有机物的光化学分解为主。采用PARAFAC分析CDOM三维荧光光谱特征,得到1种类腐殖质组分C2（252（310 nm）/405 nm）及2种类蛋白组分C1（224（276 nm）/335 nm）和C3（224（260 nm）/300 nm）,其中类腐殖质荧光组分占总荧光强度的11%~22%,蛋白质荧光组分占总荧光强度的78%~89%,蛋白质荧光中类色氨酸和类络氨酸组分对荧光强度的贡献相当。洋流在大尺度上控制西太平洋CDOM的分布特征,两流交界处和环流形成区域的CDOM相对含量较高,荧光信号较强。西太上层水体CDOM相对含量和荧光信息,与温度、盐度、DO和营养盐等理化因素之间的相关分析结果表明,CDOM主要成分类蛋白质的产生主要受上层水体初级生产过程控制。     

Chromophoric dissolved organic matter and dissolved organic carbon in Chesapeake Bay

Chromophoric dissolved organic matter (CDOM) is the light absorbing fraction of dissolved organic carbon (DOC). The optical properties of CDOM potentially permit remote sensing of DOC and CDOM, and correction for CDOM absorption is essential for remote sensing of chlorophyll a (chl a) in coastal and estuarine waters. To provide data for this purpose, we report the distributions of CDOM, DOC, and chl a from seven cruises in Chesapeake Bay in 1994–1997. We observed non-conservative distributions of chl a and DOC in half of the cruises, indicating net accumulations within the estuary; however, there were no net accumulations or losses of CDOM, measured as absorption at 355 nm or as fluorescence. Freshwater end member CDOM absorption varied from 2.2 to 4.1 m⁻¹. Coastal end member CDOM absorption was considerably lower, ranging over 0.4–1.1 m⁻¹. The fluorescence/absorption ratio was similar to those reported elsewhere for estuarine and coastal waters; however, in the lower salinity/high CDOM region of the Bay, the relationship was not constant, suggestive of the mixing of two or more CDOM sources. Chl a was not correlated with the absorption for most of the cruises nor for the data set as a whole; however, CDOM and DOC were significantly correlated, with two groups evident in the data. The first group had high CDOM concentrations per unit DOC and corresponded to the conservative DOC values observed in the transects. The second group had lower CDOM concentrations per unit DOC and corresponded to the non-conservative DOC values associated with net DOC accumulation near the chl a maximum on the salinity gradient. This indicates the production of non-chromophoric DOC in the region of the chl a maximum of Chesapeake Bay. In terms of remote sensing, these data show that (1) the retrieval of the absorption coefficient of CDOM from fluorescence measurements in the Bay must consider the variability of the fluorescence/absorption relationship, and (2) estimates of DOC acquired from CDOM absorption will underestimate DOC in regions with recent, net accumulations of DOC.     

闽江冲淡水扩展范围的季节变化特征

利用2006～2007年春、夏、冬季3个航次的CTD资料分析闽江冲淡水扩展范围季节变化特征.通过分析闽江口及邻近海域温度、盐度的平面、垂直分布特征,结果表明:闽江冲淡水的扩展范围具有明显的季节变化.春季由于闽江口附近受到闽浙沿岸流的影响,冲淡水的扩展范围难以确定,但从盐度分布情况不难看到闽江口表层被盐度低于25的具有明显冲淡水特征的水体覆盖.夏季闽江口被温度高于27℃、盐度低于25的冲淡水覆盖,冲淡水前沿向东北扩展至罗源湾、三沙湾.冬季闽江径流减弱,闽江口主要受到闽浙沿岸水的影响,基本看不到闽江冲淡水的痕迹.春季、夏季闽江冲淡水明显的影响闽江口时,闽江口附近海域被25盐度等值线包围,25盐度等值线可作为闽江口明显受冲淡水影响的指征.     

长江口及其邻近海区环流和温、盐结构动力学研究Ⅱ.环流的基本特征

利用POM模型,以研究海区的海面风应力、温度和盐度资料作为海面边界条件,以与外海界面处的温度和盐度资料作为侧向液边界条件,并考虑长江径流、台湾暖流和东海沿岸流的影响,对长江口及其邻近海区各季节的三维斜压环流和温、盐结构进行了数值模拟。环流的数值结果表明,冬季和秋季研究海区的水平环流主要由长江径流、东海沿岸流、台湾暖流、杭州湾环流和沿岸流与台湾暖流之间的气旋和反气旋涡构成;东海沿岸流与长江径流顺岸南下,随着自北往南岸界地形坡度的增大,其流幅变窄,流速增强;台湾暖流沿陡坡及其外缘蜿蜒北上,随着自南往北水深的变浅,其流幅由宽变窄继而又由窄变宽,流速却一直由强变弱。冬季和秋季海区纬度断面垂直环流的总趋势由近岸向外海流动,海底地形变化缓慢区离岸流产生波动,海底地形变化显著的陡坡区离岸流产生剧烈振荡而生成强升降流。春季和夏季研究海区的水平环流主要由长江径流与东海沿岸流汇合流、台湾暖流、杭州湾环流、舟山群岛附近及长江径流和东海沿岸流汇合流与台湾暖流之间的气旋和反气旋涡构成;长江径流和台湾暖流平行北上并在长江口以北产生顺时针偏转。由海区水平环流特征和变化趋势证实,春季长江冲淡水已开始向东北偏转,夏季冲淡水的偏转程度、伸展距离和扩展范围都更甚于春季;春季在长江口近岸存在弱上升流,夏季长江口外的陡坡区出现下降流,而长江口以北和以南的陡坡区出现上升流。     

胶州湾海水中一氧化碳光致生成影响因素的研究

海水中的一氧化碳(CO)主要由溶解有色有机物(CDOM)光降解产生,且CO的光致生成量受到环境因素的影响。采集了胶州湾及其河口表层水样,通过实验室模拟实验开展了不同的环境条件(辐射强度、光照时间、温度、pH和盐度)以及水体中CDOM的来源对CO光致生成的影响研究。结果表明胶州湾海水中CO的光致生成速率随辐射强度的增强、水温的升高和水体pH的增大而增大;随着光照时间的延长、水体盐度的增大而逐渐减小;不同来源的CDOM对水体中CO的光致生成速率产生了不同的影响。     

胶州湾海水中一氧化碳光致生成影响因素的研究

海水中的一氧化碳(CO)主要由溶解有色有机物(CDOM)光降解产生,且CO的光致生成量受到环境因素的影响。采集了胶州湾及其河口表层水样,通过实验室模拟实验开展了不同的环境条件(辐射强度、光照时间、温度、pH和盐度)以及水体中CDOM的来源对CO光致生成的影响研究。结果表明胶州湾海水中CO的光致生成速率随辐射强度的增强、水温的升高和水体pH的增大而增大;随着光照时间的延长、水体盐度的增大而逐渐减小;不同来源的CDOM对水体中CO的光致生成速率产生了不同的影响。     

珠江口海域a_g(440)遥感模式研究及应用

有色可溶性有机物质(Coloured Dissolved Organic Matter,CDOM)直接影响着海水的光学性质,为水体有机污染物含量遥感反演的基础参数之一。在水色遥感研究领域,一般用440nm波长位置的吸收系数ag(440)来表示其浓度,因而建立ag(440)遥感反演模式,对于掌握相关海域CDOM浓度的空间变化规律及进一步提取其他水环境参数具有重要的作用。利用2013年11月和2014年2月两个航次在珠江口海域现场采集的表观光学量及固有光学量数据,建立了基于HJ-1/CCD卫星传感器的ag(440)遥感反演模型,并应用于珠江口海域,得到该区域2012年1月至2014年6月晴天CDOM浓度空间动态分布图。研究结果表明:(1)现场测定的珠江口海域ag(440)在0.1~0.3m-1,且不同断面ag(440)呈现出一定的规律性变化;(2)利用现场实测数据对遥感模式估算值进行验证,计算出估算值的相对误差为9%,表明所建立模式具有较高的准确率;(3)遥感反演的CDOM空间分布数据与实测数据得到的分布特征基本吻合,整个珠江口及其邻近海域ag(440)的数值范围为0.07~0.31m-1,而且珠江口西部海域ag(440)高于中部和东部海域。     

~(234)U/~(238) as a potential tracer for tracking water masses mixing in the northern East China Sea

The optimum multiparameter(OMP) method was often used to determine the percentages of water masses based on temperature, salinity and other parameters, like nutrient or dissolved oxygen(DO). There are a number of water masses in the East China Sea(ECS), a marginal sea of the western Pacific Ocean. However, it is difficult to clarify the proportion of water masses using traditional parameters, such as temperature, salinity, nutrient or DO because of the occurring of intensive biogeochemical processes in the near shore and shelf areas. Here, we reported the use of ~(234)U/~(238)Uactivity ratio embedded in the OMP method. The results indicate that seawater in the northern ECS mainly consisted of the estuarine water of Changjiang River(CEW), Kuroshio water(KW), and Yellow Sea Coastal Current(YSCC). In March 2017, the CEW only influenced the offshore waters shallower than30 m; the KW affected the east edge and the YSCC contributed more than 75% in the northern ECS.     

Spatial and Temporal Distribution of Coloured Dissolved Organic Matter (CDOM) in Narragansett Bay,Rhode Island: Implications for Phytoplankton in Coastal Waters

One indicator of health in estuarine and coastal ecosystems is the ability of local waters to transmit sunlight to planktonic, macrophytic, and other submerged vegetation for photosynthesis. The concentration of coloured dissolved organic matter (CDOM) is a primary factor affecting the absorption of incident sunlight in coastal and estuarine waters. In estuaries, CDOM concentrations vary due to changes in salinity gradients, inflows of industrial and domestic effluents, and the production of new dissolved organic matter from marine biologic activity. CDOM absorption data have been collected from a variety of waters. However, there are a limited number of measurements along the US east coast and a general lack of data from New England waters.This study characterized the temporal and spatial variability of CDOM absorption over an annual cycle in Narragansett Bay and Block Island Sound (Rhode Island). Results suggested that, in Narragansett Bay, the magnitude of CDOM absorption is related to the seasonal variability of freshwater input from surrounding watersheds and new CDOM production from in situ biologic activity. The data show that the average CDOM absorption coefficient at 412 nm was 0·45 m⁻¹ and the average spectral slope was 0·020 nm⁻¹.     

Photobleaching of chromophoric dissolved organic matter in natural waters: kinetics and modeling

The effects of monochromatic and polychromatic UV and visible (VIS) radiation on the optical properties (absorption and fluorescence) of chromophoric dissolved organic matter (CDOM) were examined for a Suwannee River fulvic acid (SRFA) standard and for water from the Delaware and Chesapeake Bays. The primary (direct) loss of absorption and fluorescence occurred at the irradiation wavelength(s), with smaller secondary (indirect) losses occurring outside the irradiation wavelength(s). The efficiency of both direct and indirect photobleaching decreased monotonically with increasing wavelength. Exposure to polychromatic light increased the CDOM absorption spectral slope (S), consistent with previous field measurements. An analysis of the monochromatic photobleaching kinetics argues that a model based on a simple superposition of multiple chromophores undergoing independent photobleaching cannot apply; this conclusion further implies that the absorption spectrum of CDOM cannot arise solely from a simple superposition of the spectra of numerous independent chromophores. The kinetics of CDOM absorption loss with the monochromatic irradiation were employed to create a simple, heuristic model of photobleaching. This model allowed us to examine the importance of the indirect photobleaching losses in determining the overall photobleaching rates as well as to model the photobleaching of natural waters under polychromatic light fields. Application of this model to natural waters closely predicted the change in the CDOM spectral shape caused by photodegradation. The time scale of this process was consistent with field observations acquired during the summertime for coastal waters in the Middle Atlantic Bight (MAB). The results indicate that the ratio of the photodegradation depth to the mixed layer depth is a key parameter controlling the rate of the photobleaching in surface waters.     

长江口及邻近海域有色溶解有机物的分布及河口混合行为的季节变化研究

于2019年3月、7月和10月对长江口及邻近海域有色溶解有机物(CDOM)的分布及河口混合行为进行分析研究。通过对盐度、吸收光谱斜率S_275～295、吸收系数a_CDOM(355)以及叶绿素a的分析发现,在河口内低盐度区,7月淡水流量大,陆源输入量最大,a_CDOM(355)值最高,3月CDOM来源主要受陆源输入和浮游植物生产活动的影响,a_CDOM(355)值较10月高;在口外高盐度区,3月和7月的a_CDOM(355)值相近,均低于10月,CDOM分布主要受浮游植物生产活动的影响。利用三维荧光光谱?平行因子分析方法共鉴定出4个荧光组分:类蛋白质组分C1(280/330 nm)、类腐殖质组分C2(300/350 nm)、类腐殖质组分C3(260/465 nm)和类腐殖质组分C4(320/410 nm)。在3月、7月及10月,4个荧光组分强度由长江口内到口外呈递减趋势,受陆源输入和浮游植物生产活动的影响,平均荧光强度的季节变化总体上来说,由大到小依次为7月、10月、3月。3个季节CDOM荧光组分均存在偏离理论稀释线的现象,说明CDOM的来源(陆源输入、沉积物再悬浮和现场生物活动)和去除(被颗粒物吸附、光降解和细菌降解)机制复杂多变,揭示了长江口区域CDOM在不同时空下的不保守混合行为。     

Isotope parameters (δD, δ<Superscript>18</Superscript>O) and sources of freshwater input to Kara Sea

The isotope characteristics (δD, δ¹⁸О) of Kara Sea water were studied for quantitative estimation of freshwater runoff at stations located along transect from Yamal Peninsula to Blagopoluchiya Bay (Novaya Zemlya). Freshwater samples were studied for glaciers (Rose, Serp i Molot) and for Yenisei and Ob estuaries. As a whole, δD and δ¹⁸O are higher in glaciers than in river waters. isotope composition of estuarial water from Ob River is δD =–131.4 and δ18O =–17.6‰. Estuarial waters of Yenisei River are characterized by compositions close to those of Ob River (–134.4 and–17.7‰), as well as by isotopically “heavier” compositions (–120.7 and–15.8‰). Waters from studied section of Kara Sea can be product of mixing of freshwater (δD =–119.4, δ¹⁸O =–15.5) and seawater (S = 34.9, δD = +1.56, δ¹⁸O = +0.25) with a composition close to that of Barents Sea water. isotope parameters of water vary significantly with salinity in surface layer, and Kara Sea waters are desalinated along entire studied transect due to river runoff. concentration of freshwater is 5–10% in main part of water column, and <5% at a depth of >100 m. maximum contribution of freshwater (>65%) was recorded in surface layer of central part of sea.     

Optical properties of estuarine dissolved organic matter isolated using cross-flow ultrafiltration

Dissolved organic matter(DOM) from freshwater, mid-salinity, and seawater endmember samples in the Jiulong River Estuary, China were fractionated using cross-flow ultrafiltration with a 10-kDa membrane. The colloidal organic matter(COM; 10 kDa–0.22 μm) retentate, low molecular weight(LMW) DOM(10 kDa) permeate, and bulk samples were analyzed using absorption spectroscopy and three-dimensional fluorescence excitation-emission-matrix spectroscopy. The UV-visible spectra of COM were very similar to those obtained for permeate and bulk samples, decreasing monotonically with increasing wavelength. Most of the chromophoric DOM(CDOM, expressed as the absorption coefficient a355) occurred in the LMW fraction, while the percentage of CDOM in the colloidal fraction was substantially higher in the freshwater endmember(13.4% of the total) than in the seawater endmember(6.8%). The bulk CDOM showed a conservative mixing behavior in the estuary, while there was removal of the COM fraction and a concurrent addition of the permeate fraction in the mid-salinity sample, implying that part of the colloidal CDOM was transformed into LMW CDOM. Two humic-like components(C1: 250, 325/402 nm; and C2: 265, 360/458 nm) and one protein-like component(C3: 275/334 nm) were identified using parallel factor analysis. The contributions of the C1, C2, and C3 components of the COM fraction to the bulk sample were 2.5%–8.7%, 4.8%–12.6%, and 7.4%–14.7%, respectively, revealing that fluorescent DOM occurred mainly in the LMW fraction in the Jiulong River Estuary. The C1 and C2 components in the retentate and permeate samples showed conservative mixing behavior, but the intensity ratio of C2/C1 was higher in the retentate than in the permeate fractions for all salinity samples, showing that the humic component was more enriched in the COM than the fulvic component. The intensity ratio of C3/(C1+C2) was much higher in the retentate than in the permeate fraction for mid-salinity and seawater samples, revealing that the protein-like component was relatively more enriched in COM than the humic-like component. The contribution of the protein-like component(C3) to the total fluorescence in the retentate increased from 14% in the freshwater endmember to 72% for the seawater endmember samples, clearly indicating the variation of dominance by the humic-like component compared to the protein-like component during the estuarine mixing process of COM.