Spectroscopic characterization and molecular weight distribution of dissolved organic matter in sediment porewaters from Lake Erhai, Southwest China

Dissolved organic matter (DOM) in sediment porewaters from Lake Erhai, Southwest China was investigated using dissolved organic carbon (DOC) concentration, UV absorbance, fluorescence and molecular weight distribution. DOC exhibited a high concentration at the sediment–water interface with a rapid decrease to the oxic–anoxic interface at approximately 7 cm, and then increased with depth. Similar trends were also found for the UV absorption coefficients at 254 and 280 nm in the porewaters. DNA in the sediment was also measured, which confirmed the high abundance of aerobic bacteria in the upper layer of the sediment. Both humic-like (peaks A and C) and protein-like (peaks B and D) fluorescence were observed in the porewater DOM, and their fluorescence intensities exhibited a similar porewater profile as DOC concentration. A strong correlation was found between the peak fluorescence intensity ratio r(A, C) and r(D, B). Both the fluorescence index and UV absorption coefficient at 254 nm suggested a dramatic increase in aromaticity of porewater DOM across the oxic–anoxic interface. Porewater DOM exhibited a multimodal distribution of molecular weight with a relatively low polydispersity. The results of this study offer significant insight into the nature and properties of DOM in freshwater ecosystems.     

白洋淀沉水植物腐解释放溶解性有机物光谱特性

利用水生植物修复受污染水体中,水生植物在秋冬季节腐烂分解会释放大量溶解性有机物(DOM),DOM可影响水环境中污染物的迁移转化,对水体中的化学和生物过程产生一定影响。因此利用紫外可见光光谱(UV-vis)结合平行因子分析法(PARAFAC)和主成分分析法(PCA)来表征和分析水生植物腐解产物中DOM的组分及其特点。UV-vis的研究表明随着腐解时间的增加,样品中DOM的腐殖化程度逐渐升高,当腐殖化程度达到最高值时,腐解进入矿化阶段,此时腐殖化程度逐渐降低。通过PARAFAC分析可以分离出3种类蛋白组分(C1、C2和C5)和2种类腐殖酸组分(C3和C4)。由PCA可以得出在腐解初期,类蛋白组分占据主导地位;随着腐解时间的增加,类蛋白组分含量逐渐降低,而类腐殖酸组分含量逐渐升高。     

Spatiotemporal Distribution,Sources, and Photobleaching Imprint of Dissolved Organic Matter in the Yangtze Estuary and Its Adjacent Sea Using Fluorescence and Parallel Factor Analysis

To investigate the seasonal and interannual dynamics of dissolved organic matter (DOM) in the Yangtze Estuary, surface and bottom water samples in the Yangtze Estuary and its adjacent sea were collected and characterized using fluorescence excitation-emission matrices (EEMs) and parallel factor analysis (PARAFAC) in both dry and wet seasons in 2012 and 2013. Two protein-like components and three humic-like components were identified. Three humic-like components decreased linearly with increasing salinity (r>0.90, p<0.001), suggesting their distribution could primarily be controlled by physical mixing. By contrast, two protein-like components fell below the theoretical mixing line, largely due to microbial degradation and removal during mixing. Higher concentrations of humic-like components found in 2012 could be attributed to higher freshwater discharge relative to 2013. There was a lack of systematic patterns for three humic-like components between seasons and years, probably due to variations of other factors such as sources and characteristics. Highest concentrations of fluorescent components, observed in estuarine turbidity maximum (ETM) region, could be attributed to sediment resuspension and subsequent release of DOM, supported by higher concentrations of fluorescent components in bottom water than in surface water at two stations where sediments probably resuspended. Meanwhile, photobleaching could be reflected from the changes in the ratios between fluorescence intensity (F_max) of humic-like components and chromophoric DOM (CDOM) absorption coefficient (a355) along the salinity gradient. This study demonstrates the abundance and composition of DOM in estuaries are controlled not only by hydrological conditions, but also by its sources, characteristics and related estuarine biogeochemical processes.     

Dissolved organic matter (DOM) concentration and quality in a forested mid-Atlantic watershed, USA

Understanding the quantity and quality of dissolved organic matter (DOM) in potential watershed sources is critical for explaining and quantifying the exports of DOM in stream runoff. Here, we examined the concentration and quality of DOM for ten watershed sources in a 12?ha forested catchment over a two-year period. DOM composition was evaluated for: throughfall, litter leachate, soil water (zero and tension), shallow and deep groundwater, stream water, hyporheic zone, and groundwater seeps. DOM quality was measured using a suite of optical indices including UV–visible absorbance and PARAFAC modeling of fluorescence excitation-emission matrices (EEMs). DOM concentrations and quality displayed a pronounced trend across watershed sources. Surficial watershed sources had higher DOM concentrations and more humic-like DOM with higher molecular weight whereas deeper groundwater sources were rich in % protein-like fluorescence. The greater % contribution of protein-like fluorescence in groundwater suggested that a larger fraction of groundwater DOM may be bioavailable. DOM for wetland groundwater was more aromatic and humic-like than that at the well-drained riparian location. Principal component analyses (PCA) revealed that the differences in surficial watershed compartments were dictated by humic-like components while groundwater sources separated out by % protein-like fluorescence. Observations from optical indices did not provide any conclusive evidence for preferential association of dissolved organic carbon (DOC) or dissolved organic nitrogen (DON) with any particular DOM quality pools.     

灞河流域溶解性有机质分子量分级表征及其与水质的相关性

溶解性有机质是水体中有机质分解与营养盐再生的核心载体之一,是碳、氮等生源要素生物地球化学循环的重要环节,也是水环境科学研究的重点内容.本研究应用液相-有机碳-有机氮检测(LC-OCD-OND)技术研究了西安市灞河流域水体中溶解性有机质(DOM)不同分子量组分特征,分析其与河水水质的相关性.结果表明: 河水DOM按照分子量分布,主要由生物大分子、腐殖质类物质、腐殖质降解产物、低分子中性物质和低分子酸组成,各组分平均浓度分别为0.15、1.75、0.48、0.36和0.002 mg·L^-1,河水中DOM总体含量水平由高到低的顺序为城市河段＞城镇河段＞源区河段.组分中分子量介于1000~20000 Da的腐殖质类物质占DOM总量的49.0%,含量及丰度从高到低依次为中游城镇段＞污水厂出口段＞污水厂下游河口段＞上游源头段;分子量＞20000 Da的生物大分子约占DOM总量的5.1%,丰度由高到低依次为污水厂出口＞污水厂下游河口＞上游源头＞中游城镇段,污水处理厂出水所产生的外生源有机质对河流DOM的贡献最大.DOM不同分子量组分与水质的相关性明显,表明基于LC-OCD-OND分级表征的DOM各分子量组分和丰度不仅可以作为水质监测的一个综合性指标,也可以用来表征河流水质的空间异质性,并能对污染物各组分进行定量化判别和来源解析.     

Molecular characteristics of microbially mediated transformations of Synechococcus-derived dissolved organic matter as revealed by incubation experiments

In this study, we investigated the microbially mediated transformation of labile Synechococcus-derived DOM to RDOM using a 60-day experimental incubation system. Three phases of TOC degradation activity (I, II and III) were observed following the addition of Synechococcus-derived DOM. The phases were characterized by organic carbon consumption rates of 8.77, 1.26 and 0.16 μmol L⁻¹ day⁻¹, respectively. Excitation emission matrix analysis revealed the presence of three FDOM components including tyrosine-like, fulvic acid-like, and humic-like molecules. The three components also exhibited differing biological availabilities that could be considered as labile DOM (LDOM), semi-labile DOM (SLDOM) and RDOM, respectively. DOM molecular composition was also evaluated using FT-ICR MS. Based on differing biological turnover rates and normalized intensity values, a total of 1704 formulas were identified as candidate LDOM, SLDOM and RDOM molecules. Microbial transformation of LDOM to RDOM tended to proceed from high to low molecular weight, as well as from molecules with high to low double bond equivalent (DBE) values. Relatively higher aromaticity was observed in the formulas of RDOM molecules relative to those of LDOM molecules. FDOM components provide valuable proxy information to investigate variation in the bioavailability of DOM. These results suggest that coordinating fluorescence spectroscopy and FT-ICR MS of DOM, as conducted here, is an effective strategy to identify and characterize LDOM, SLDOM and RDOM molecules in incubation experiments emulating natural systems. The results described here provide greater insight into the metabolism of phytoplankton photosynthate by heterotrophic bacteria in marine environments.     

黄山松土壤可溶性有机质对氮添加的响应及其与细菌群落的关联

为探究黄山松土壤可溶性有机质(DOM)数量和质量对短期氮(N)添加的响应及其与细菌群落的关联,在福建戴云山自然保护区设置不同N添加水平(0、40和80 kg N·hm^-2·a^-1)试验,采用三维荧光与平行因子联用法,并结合高通量测序手段分别对土壤DOM和细菌群落进行分析。结果表明: 与对照相比,N添加整体降低了0～10和10～20 cm土层可溶性有机碳(DOC)含量和DOM腐殖化指数(HIX),其中,高氮(80 kg N·hm^-2·a^-1)添加下均显著降低。平行因子分析法进一步表明N添加下DOM中类腐殖质组分(C1、C2)的相对含量降低。此外,N添加减少了富营养细菌(变形菌门、酸微菌纲)的相对丰度,而增加了贫营养细菌(斯巴达杆菌纲)的相对丰度。富营养细菌的相对丰度与HIX、C1、C2呈显著正相关,与相对易分解的类富里酸组分(C3)呈显著负相关;而贫营养细菌的情况则相反。说明N添加下不同生活策略的细菌类群对DOM中难分解和易分解组分存在明显的偏好性。我们推测N沉降加剧背景下土壤微生物生活策略的转变可能有助于DOM组分的塑造。     

Fluorescence and amino acid characteristics of molecular size fractions of DOM in the waters of Lake Biwa

Dissolved organic matter (DOM) in the waters from Lake Biwa, Japan was fractionated using tangential flow ultrafiltration, and subsequently characterized by fluorescence properties and amino acids. While major dissolved organic carbon (DOC), UV absorbance (Abs), humic-like fluorescence (Flu) and total hydrolyzed amino acids (THAA) occurred in the less than 5 kDa molecular size fraction, they were not evenly distributed among various molecular size fractions. Flu/Abs ratios increased, and THAA/DOC ratios decreased with decreasing molecular size. Humic-like fluorescence occurred in all molecular size fractions, but protein-like fluorescence only occurred in the 0.1 m-GF/F fraction. Subtle differences in amino acid compositions (both individuals and functional groups) were observed between various molecular size fractions, this may indicate the occurrence of DOM degradation from higher to lower molecular weight. The results reported here have significance for further understanding the sources and nature of DOM in aquatic environments.     

Bioavailability and size-fraction of dissolved organic carbon, nitrogen, and sulfur at the Arbutus Lake watershed, Adirondack Mountains, NY

The heterogeneity of DOM is closely linked with the various sources and the diversity of biogeochemical processes. We studied the spatial and temporal patterns of the quantity (bioavailable [B-] and refractory concentrations by laboratory incubations) and quality (δ¹³C, aromaticity, and size-fraction by ultrafiltration [low molecular weight (LMW) < 1 kDa and high molecular weight (HMW) > 1 kDa]) of dissolved organic C (DOC), N (DON), and S (DOS) for surface waters (two upland streams, two wetland-affected streams, and lake outlet) over a 14-month period within the Arbutus Lake watershed in the Adirondack Mountains of New York State, USA. The % BDOC and % BDON of this watershed averages ranged from 6 to 18 % and from 12 to 43 %, respectively. The DOC and DON concentrations increased as water was transported through wetland areas of the Arbutus Lake watershed. DOC and DON constituents in the surface waters passing through a wetland were composed mostly of refractory HMW components (% HMWDOC: 55 %, % HMWDON: 60 %) with a higher level of aromaticity compared to upland streams (% HMWDOC: approximately 35 %, % HMWDON: approximately 30 %). DOS was dominated by the refractory (% BDOS range 6–13 %) and LMW (% LMWDOS range 62–96 %) form and we suggest that bacterial dissimilatory sulfate reduction might play an important role in generating this distinct DOS biogeochemistry. The aromaticity was positively related to total DOC concentration, but negatively to % BDOC. Arbutus Lake DOM was dominated by bioavailable and LMW characteristics, compared to wetland-affected streams where refractory and HMW DOM fractions were more prevalent. Our study suggests the different variability of DOM characteristics among elements (C, N, S) using a “bioavailability-molecular size model” showing a diagenetic perspective due to the relative refractoriness of the LMW DOS. This study also highlights the importance of multiple approaches for understanding DOM biogeochemistry with respect to molecular size, bioavailability, aromaticity, stoichiometry, isotopic values, and elemental concentrations.     

Compositional changes and molecular transformations of dissolved organic matter during the arctic spring floods in the lower Churchill watershed (Northern Manitoba,Canada)

The spring flood season in Northern aquatic systems is characterized by rapid changes in river discharge and fluxes of organic and inorganic species, yet remains under sampled. In this study, we examine the molecular characteristics and transformations of dissolved organic matter (DOM) during the Arctic spring season at three locations in the lower Churchill River watershed. Characteristics including dissolved organic carbon concentrations, optical properties (i.e. absorbance and, excitation-emission matrix fluorescence coupled with parallel factor analysis, EEM-PARAFAC) and molecular characterization using high resolution mass spectrometry (HR-MS) were assessed. As spring floods progressed, a significant reduction in DOM spectral slope (S_275–295) and an increase in absorbance (a₂₇₀) were observed suggesting an increase in DOM molecular weight and concentrations at all sampled locations. A maximum in HR-MS lignin-class compounds coincided with peak flow at all sampled locations, further validated with an increase in PARAFAC humic-like components and a minimum in S_275–295. Principal component analyses revealed that both protein and aromatic humic-like material measured by HR-MS and EEM-PARAFAC, respectively, were significantly correlated (ANOVA, p < 0.05), demonstrating similarities in both techniques. Based on network analysis of HR-MS assigned formula, the highest peak flow day was characterized by a maximum in homologous DOM of C₂H₂O and C₂H₄ additions, suggesting structural similarities in newly deposited lignin and humic material. Overall, the progression of the spring freshet shifted the lower Churchill River watershed from an environment rich in proteins and lipids to an aquatic system more abundant in lignin, aromatic and humic DOM.     

冰川退缩区不同演替阶段土壤溶解性有机质光谱特征及环境响应

溶解有机质(DOM)作为土壤中最活跃的有机组分,在土壤生物地球化学过程中起着关键作用,探讨植被演替过程中DOM的来源、组成、环境响应与累积规律,对预测土壤碳循环过程具有重要意义。本研究从海螺沟冰川退缩区植被原生演替序列选取演替年龄分别为12、30、40、50、80、120年的样地采集表层和亚表层土壤样本,测定DOM浓度并进行紫外-可见光光谱和三维荧光光谱分析,研究原生演替过程中DOM含量和组成的变化特征及其影响因素。结果表明: 土壤DOM浓度随演替年龄的增加而显著增加。土壤DOM中类蛋白组分、荧光指数和生物指数随演替时间的增加而减小,类腐殖质组分和腐殖化程度随演替过程不断增加,土壤DOM芳香化程度先增加后减小。pH值、铵态氮含量解释了62.2%的表层土壤DOM组分变异,土壤含水率和pH值解释了64.3%的亚表层土壤DOM组分变异,说明环境因素是影响海螺沟冰川退缩区原生演替过程中土壤DOM数量和组成的重要因子。     

Distinct Optical Chemistry of Dissolved Organic Matter in Urban Pond Ecosystems

Urbanization has the potential to dramatically alter the biogeochemistry of receiving freshwater ecosystems. We examined the optical chemistry of dissolved organic matter (DOM) in forty-five urban ponds across southern Ontario, Canada to examine whether optical characteristics in these relatively new ecosystems are distinct from other freshwater systems. Dissolved organic carbon (DOC) concentrations ranged from 2 to 16 mg C L^-1 across the ponds with an average value of 5.3 mg C L^-1. Excitation-emission matrix (EEM) spectroscopy and parallel factor analysis (PARAFAC) modelling showed urban pond DOM to be characterized by microbial-like and, less importantly, by terrestrial derived humic-like components. The relatively transparent, non-humic DOM in urban ponds was more similar to that found in open water, lake ecosystems than to rivers or wetlands. After irradiation equivalent to 1.7 days of natural solar radiation, DOC concentrations, on average, decreased by 38% and UV absorbance decreased by 25%. Irradiation decreased the relative abundances of terrestrial humic-like components and increased protein-like aspects of the DOM pool. These findings suggest that high internal production and/or prolonged exposure to sunlight exerts a distinct and significant influence on the chemistry of urban pond DOM, which likely reduces its chemical similarity with upstream sources. These properties of urban pond DOM may alter its biogeochemical role in these relatively novel aquatic ecosystems.     

冻融条件和土壤湿度对森林土壤渗漏液溶解性有机质含量与光谱结构特征的影响

土壤溶解性有机质(DOM)含量及其稳定性影响土壤碳氮循环关键过程,目前气候变化下森林土壤DOM含量及其光谱结构特征仍不明确.本研究利用长白山阔叶红松混交林和次生白桦林表层土壤进行室内冻融模拟试验,结合三维荧光光谱-平行因子分析方法,研究冻融强度和冻融循环次数及其交互作用对不同湿度温带森林土壤渗漏液DOM含量、组分和光谱结构特征的影响.结果表明: 森林土壤渗漏液DOM含量及其组分因林分类型、土壤湿度、冻融强度、冻融循环次数不同而存在差异.2种林分土壤渗漏液DOM含量均在中湿度下最低,并受高强度冻融影响显著,且随冻融循环次数增加呈现先增加后降低的趋势.可鉴别DOM的3个荧光组分:胡敏酸类DOM、富里酸类DOM和蛋白类DOM;阔叶红松混交林土壤渗漏液DOM组分以富里酸类物质为主,腐殖化程度较高;而次生白桦林土壤渗漏液DOM组分以胡敏酸类物质为主,3组分受冻融强度显著影响,稳定性较低.经冗余分析(RDA)发现,林分在很大程度上决定森林土壤DOM属性变化,次生白桦林土壤渗漏液DOM含量及其3组分荧光强度大于阔叶红松混交林;土壤湿度显著影响DOM芳香性,2种林分土壤渗漏液DOM芳香性均呈中湿度>高湿度>低湿度的趋势;随冻融强度增加,阔叶红松混交林土壤渗漏液DOM芳香性显著降低;多次冻融循环显著提高2种林分土壤渗漏液DOM腐殖化程度.因此,不同冻融作用下,低湿度温带森林土壤渗漏液DOM含量及其生物有效性呈现增加的趋势,尤其是次生白桦林土壤,可能会增加春季冻融期温带森林土壤溶解性有机质淋溶损失.这些结果可为深入研究野外冻融期温带森林土壤溶解性有机质周转机制提供参考.     

Effects of Watershed History on Dissolved Organic Matter Characteristics in Headwater Streams

Dissolved organic matter (DOM) is recognized as a major component in the global carbon cycle and is an important driver in aquatic ecosystem function. Climate, land use, and forest cover changes all impact stream DOM and alter biogeochemical cycles in terrestrial environments. We determined the temporal variation in DOM quantity and quality in headwater streams at a reference watershed (REF), a watershed clear-cut 30 years ago (CC), and a watershed converted to a white pine plantation 50 years ago (WP) at the US Forest Service, Coweeta Hydrologic Laboratory, in the Nantahala Mountains of western North Carolina, USA. Average stream dissolved organic carbon (DOC) concentrations in CC or WP were 60 and 80% of those in REF, respectively. Stream DOM composition showed that the difference was mainly due to changes in humic-like components in chromophoric DOM. In addition, excitation–emission matrix fluorescence data with parallel factor analysis indicate that although the concentration of protein-like components did not differ significantly among watersheds, their relative abundance showed an enrichment in CC and WP compared to REF. The ratio of humic acid-type to fulvic acid-type components was highest and lowest at REF and WP, respectively. Our data suggest that forest ecosystem disturbance history affects the DOM quantity and quality in headwater streams over decades as a result of changes in watershed soil organic matter characteristics due to differences in organic matter inputs.     

Tracking variations in fluorescent-dissolved organic matter in an aerobic submerged membrane bioreactor using excitation–emission matrix spectra combined with parallel factor analysis

In this study, the variations in the fluorescent components of dissolved organic matter (DOM) were tracked for an aerobic submerged membrane bioreactor (MBR) at three different operation stages (cake layer formation, condensation, and after cleaning). The fluorescent DOM was characterized using excitation–emission matrix (EEM) spectroscopy combined with parallel factor analysis (PARAFAC). Non-aromatic carbon structures appear to be actively involved in the membrane fouling for the cake layer formation stage as revealed by much higher UV-absorbing DOM per organic carbon found in the effluent versus those inside the reactor. Four fluorescent components were successfully identified from the reactor and the effluent DOMs by EEM-PARAFAC modeling. Among those in the reactor, microbial humic-like fluorescence was the most abundant component at the cake layer formation stage and tryptophan-like fluorescence at the condensation stage. In contrast to the reactor, relatively similar composition of the PARAFAC components was exhibited for the effluent at all three stages. Tryptophan-like fluorescence displayed the largest difference between the reactor and the effluent, suggesting that this component could be a good tracer for membrane fouling. It appears that the fluorescent DOM was involved in membrane fouling by cake layer formation rather than by internal pore adsorption because its difference between the reactor and the effluent was the highest among all the four components, even after the membrane cleaning. Our study provided an insight into the fate and the behavior fluorescent DOM components for an MBR system, which could be an indicator of the membrane fouling.     

Iron and fluorescent dissolved organic matter in an estuarine and coastal system in Japan

Terrestrially derived dissolved organic matter (DOM) plays an important role in providing ligands that regulate iron transport to coastal ecosystems. Fluorescent DOM (FDOM) is widely used to evaluate DOM behavior, but the relationship between iron and FDOM distributions in estuarine and coastal systems is poorly understood. In this study, we investigated the distributions of dissolved iron (DFe < 0.7-µm fraction) and FDOM as measured using fluorescence excitation-emission matrices in the Yura River–Tango Bay system. The DFe distribution was not closely correlated with that of the humic-like FDOM, indicating that humic-like FDOM is not the main regulator of the DFe concentration in this system. The relatively low fluorescence intensity of humic-like FDOM demonstrates that colloidal inorganic iron is the major component of riverine DFe. A large proportion of the riverine DFe (> 94%) was removed in the mixing process, but humic-like FDOM likely supports the transport of DFe to the coastal zone.     

Characterisation and reactivity continuum of dissolved organic matter in forested headwater catchments of Andean Patagonia

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2005年4月对长江口全区域潮下带共10个采样站位的大型底栖动物进行了调查。调查采获大型底栖动物38种,分属5个生态类型,种类数较少,河口外缘站位种类数多于口内站位。各站位大型底栖动物的平均丰度为32.9个/m2、平均生物量为5.035g/m2(湿重);与20世纪七八十年代相比,平均生物量显著降低;口外缘站位的总丰度和总生物量均高于口内站位。环境因子相关分析表明,盐度是决定长江口大型底栖动物种类分布最重要的环境因子。群落聚类、标序分析显示,春季长江口潮下带大型底栖动物群落结构空间分异明显,完全符合目前长江口支、港、槽“三级分汊”的空间格局。其中,北支的大型底栖动物以混合高盐水种类为主,而南支则以淡水和半咸水种类为主。南支的南北槽分界处内外站位的群落差异也由盐度决定,因为靠近口内的群落均受长江冲淡水影响较大;而口外站位群落则受咸淡水影响。南支的南北港分界点内外的群落差异则主要受长江来水的影响,原因在于处在港分界点以内的群落所在区域,直接受长江来水的冲刷,底质环境极不稳定;而港、槽分界点之间的群落所在区域由于河口上段的诸多明暗沙体的阻挡,水势较为稳定,所以底质环境较稳定,从而使得港、槽分界点之间的群落出现了更多的沙蚕等底质环境类型种类。     

Sources and seasonal patterns of dissolved organic matter (DOM) in the hyporheic zone

The hyporheic zone is a region underneath streambeds that integrates surface and groundwater. Although its location is central to biogeochemical linkages between the riparian zone, dissolved nutrients, and benthic biota, the seasonal quality and likely sources of dissolved organic matter (DOM) in the hyporheic zone are not well understood. To investigate DOM characteristics in the hyporheic zone, water from the surface and subsurface (at depths 20, 60, and 100 cm below the streambed) was sampled every 4 weeks from 2007 to 2008 in a third-order stream in southern Ontario. Using UV spectroscopy, measures of spectral slopes, aromaticity, and A ₂₅₄/A ₃₆₅ ratios (molecular weight) were obtained. Temporal changes in these measures were consistent with watershed processes such as shedding of leaf litter in the fall, and photochemical and biofilm influence in the spring and summer. The fluorescence index (a measure of relative DOM source) suggested that at the surface and in the downwelling zone, DOM microbial sources increased with depth in the sediment, regardless of the season. Excitation–emission matrices (EEMs) showed seasonally distinct, protein-like DOM components of bacterial origin that were stronger in the fall. Leachates from specific allochthonous DOM sources—leaf litter from Betula papyrifera (white birch) and Thuja occidentalis (white cedar)—and an autochthonous source, biofilm, were isolated and incubated with unfiltered surface water. EEMs from these leachates indicated that these sources could indeed help explain observed patterns of DOM in surface and subsurface waters. These results suggest that although DOM sources were relatively constant, biogeochemical processing within the hyporheic zone resulted a DOM pool that was temporally dynamic and altered the nature of organic matter transported downstream into lakes and rivers.     

Molecular weight fractionation of dissolved organic matter (DOM) released by phytoplankton

The application of an ultrafiltration procedure for fractionation of molecular weight of dissolved organic matter (DOM) extracellularly released by phytoplankton is described. Seven ultrafiltration membranes Diaflo (Amicon Corp. USA), with range 500--300,000 molecular weight (MW), were used for separation of different molecular weight compounds released by phytoplankton in Rhode River estuary of Chesapeake Bay, and their composition was determined. Percentage of extracellular release of DOM by phytoplankton varied from 3.92--68.07% of total carbon fixed in photosynthesis. The composition of algal extracellular products varied with different phytoplankton populations. However, two fractions of molecular weight compounds dominated in the composition of DOM released, i.e. a low molecular weight fraction of less than 1,000 MW and the fraction between 10,000--30,000 MW. The ultrafiltration procedure is effective for studying the composition of DOM released by phytoplankton in natural waters.