Chromophoric Dissolved Organic Matter Composition Research Articles

Implications of irradiance exposure and non-photochemical quenching for multi-wavelength (bbe FluoroProbe) fluorometry

Multi-wavelength fluorometers, such as the bbe FluoroProbe (FP), measure excitation spectra of chlorophyll a (Chl-a) fluorescence to infer the abundance and composition of phytoplankton communities as well as the concentration of chromophoric dissolved organic matter (CDOM). Experiments were conducted on laboratory cultures and on natural communities of freshwater phytoplankton to determine how the response of phytoplankton to high irradiance might affect fluorometric estimates of community composition and concentrations of Chl-a and CDOM. Cultures of a representative cyanobacterium, bacillariophyte, synurophyte, cryptophyte, and chlorophyte revealed changes in Chl-a excitation spectra as irradiance was increased to saturating levels and non-photochemical quenching (NPQ) increased. The degree of change and resulting classification error varied among taxa, being strong for the synurophyte and cryptophyte but minimal for the cyanobacterium. Acute-exposure experiments on phytoplankton communities of varying taxonomic composition from five lakes yielded variable results on apparent community composition. There was a consistent decrease in CDOM estimates, whereas Chl-a estimates were generally increased. Subsequent exposure to low PAR relaxed NPQ and tended to reverse the effects of high irradiance on composition, total Chl-a, and CDOM estimates. Relaxation experiments on near-surface communities in a sixth, large lake, Georgian Bay, showed that total Chl-a estimates increased by 44% on average when dark treatments were used to relax NPQ, though, in contrast to the findings from the small lakes, there was little effect on CDOM estimates. We observed a statistically-significant, negative linear relationship between the photon flux density of in situ irradiance and the accuracy of taxonomic assignment by FP in Georgian Bay. Not discounting the correlations between light intensity and the accuracy of the FP that were observed in this study, we conclude that the applicability of the reference spectra to the system under investigation is a more important consideration than variability in natural irradiance conditions.

Composition, Spatial Distribution Characteristics and Source Analysis of Chromophoric Dissolved Organic Matter in the Lanzhou Reach of the Yellow River

Chromophoric dissolved organic matter (CDOM) in riverine systems can be affected by environmental conditions and land-use, and can thus provide important information regarding anthropogenic activities in surrounding landscapes. It can modify the optical properties of waters and affect the balance and availability of dissolved nutrients and metals in water bodies. However, the characteristics of CDOM in the Lanzhou reach of the Yellow River have not yet been reported. In this study, the optical properties of water samples collected at 32 locations during April 2017 across the Lanzhou reach of the Yellow River were examined using UV-VIS and excitation-emission matrix fluorescence spectroscopy-parallel factor analysis (EEM-PARAFAC), to determine CDOM compositional changes, spatial distribution characteristics, and sources. Cluster analysis was used to categorize samples into groups of similar pollution levels within a study area. Results showed that CDOM was primarily comprised of low molecular weight organic substances with aromatic structure belonging to complex "protein-like-humic-like" substances, and dominated by protein-like substances (organism sources). Two humic-like components (C1, C4), one tryptophan-like component (C2), and one non-humic-like component (C3) were identified by PARAFAC. Tryptophan-like substances were predominant in the components of CDOM, accounting for 51.06% of average total fluorescence intensity. Humic-like materials and non-humic-like substances accounted for 36.74% and 12.20%, respectively. Weak correlations were observed between protein-like substances and humic-like substances, indicating different sources of these components. The distribution of total fluorescence intensity showed a distinct spatial pattern; trends in fluorescence intensity were weak-strong-weak along an upstream to downstream continuum, mainly affected by changes in the content of protein-like substances. The spatial variation of the CDOM in the Lanzhou reach of the Yellow River can therefore be assessed based on protein-like materials dynamics. Public spaces along rivers offer opportunities for community gatherings and recreational activities. However, high-intensity anthropogenic activities strongly influence CDOM concentration and composition in this area in different ways; sources include increased residential/commercial wastewater, catering, water recreation facilities pollution, shipping, and a small amount of industrial discharge. In addition, it was concluded that endogenetic pollution may become the main source of internal loading in the Lanzhou reach of the Yellow River, implying that stronger endogenetic pollution control is needed to alleviate CDOM pollution and improve water quality.

Distinguishing the Compositions and Sources of the Chromophoric Dissolved Organic Matter in a Typical Karst River During the Dry Season: A Case Study in Bitan River, Jinfo Mountain

Since resistant dissolved organic matter (RDOM) plays a critically important role in a karst carbon sink, one of the most important continental carbon sinks, research focusing on the origination, transportation, and translation of RDOM in a karst water system is important. Currently, 3D-fluorescence EEMs are used to detect the composition and origination of chromophoric dissolved organic matter (CDOM), an important part of RDOM. This is a very fast and efficient method for CDOM analysis. In this study, 3D-fluorencence EEMs combined with UV-visible absorption spectrum were used to analyze the composition and origination of CDOM in the Bitan River at Jinfo Mountain. Samples were collected from nine sampling sites from January to March 2017 and analyzed with CDOM EEMs and UV-visible absorption spectrums. In addition hydrochemical characteristics were determined and then samples were stimulated with PARAFAC to detect the chromophoric fluorescent groups and indexes. The PARAFAC stimulation revealed three chromophoric fluorescent groups in which fulvic acid was the largest component, accounting for about 44%, with a humic acid content of about 32% and tyrosine-like acid content of about 24%. Four indexes: FI, BIX, HIX, and β∶α, were calculated, and the mean values were 2.06, 0.87, 4.35 and 0.69, which showed relatively high FI, BIX, and β∶α values and a low HIX value, implying that the CDOM was autochthonous and originated from microbes and aquatic plants in the dry season. The spatial dynamic of the index revealed an increased BIX and decreased HIX from the upstream area to the downstream area, implying the impact of land-use and human activities. The forest soil input more humic acid and agriculture input more N and P resulting in flourishing aquatic plants and microbes. Moreover, the correlation coefficients of DIC and humic acid, tyrosine-like acid were 0.515 (P<0.05) and 0.644 (P<0.01), from which it could be inferred that DIC contributed to CDOM formation. The conclusions of this study revealed that DIC would be fixed by karst water aquatic plants and microbes and then sink as autochthonous CDOM and become part of karst water carbon sink.

Impacts of cyanobacterial blooms outbreak and decline on chromophoric dissolved organic matter in Lake Taihu

利用平行因子分析（PARAFAC）技术对藻华暴发季节太湖梅梁湾和开敞区水样中有色可溶性有机物（CDOM）的三维荧光光谱进行分析，获得代表类酪氨酸、类色氨酸和类腐殖酸等3种荧光组分，这3个组分的荧光得分值均与叶绿素a浓度呈极显著正相关，其中类腐殖酸物质荧光得分值占总分值的比例也与叶绿素a浓度极显著正相关，由此可以推测，蓝藻水华可能是太湖CDOM的一个重要来源，并极大地影响了湖泊CDOM的组成结构.为进一步探索蓝藻水华对湖泊CDOM的影响，于太湖梅梁湾湖岸进行藻华暴发和衰亡模拟试验.PARAFAC结果显示，模拟试验水样CDOM的三维荧光图谱同样含有3个组分，其中类酪氨酸和类色氨酸荧光峰位置与湖泊水样基本相同，而类腐殖酸物质荧光峰则有一定程度的蓝移，表明培养试验中有新的内源类腐殖酸物质产生.利用PARAFAC模型中的得分值计算后发现，初始叶绿素a浓度越高，类腐殖酸物质所占比例越大，而实验结束时添加了高浓度和中等浓度藻的2个处理中类腐殖酸物质对荧光的贡献率也有显著提高.因此，藻华的长期暴发可以显著改变湖泊CDOM的组成，导致类腐殖酸物质比例提高.;Parallel Factor Analysis (PARAFAC) was applied to assess the composition of chromophoric dissolved organic matter (CDOM) from the excitation-emission matrix (EEM) fluorescence spectra of the water samples collected from Meiliang Bay and the open area in Lake Taihu during a cyanobacterial bloom event, and three fluorescent components were identified:a tyrosine-like component, a tryptophan-like component and a humic-like component. Correlation analysis showed that the concentrations of chlorophyll-a(Chl.a) were significantly correlated with the fluorescence intensities of the three components, respectively, as well as the proportion of humic-like to total fluorescence intensities, suggesting that cyanobacteria might be an important source of CDOM and can greatly change the composition of CDOM in Lake Taihu. For further investigation, a simulation experiment was performed on the shore of Meiliang Bay. PARAFAC model showed that the number of components and the spectral characteristics of CDOM EEM data from the simulation experiment are similar in terms of the number of fluorescence peaks and their position to previously identified components in the water samples from Lake Taihu, except that the fluorescence peaks of humic acids showed a certain degree of blue shift, which suggested that endogenous humic substances were produced during the simulation experiment. It was found that the higher initial concentrations of Chl.a, the greater contribution of humic acids to total CDOM fluorescence intensity derived by the PARAFAC model. Moreover, at the end of the experiment, the ratios of humic substances to total fluorescent substances increased significantly in the two treatments with relatively high concentrations of cyanobacteria. Thus, the long-term outbreak of cyanobacterial blooms can significantly alter the composition of the CDOM in lakes, resulting in an increase of the proportion of humic-like substances.

Effects of rainstorm on the spectral absorption properties of chromophoric dissolved organic matter and particles in Lake Qiandao

为了研究暴雨事件对千岛湖有色可溶性有机物（CDOM）和颗粒物吸收光谱的影响，利用2016年暴雨前（3月1-6日）和暴雨后（4月6-11日）采集的水样，对暴雨前、后千岛湖水体CDOM、浮游藻类和非藻类颗粒物的吸收光谱特征进行分析，探讨暴雨事件对其造成的影响.结果表明：千岛湖作为典型的深水型内陆湖泊，其CDOM、浮游藻类颗粒物和非藻类颗粒物的吸收强度较太湖等浅水型湖泊弱.暴雨前，CDOM光谱吸收系数a_CDOM（λ）值在0~0.6 m^-1范围内变化，其光谱拟合系数S_CDOM的均值为0.0158±0.00145 nm^-1.暴雨前浮游藻类光谱吸收在总颗粒物中占主导，a_ph（λ）在0~0.35 m^-1范围内变化，非藻类颗粒物光谱吸收系数a_NAP（λ）在0~0.15 m^-1范围内变化，其光谱拟合系数S_NAP均值为5.62±0.57 μm^-1；暴雨后CDOM光谱吸收系数a_CDOM（λ）值在0~1.6 m^-1范围内变化，其光谱拟合系数S_CDOM的均值为0.0157±0.00101 nm^-1.暴雨后浮游藻类光谱吸收系数a_ph（λ）在0~2.5 m^-1范围内变化，非藻类颗粒物光谱吸收在部分区域已占据主导地位，a_NAP（λ）在0~0.8 m^-1范围内变化，其光谱拟合系数S_NAP均值为5.72±0.68 μm^-1.由CDOM吸收特征值相对分子质量M值得出，暴雨前、后千岛湖不同区域CDOM组成都以富里酸为主，且暴雨前M值分布较均匀，暴雨后M值呈现从新安江向缓冲区、东南区递增的趋势，这说明西北区随暴雨输入的腐殖酸增加了CDOM的相对分子质量.暴雨对S_NAP值影响较大的区域为西北区、西南区、东北区，对西南区影响最小.本研究为使用光学手段深入探讨暴雨事件对千岛湖水环境的影响提供重要依据.;To study the influence of heavy rainfall on absorption characteristics of chromophoric dissolved organic matter (CDOM) and particles in Lake Qiandao, the samples were collected on March 1-6, 2016 (before the rainstorm) and April 6-11, 2016 (after the rainstorm). The spectral absorption of CDOM, phytoplankton and non-algal particles (NAP) in the lake were analyzed and the influence of rainstorm was discussed. The results showed that Lake Qiandao, as a typical deep inland lake, its spectral absorption coefficients of CDOM, phytoplankton and NAP is small, compared with shallow lakes like Lake Taihu. The CDOM spectral absorption coefficients varied in the range of 0-0.6 m^-1, the mean value of the spectral fitting coefficient of CDOM(S_CDOM) was 0.0158±0.00145 nm^-1 before the rainstorm. The spectral absorption of phytoplankton before the rainstorm dominated the absorption of total particles, varied in the range of 0-0.35 m^-1. The spectral absorption coefficients of NAP varied in the range of 0-0.15 m^-1, and the mean value of spectral fitting coefficient of NAP(S_NAP) was 5.62±0.57 μm^-1 before the rainstorm; The CDOM spectral absorption coefficients varied in the range of 0-1.6 m^-1 S_CDOM was 0.0157±0.00101 nm^-1 after the rainstorm. The spectral absorption of phytoplankton varied in the range of 0-2.5 m^-1. NAP absorption dominated the absorption of particles in some regions after the rainstorm, a_NAP(λ)varied in the range of 0-0.8 m^-1 and S_NAP was 5.72±0.68 μm^-1. The M value showed that CDOM composition of the different regions of the lake is mainly fulvic acid both before and after the rainstorm, the spatial distribution of M value is relative even before the rainstorm. However, the M value showed an increasing trend from the Xin'anjiang to the buffer area and southeastern area after the rainstorm, which indicates that humic acids increased the relative molecular mass of CDOM in the northwestern area. Rainstorm had the greatest impact on the northwestern, southwestern and northeastern areas regarding the value of S_NAP, with the smallest impact on the southwestern area. This study provides an important basis for the use of optical means to further explore the impact of heavy rainfall on aquatic environment of Lake Qiandao.

Optical properties and composition changes in chromophoric dissolved organic matter along trophic gradients: Implications for monitoring and assessing lake eutrophication.

Chromophoric dissolved organic matter (CDOM) is an important optically active substance in aquatic environments and plays a key role in light attenuation and in the carbon, nitrogen and phosphorus biogeochemical cycles. Although the optical properties, abundance, sources, cycles, compositions and remote sensing estimations of CDOM have been widely reported in different aquatic environments, little is known about the optical properties and composition changes in CDOM along trophic gradients. Therefore, we collected 821 samples from 22 lakes along a trophic gradient (oligotrophic to eutrophic) in China from 2004 to 2015 and determined the CDOM spectral absorption and nutrient concentrations. The total nitrogen (TN), total phosphorus (TP), and chlorophyll a (Chla) concentrations and the Secchi disk depth (SDD) ranged from 0.02 to 24.75 mg/L, 0.002-3.471 mg/L, 0.03-882.66 μg/L, and 0.05-17.30 m, respectively. The trophic state index (TSI) ranged from 1.55 to 98.91 and covered different trophic states, from oligotrophic to hyper-eutrophic. The CDOM absorption coefficient at 254 nm (a(254)) ranged from 1.68 to 92.65 m-1. Additionally, the CDOM sources and composition parameters, including the spectral slope and relative molecular size value, exhibited a substantial variability from the oligotrophic level to other trophic levels. The natural logarithm value of the CDOM absorption, lna(254), is highly linearly correlated with the TSI (r2 = 0.92, p < .001, n = 821). Oligotrophic lakes are distinguished by a(254)<4 m-1, and mesotrophic and eutrophic lakes are classified as 4 ≤ a(254)≤10 and a(254)>10 m-1, respectively. The results suggested that the CDOM absorption coefficient a(254) might be a more sensitive single indicator of the trophic state than TN, TP, Chla and SDD. Therefore, we proposed a CDOM absorption coefficient and determined the threshold for defining the trophic state of a lake. Several advantages of measuring and estimating CDOM, including rapid experimental measurements, potential in situ optical sensor measurements and large-spatial-scale remote sensing estimations, make it superior to traditional TSI techniques for the rapid monitoring and assessment of lake trophic states.

Characterizing spatiotemporal variations of chromophoric dissolved organic matter in headwater catchment of a key drinking water source in China.

Natural surface drinking water sources with the increasing chromophoric dissolved organic matter (CDOM) have profound influences on the aquatic environment and drinking water safety. Here, this study investigated the spatiotemporal variations of CDOM in Fengshuba Reservoir and its catchments in China. Twenty-four surface water samples, 45 water samples (including surface water, middle water, and bottom water), and 15 pore water samples were collected from rivers, reservoir, and sediment of the reservoir, respectively. Then, three fluorescent components, namely two humic-like components (C1 and C2) and a tryptophan-like component (C3), were identified from the excitation-emission matrix coupled with parallel factor analysis (EEM-PARAFAC) for all samples. For spatial distributions, the levels of CDOM and two humic-like components in the reservoir were significantly lower than those in the upstream rivers (p<0.01), indicating that the reservoir may act as a reactor to partly reduce the levels of exogenous input including CDOM and humic-like matters from the surrounding catchment. For temporal variations, the mean levels of CDOM and three fluorescent components did not significantly change in rivers, suggesting that perennial anthropic activity maybe an important factor impacting the concentration and composition of river CDOM but not the precipitation and runoff. However, these mean values of CDOM for the bulk waters of the reservoir changed markedly along with seasonal variations, indicating that the hydrological processes in the reservoir could control the quality and quantity of CDOM. The different correlations between the fluorescent components and primary water parameters in the river, reservoir, and pore water samples further suggest that the reservoir is an important factor regulating the migration and transformation of FDOM along with the variations of different environmental gradients.

Hydraulic connectivity and evaporation control the water quality and sources of chromophoric dissolved organic matter in Lake Bosten in arid northwest China

Lake Bosten is the largest oligosaline lake in arid northwestern China, and water from its tributaries and evaporation control the water balance of the lake. In this study, water quality and chromophoric dissolved organic matter (CDOM) absorption and fluorescence were investigated in different seasons to elucidate how hydraulic connectivity and evaporation may affect the water quality and variability of CDOM in the lake. Mean suspended solids and turbidity were significantly higher in the upstream tributaries than in the lake, the difference being notably more pronounced in the wet than in the dry season. A markedly higher mean first principal component (PC1) score, which was significantly positively related to protein-like components, and a considerably lower fluorescence peak integration ratio – IC:IT, indicative of the terrestrial humic-like CDOM contribution percentage, were observed in the lake than in the upstream tributaries. Correspondingly, notably higher contribution percentages of terrestrial humic-like components were observed in the river mouth areas than in the remaining lake regions. Furthermore, significantly higher mean turbidity, and notably lower mean conductivity and salinity, were recorded in the southwestern Kaidu river mouth than in the remaining lake regions in the wet season. Notably higher mean salinity is recorded in Lake Bosten than in upstream tributaries. Autochthonous protein-like associated amino-acids and also PC1 scores increased significantly with increasing salinity. We conclude that the dynamics of water quality and CDOM composition in remote arid Lake Bosten are strongly driven by evaporation and also the hydraulic connectivity between the upstream tributaries and the downstream lake.

Assessment of the Spatial-temporal Distribution Characteristics and Main Affecting Factors of Chromophoric Dissolved Organic Matter in Spring and Summer at the Changjiang Estuary and Adjacent Areas

The composition and distribution characteristics of chromophoric dissolved organic matter(CDOM) of Changjiang Estuary and its adjacent waters in spring (March 2015) and summer (July 2015) were evaluated by excitation-emission matrix spectroscopy (EEMs) in combination with parallel factor(PARAFAC) analysis. Three humic-like components[C1(370/495 nm),C2(330/405 nm),C3(365/440 nm)] and one protein-like component[C4(295/345 nm)] were identified. The distribution patterns of the four components in spring and summer were almost the same, showing a decreasing trend from Changjiang Estuary to adjacent waters. In spring, the high value areas of three humic-like components were located inside of Changjiang Estuary, and the high value areas of the protein-like component were located in the South Passage of Changjiang Estuary. In the surface layer, the CDOM was mainly from terrestrial input and human activities; the influence of terrestrial weakened in the middle layer because of the lower fluorescence intensity values; and in the bottom layer the fluorescence values were higher than those in the middle layer, which was caused by sediment resuspension. There were high value areas in the Daishan Island, which were related to human activities. In summer, the high value areas of CDOM were almost the same as those in spring, and all layers had similar fluorescence intensity values, which indicated that they were mixed evenly. CDOM in spring and summer had high humification index (HIX) values in the Changjiang Estuary, whereas for biological index (BIX), its high values appeared in the adjacent waters. Four components (C1-C4), a355 and DOC, TN, TP, DO, Chl-a, Salinity were evaluated by redundancy analysis (RDA), which indicated that C1-C4 and TN, TP were mainly affected by terrestrial input and human activities, and DOC was affected by terrestrial and marine source together. The use of EEMs-PARAFAC in combination with multivariate analysis to parse the composition of CDOM revealed the source of CDOM and main influencing factors in Changjiang Estuary and adjacent waters, and plays an important role in the biogeochemical research of biogenic elements in estuarine waters.

Spatio-seasonal variability of chromophoric dissolved organic matter absorption and responses to photobleaching in a large shallow temperate lake

Abstract. The development and validation of remote-sensing-based approaches for the retrieval of chromophoric dissolved organic matter (CDOM) concentrations requires a comprehensive understanding of the sources and magnitude of variability in the optical properties of dissolved material within lakes. In this study, spatial and seasonal variability in concentration and composition of CDOM and the origin of its variation was studied in Lake Balaton (Hungary), a large temperate shallow lake in central Europe. In addition, we investigated the effect of photobleaching on the optical properties of CDOM through in-lake incubation experiments. There was marked variability throughout the year in CDOM absorption in Lake Balaton (aCDOM(440) = 0. 06–9.01 m−1). The highest values were consistently observed at the mouth of the main inflow (Zala River), which drains humic-rich material from the adjoining Kis-Balaton wetland, but CDOM absorption decreased rapidly towards the east where it was consistently lower and less variable than in the westernmost lake basins. The spectral slope parameter for the interval of 350–500 nm (SCDOM(350–500)) was more variable with increasing distance from the inflow (observed range 0.0161–0.0181 nm−1 for the mouth of the main inflow and 0.0158–0.0300 nm−1 for waters closer to the outflow). However, spatial variation in SCDOM was more constant exhibiting a negative correlation with aCDOM(440). Dissolved organic carbon (DOC) was strongly positively correlated with aCDOM(440) and followed a similar seasonal trend but it demonstrated more variability than either aCDOM or SCDOM with distance through the system. Photobleaching resulting from a 7-day exposure to natural solar UV radiation resulted in a marked decrease in allochthonous CDOM absorption (7.04 to 3.36 m−1, 42 % decrease). Photodegradation also resulted in an increase in the spectral slope coefficient of dissolved material.

Comparison of bacterial growth in response to photodegraded terrestrial chromophoric dissolved organic matter in two lakes.

Terrestrial chromophoric dissolved organic matter (CDOM) could subsidize lake food webs. Trophic state and altitude have a pronounced influence on the CDOM concentration and composition of a lake. The impact of future changes in solar radiation on high-altitude lakes is particularly alarming because these aquatic ecosystems experience the most pronounced radiation variation worldwide. Photodegradation experiments were conducted on terrestrial CDOM samples from oligotrophic alpine Lake Tiancai and low-altitude eutrophic Lake Xiaohu to investigate the response of bacterial growth to photodegraded CDOM. During the photo-irradiation process, the fluorescent CDOM intensity evidently decreased in an inflowing stream of Lake Tiancai, with the predominance of humic-like fluorescence. By contrast, minimal changes were observed in the riverine CDOM of Lake Xiaohu, with the predominance of protein-like fluorescence. The kinetic constants of photodegradation indicated that the degradation rate of terrestrial (soil) humic acid in Lake Tiancai was significantly higher than that in Lake Xiaohu (p<0.001). Soil humic and fulvic acids irradiated in the simulated experiment were applied to incubated bacteria. The specific growth rate of bacteria incubated with soil humic substances was significantly higher in Lake Tiancai than in Lake Xiaohu (p<0.05). Furthermore, the utilizing rate of dissolved oxygen (DO) confirmed that the DO consumption by bacteria incubated with terrestrial CDOM in Lake Tiancai was significantly greater than that in Lake Xiaohu (p<0.05). In summary, the exposure of terrestrial CDOM to light significantly enhances its availability to heterotrophic bacteria in Lake Tiancai, an oligotrophic alpine lake, which is of importance in understanding bacterial growth in response to photodegraded terrestrial CDOM for different types of lakes.

Characteristics and sources analysis of riverine chromophoric dissolved organic matter in Liaohe River, China

Chromophoric dissolved organic matter (CDOM) in riverine systems can be affected by environmental conditions and land-use, and thus could provide important information regarding human activities in surrounding landscapes. The optical properties of water samples collected at 42 locations across the Liaohe River (LHR, China) watershed were examined using UV-Vis and fluorescence spectroscopy to determine CDOM characteristics, composition and sources. Total nitrogen (TN) and total phosphorus (TP) concentrations at all sampling sites exceeded the GB3838-2002 (national quality standards for surface waters, China) standard for Class V waters of 2.0 mg N/L and 0.4 mg P/L respectively, while trophic state index (TSIM) indicated that all the sites investigated were mesotrophic, 64% of which were eutrophic at the same time. Redundancy analysis showed that total suspended matter (TSM), dissolved organic carbon (DOC), and turbidity had a strong correlation with CDOM, while the other parameters (Chl a, TN, TP and TSIM) exhibited weak correlations with CDOM absorption. High spectral slope values and low SUVA254 (the specific UV absorption) values indicated that CDOM in the LHR was primarily comprised of low molecular weight organic substances. Analysis of excitation-emission matrices contour plots showed that CDOM in water samples collected from upstream locations exhibited fulvic-acid-like characteristics whereas protein-like substances were most likely predominant in samples collected in estuarine areas and downstream from large cities. These patterns were interpreted as indicative of water pollution from urban and industrial activities in several downstream sections of the LHR watershed.

Changes in optical characteristics of surface microlayers hint to photochemically and microbially mediated DOM turnover in the upwelling region off the coast of Peru

Abstract. The coastal upwelling system off the coast of Peru is characterized by high biological activity and a pronounced subsurface oxygen minimum zone, as well as associated emissions of atmospheric trace gases such as N2O, CH4 and CO2. From 3 to 23 December 2012, R/V Meteor (M91) cruise took place in the Peruvian upwelling system between 4.59 and 15.4° S, and 82.0 to 77.5° W. During M91 we investigated the composition of the sea-surface microlayer (SML), the oceanic uppermost boundary directly subject to high solar radiation, often enriched in specific organic compounds of biological origin like chromophoric dissolved organic matter (CDOM) and marine gels. In the SML, the continuous photochemical and microbial recycling of organic matter may strongly influence gas exchange between marine systems and the atmosphere. We analyzed SML and underlying water (ULW) samples at 38 stations focusing on CDOM spectral characteristics as indicator of photochemical and microbial alteration processes. CDOM composition was characterized by spectral slope (S) values and excitation–emission matrix fluorescence (EEMs), which allow us to track changes in molecular weight (MW) of DOM, and to determine potential DOM sources and sinks. Spectral slope S varied between 0.012 to 0.043 nm−1 and was quite similar between SML and ULW, with no significant differences between the two compartments. Higher S values were observed in the ULW of the southern stations below 15° S. By EEMs, we identified five fluorescent components (F1–5) of the CDOM pool, of which two had excitation/emission characteristics of amino-acid-like fluorophores (F1, F4) and were highly enriched in the SML, with a median ratio SML : ULW of 1.5 for both fluorophores. In the study region, values for CDOM absorption ranged from 0.07 to 1.47 m−1. CDOM was generally highly concentrated in the SML, with a median enrichment with respect to the ULW of 1.2. CDOM composition and changes in spectral slope properties suggested a local microbial release of DOM directly in the SML as a response to light exposure in this extreme environment. In a conceptual model of the sources and modifications of optically active DOM in the SML and underlying seawater (ULW), we describe processes we think may take place (Fig. 1); the production of CDOM of higher MW by microbial release through growth, exudation and lysis in the euphotic zone, includes the identified fluorophores (F1, F2, F3, F4, F5). Specific amino-acid-like fluorophores (F1, F4) accumulate in the SML with respect to the ULW, as photochemistry may enhance microbial CDOM release by (a) photoprotection mechanisms and (b) cell-lysis processes. Microbial and photochemical degradation are potential sinks of the amino-acid-like fluorophores (F1, F4), and potential sources of reworked and more refractory humic-like components (F2, F3, F5). In the highly productive upwelling region along the Peruvian coast, the interplay of microbial and photochemical processes controls the enrichment of amino-acid-like CDOM in the SML. We discuss potential implications for air–sea gas exchange in this area.

Characterization of chromophoric dissolved organic matter and relationships among PARAFAC components and water quality parameters in Heilongjiang, China.

Chromophoric dissolved organic matter (CDOM) is an important optically active substance that can transports nutrients and pollutants from terrestrial to aquatic systems. Additionally, it is used as a measure of water quality. To investigate the source and composition of CDOM, we used chemical and fluorescent analyses to characterize CDOM in Heilongjiang. The composition of CDOM can be investigated by excitation-emission matrix (EEM) fluorescence and parallel factor analysis (PARAFAC). PARAFAC identified four individual components that were attributed to microbial humic-like (C1) and terrestrial humic-like (C2-4) in water samples collected from the Heilongjiang River. The relationships between the maximum fluorescence intensities of the four PARAFAC components and the water quality parameters indicate that the dynamic of the four components is related to nutrients in the Heilongjiang River. The relationships between the fluorescence component C3 and the biochemical oxygen demand (BOD5) indicates that component C3 makes a great contribution to BOD5 and it can be used as a carbon source for microbes in the Heilongjiang River. Furthermore, the relationships between component C3, the particulate organic carbon (POC), and the chemical oxygen demand (CODMn) show that component C3 and POC make great contributions to BOD5 and CODMn. The use of these indexes along with PARAFAC results would be of help to characterize the co-variation between the CDOM and water quality parameters in the Heilongjiang River.

Using a Gaussian decomposition approach to model absorption spectra of chromophoric dissolved organic matter

The chromophoric dissolved organic matter (CDOM) is a significant water constituent influencing inherent and apparent optical properties of natural waters and plays a key role in ecosystem functioning. The spectral slope (S) describing the approximate exponential decline in CDOM absorption with increasing wavelength is widely used for tracing changes in the chemical composition of CDOM. The currently accepted method of characterizing CDOM absorption (i.e., fitting a simple exponential model) can lead to loss of information and large errors. We propose a better method for modeling CDOM absorption spectra based on a Gaussian decomposition approach that removes the errors associated with the choice of the spectral range used to estimate S. Using artificially generated spectra with known parameters (n=1000), we show that our method provides robust estimates of S closely resembling the original values. On average, the error on S estimations was 0.16% for the proposed method compared to 27% and 11% for the traditional modeling approaches fitted over 300–700nm and 240–700nm respectively. We further demonstrate the ability of the method to decompose and model chromophores present in complex spectra from oceanic water samples from around the world. The proposed method opens avenues for long-term or cross-site comparison studies of the dynamics of the CDOM pool and constitutes a promising supplement to techniques based on CDOM fluorescence.

Investigation of seasonal variability of CDOM fluorescence in the southern changjiang river estuary by EEM-PARAFAC

The southern Changjiang River Estuary has attracted considerable attention from marine scientists because it is a highly biologically active area and is biogeochemically significant. Moreover, land-ocean interactions strongly impact the estuary, and harmful algal blooms (HABs) frequently occur in the area. In October 2010 and May 2011, water samples of chromophoric dissolved organic matter (CDOM) were collected from the southern Changjiang River Estuary. Parallel factor analysis (PARAFAC) was used to assess the samples’ CDOM composition using excitation-emission matrix (EEM) spectroscopy. Four components were identified: three were humic-like (C1, C2 and C3) and one was protein-like (C4). Analysis based on spatial and seasonal distributions, as well as relationships with salinity, Chl a and apparent oxygen utilization (AOU), revealed that terrestrial inputs had the most significant effect on the three humic-like Components C1, C2 and C3 in autumn. In spring, microbial processes and phytoplankton blooms were also important factors that impacted the three components. The protein-like Component C4 had autochthonous and allochthonous origins and likely represented a biologically labile component. CDOM in the southern Changjiang River Estuary was mostly affected by terrestrial inputs. Microbial processes and phytoplankton blooms were also important sources of CDOM, especially in spring. The fluorescence intensities of the four components were significantly higher in spring than in autumn. On average, C1, C2, C3, C4 and the total fluorescence intensity (TFI) in the surface, middle and bottom layers increased by 123%–242%, 105%–195%, 167%–665%, 483%–567% and 184%–245% in spring than in autumn, respectively. This finding corresponded with a Chl a concentration that was 16–20 times higher in spring than in autumn and an AOU that was two to four times lower in spring than in autumn. The humification index (HIX) was lower in spring that in autumn, and the fluorescence index (FI) was higher in spring than in autumn. This result indicated that the CDOM was labile and the biological activity was intense in spring.

Retrieval of CDOM and DOC Using In Situ Hyperspectral Data: A Case Study for Potable Waters in Northeast China

Chromophoric dissolved organic matter (CDOM), the light absorbing fraction of dissolved organic carbon (DOC), together with phytoplankton and total suspended matter are the main optically active components could be retrieved by remote sensing data. Generally, different composition of DOC and CDOM corresponds to different water surface reflectance. Absorption properties of CDOM and retrieval models for CDOM and DOC were investigated with data from potable reservoirs located in the central of Jilin Province. Water sampling field surveys were conducted on 15, 16 and 19 of September 2012 across the Shitoukoumen, Erlonghu and Xilicheng reservoirs, respectively. Both empirical regression (single band model and band ratio model) and partial least squares coupled with back-propagation artificial neural models (PLSBPNN) were established to estimate CDOM absorption coefficient at 355 nm [aCDOM(355)] and DOC concentration with in situ measured remote sensing reflectance. It was found that the band ratio models and PLSBPNN model performed well for estimating DOC concentration while the band ratio models yielded the best result in retrieval CDOM. Moreover, all the three models performed better on the DOC concentration estimation than the performance on aCDOM(355). Band ratio models outperformed (R 2 = 0.55) other models for estimating CDOM absorption coefficient, while PLSBPNN model outperformed other models with respect to DOC estimation (R 2 = 0.93). High spectral slope values indicated that CDOM in the potable waters primarily comprised low molecular weight organic substances; while sources of DOC were mainly coming from exogenous input, which was the main reason lead to the difference of model performances on DOC and aCDOM(355) estimation. The algorithms developed in this study is needed to be tested and refined with more in situ spectral data, also future work is still needed to be undertaken for characterizing the dynamic of the potable water quality with remotely sensed imagery.

Seasonal Variations of CDOM Optical Properties in Rivers Across the Liaohe Delta

Absorption and fluorescence properties of chromophoric dissolved organic matter (CDOM) in rivers across the Liaohe River Delta, a large estuary located in the southern region of northeast China, were investigated using spectroscopy and fluorescence to analyze CDOM characteristics, composition and sources in winter (January), spring (April and May) and autumn (September) 2013. Results indicated that CDOM absorption of ice samples was lower compared to water samples. CDOM absorption also showed significant spatial variation but not temporal variation. In contrast, dissolved organic carbon (DOC) concentrations showed obvious temporal characteristics. A stronger linear relationship was found between CDOM absorption and DOC concentration in winter (water, R 2 = 0.95, p < 0.001; ice, R 2 = 0.85, p < 0.001) compared to samples from other seasons (April, R 2 = 0.51, p < 0.01; May, R 2 = 0.34, p < 0.05; September, R 2 = 0.45, p < 0.01). CDOM fluorescence varied over a large range across seasons, with the highest levels observed in January at the Xisha River (XSR). Moreover, strong linear relationships were also observed between CDOM absorption and fluorescence intensity at 355 nm [Fn(355)] in January (R 2 = 0.87, p < 0.001), May (R 2 = 0.76, p < 0.001) and September (R 2 = 0.94, p < 0.001). Also based on study findings, CDOM fluorophores identified by 3-D excitation-emission matrices (EEMs) illustrated that the rivers across the Liaohe Delta were seriously polluted by anthropogenic disturbances, exhibiting strong protein-like fluorescence of CDOM in the water column. In addition, the results also confirm that absorption and EEMs would be useful tools for tracing the sources and characteristics of CDOM and monitoring riverine water quality.

A new algorithm to retrieve chromophoric dissolved organic matter (CDOM) absorption spectra in the UV from ocean color

AbstractAccurate estimation of the absorption coefficient (ag) for chromophoric dissolved organic matter (CDOM) over ultraviolet (UV) and short visible radiation wavelengths (with λ = 275–450 nm) is crucial to provide a robust assessment of the biogeochemical significance of UV in the global ocean. Using a training data set spanning a variety of water types from the clearest open ocean to dynamic inshore waters, a novel algorithm to accurately resolve CDOM absorption spectra from ocean color is presented. Employing a suite of multivariate statistical approaches (principal component analysis, cluster analysis, and multiple linear regression), this new algorithm was developed with matched field data for CDOM spectra and remote sensing reflectance (Rrs) at Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) bands. Freed from any presupposition about CDOM spectral shape or conventional spectral extrapolations from visible data, our algorithm allows direct retrieval of a fully resolved CDOM absorption spectrum over UV wavelengths from visible Rrs and further enables a global scale view of the dynamics of CDOM over different water types. Accuracy of ag retrieval is good, with a mean absolute percent difference for ag in the UV of ∼25%. With fully resolved spectra, maps of calculated CDOM spectral slopes (S275–295, S350–400) and slope ratios (SR) are presented with the potential to provide new information about the chemical composition (e.g., molecular weight and aromaticity), sources, transformation, and cycling pathways of CDOM on global as well as regional scales. The new algorithm will contribute to improved accuracy for photochemical and photobiological rate calculations from ocean color.

Stream water carbon controls in seasonally snow-covered mountain catchments: impact of inter-annual variability of water fluxes, catchment aspect and seasonal processes

Stream water carbon (C) export is one important pathway for C loss from seasonally snow-covered mountain ecosystems and an assessment of overarching controls is necessary. However, such assessment is challenging because changes in water fluxes or flow paths, seasonal processes, as well as catchment specific characteristics play a role. For this study we elucidate the impact of: (i) changes in water flux (by comparing years of variable wetness), (ii) catchment aspect [north-facing (NF) vs. south-facing (SF)] and (iii) season (snowmelt vs. summer) on all forms of dissolved stream water C [dissolved organic C (DOC), chromophoric dissolved organic matter (CDOM) and dissolved inorganic C (DIC)] in forested catchments within the Valles Caldera National Preserve, New Mexico. The significant correlation between annual water and C fluxes (e.g. DOC r2 = 0.83, p < 0.02) confirms annual stream water discharge as the overarching control on C efflux, likely from a well-mixed ground water reservoir as indicated by previous research. However, CDOM exhibited a dominantly terrestrial fluorescence signature (59–71 %) year round, signaling a strong riparian and near stream soil control on CDOM composition. During snowmelt, the role of water as C transporter was superimposed on its control as C reservoir, when the NF stream transported significantly more soil C (40 % DOC, 56 % DIC) than the SF stream as a result of hillslope flushing. Inter-annual variations in winter precipitation were paramount in regulating annual stream C effluxes, e.g., reducing C effluxes three-fold after a dry (relative to wet) winter season. During the warmer summer months % dissolved oxygen saturation decreased, δ13CDIC increased and CDOM assumed a more microbial signature, consistent with heterotrophic respiration in the stream and riparian soils. As a result of stream C incubation and soil respiration, \( P_{{{\text{CO}}_{2} }} \) increased up to 12 times atmospheric values leading to substantial degassing.