DOC-specific Absorption Research Articles

Using CDOM spectral shape information to improve the estimation of DOC concentration in inland waters: A case study of Andean Patagonian Lakes

For the last two decades different scientific disciplines have focused on lacustrine dissolved organic matter (DOM) given its importance in the biogeochemistry of carbon and in ecosystem functioning. New satellites supply the appropriate resolutions to evaluate chromophoric dissolved organic matter (CDOM) in inland waters, opening the possibility to estimate DOM at appropriate spatiotemporal scales. This requires, however, a robust relationship between CDOM and dissolved organic carbon (DOC). In this work, we evaluated the use of CDOM as a proxy of DOC in 7 Andean Patagonian lakes. Considering the entire data set, CDOM absorption coefficients (a355 and a440) were linearly related with DOC. Shallow lakes, however, drove this relationship showing a moderate relationship, whereas, deep lakes with lower colour presented a weaker relationship. Therefore, we assessed the use of CDOM spectral shape information to improve DOC estimates regardless of observed DOM differences due to climatic seasonality and lakes' morphometry. The use of well-known CDOM spectral shape metrics (i.e., S275–295 and a250:a365 ratio) significantly improved DOC estimation. Particularly, using a Gaussian decomposition approach we found that much of the variation in the spectral shape, associated with the variability of CDOM:DOC ratio, was explained by differences in two dynamic regions centred at 270 and 320 nm. A strong nonlinear relationship was found between the a270:a320 ratio and the DOC-specific absorption coefficients a*355 and a*440. This was translated into a further improvement in DOC estimation yielding the higher R2 and lower mean absolute differences (MAPD < 16%), either considering the entire data set or shallow and deep lakes separately. Our results highlight that incorporating the CDOM spectral shape information improves the characterization of the DOC pool of inland waters, which is particularly relevant for remote and/or inaccessible sites and has significant implications for the environmental management, biogeochemical studies and future remote sensing applications.

Variable responses of dissolved organic carbon to precipitation events in boreal drinking water lakes

In boreal regions, increased concentrations of dissolved organic carbon (DOC) have been linked to extreme wet years; however, less is known about the extent to which precipitation events are altering DOC concentration and quality. We assessed the effects of rain events on a suite of six lakes in Maine, U.S.A., to better understand how events alter DOC quantity and quality. DOC concentrations and DOC quality (measured as DOC-specific absorption coefficients (Specific Ultraviolet Absorbance (SUVA254 (also a*254), a*320, and a*380)) were quantified 24 h before, and at three time points (24–48 h, 5–7 days, and 3 weeks) after five different precipitation events. Our results revealed three types of responses across the lakes: (1) an initial spike in DOC concentrations of 30–133% and in the three quality metrics of 20–86% compared to pre-storm levels, followed by return to pre-storm concentrations; (2) a sustained increase in DOC concentrations (by 4–23%) and an increase in the three DOC quality metrics (by 1–43%) through the second post-storm sampling, with concentrations falling by the third post-storm sampling compared to pre-storm levels; and (3) no change during all sampling periods. Lake residence time was a key driver of changes in DOC concentration and DOC quality in response to storm events. Our research provides evidence that precipitation events contribute to short-term abrupt changes in DOC quantity and quality that are largely driven by key landscape and lake characteristics. These changes in DOC may have important implications for management of water utilities, including alteration or implementation of treatment strategies.

Optical properties of chromophoric dissolved organic matter during a phytoplankton bloom. Implication for DOC estimates from CDOM absorption

AbstractCoastal waters of the English Channel and the North Sea are affected by a massive spring phytoplankton bloom event of the prymnesiophyte Phaeocystis globosa every year. The impact of this bloom on DOM dynamics in coastal waters of the western English Channel was investigated through a 45 d long mesocosm experiment and field survey (2012–2014). Our results highlight that, over short time scales, marine production of DOM mostly affects CDOM absorption properties in the UV domain. Specifically, it has been evidenced that the acdom spectral slope between 320 nm and 412 nm (S320‐412) can be used as a relevant indicator of the DOM marine origin. The presence of a strong linear relationship between S320‐412 and the DOC specific absorption coefficient in phytoplankton bloom conditions reveals that this optical parameter provides complementary information to those obtained from the spectral slope between 275 nm and 295 nm, S275‐295 which has been recently described as a relevant tracer of the terrestrial DOM and a good descriptor of the specific absorption coefficient of dissolved organic carbon variability in the confining river‐influenced ocean margins. This study has further demonstrated that the joint use of S275‐295 and S320‐412 significantly improves the estimation DOC‐specific absorption coefficient in the presence of varying contribution of marine and terrestrial DOM. While such bands are still not available on ocean color sensors, this methodology represents a valuable tool for estimating DOC concentration from CDOM absorption measurements from in situ data obtained from classical discrete measurements or from autonomous platforms equipped with adapted spectrophotometer.

Variations in Spectral Slope in Lake Taihu, a Large Subtropical Shallow Lake in China

Spectral slope (S), describing the exponential decrease of the absorption spectrum over a given wavelength range, is an important parameter in the study of of chromophoric dissolved organic matter (CDOM) dynamics, and also an essential input parameter in remote sensing models. Furthermore, S is often used as a proxy for CDOM composition, including the ratio of fulvic to humic acids and molecular weight. The relative broad range in S values reported in the literature can be explained by the different spectral ranges and fitting methods used. A single exponential model is used to fit the S values for 17 investigations involving 458 samples in Lake Taihu from January to October in 2004. The average S value was 15.18 ± 1.39 μm −1 for the range of 280–500 nm, which fell within the range reported in the literature. The frequency distribution of S value basically obeyed a normal distribution. Significant differences in S values between summer and other seasons showed that phytoplankton degradation was one of the important sources of CDOM in summer, whereas CDOM mainly came from the river input in other seasons. Furthermore, the estimated S value decreased with increasing wavelength range used in regression. The maximum and minimum values derived from the regression were 17.89 ± 1.25 μm −1 and 13.62 ± 2.11 μm −1 for the wavelength ranges of 280–380 nm and 400–500 nm, respectively, a decrease of 23.9%. S values significantly decreased with the increase of CDOM absorption coefficients. CDOM absorption coefficients could be more appropriately estimated from exponential model introducing the variation of S with absorption coefficients, making them useful for a remote sensing bio-optical model of Lake Taihu. DOC-specific absorption coefficient a*(λ) and the parameter M describing molecular size of the humic molecules could also be used as a proxy for the sources and types of CDOM. A general relationship was found between S and a*(λ), and M values. S increased with the decrease of DOC-specific absorption coefficient and the increase of M corresponding to the decrease of molecular weight.

Spatial distribution and absorption characteristics with relation to fluorescence of chromophoric dissolved organic matter (CDOM) in Meiliang Bay of Lake Taihu

2004年3月对太湖梅梁湾有色可溶性有机物(CDOM))的吸收和荧光等光学行为进行研究,并由此探讨了CDOM的空间分布．结果表明,溶解性有机碳(DOC)的浓度在10．48-19．72 mg/L间变化,其均值为13．20±2．79 mg/L)CDOM在280 nm,355 nm和440 nm的吸收系数分别为18．73-31．91 m^-1(平均值23．19±4．36 m^-1)、4．63-7．14 m^-1(平均值5．76±0．91 m^-1)、1．45-2．99 m^-1(平均值1．92±0．40 m^-1))355 nm波长处CDOM的比吸收系数为0．34-0．57 L/(mg·m),平均值0．44±0．06 L/(mg·m))表征CDOM分子大小的比值a(250)/a(365)变化范围为5．05-7．55)355 nm的激发波长、450 nm的发射波长处的荧光值的变化范围0．79-3 04 nm^-1(平均值1．69±0．77 nm^-1)．CDOM吸收系数、DOC浓度、荧光强度的分析显示CDOM浓度呈现从河口往湾内、湾口递减的趋势．CDOM吸收与DOC浓度的相关性随波长的降低而增加,在短波部分存在明显的正相关．355 nm处的荧光值、DOC浓度与CDOM吸收系分别存在如下显著性正相关关系:F_n(355)=0．692(±0．135)a(355)-2．297(±0．786),a(355)=0．233(±0．061)DOC+2．690(±0．816)．280-500 nm、280-360 nm、360-440 nm指数函数斜率S值分别为13．86±0．91、18．54±1．11、12．93±0．92μm^-1,S值与比吸收系数之间存在显著的负线性相关关系,而与a(25)/a(365)值则存在显著的正线性关系．比吸收系数越大,a(250)/a(365)值和S值就越小,对应的CDOM分子量就越大,腐质酸的比例就越高．;Chromophoric dissolved organic matter (CDOM) is the light absorption fraction of dissolved organic carbon (DOC) that absorbs light in both the ultraviolet and visible ranges. The spatial and temporal distribution of CDOM in aquatic ecosystems can have important effects on ecosystem productivity, negatively impacting primary productivity due to CDOM absorbs light at both ultraviolet and visible wavelengths while positively impacting secondary productivity by fueling of microbial respiration by photo-degraded CDOM. In water color remote sensing, overlaps of pigment absorption spectra with CDOM absorption at blue wavelength generally complicate the use of chlorophyll a retrieval algorithms that are based on remotely sensed ocean color and lead to overestimated chlorophyll a concentration. In addition, high concentrations of CDOM can also act as a photoprotectant against UV damage for aquatic organisms. However, the protection against UV radiation provided by dissolved humic material for aquatic biota may be diminished if photodegradation of CDOM by UV radiation and acidification increase the UV transparency in lakes. CDOM absorption measurements and their relationship with DOC, and fluorescence are presented in Meiliang Bay of Lake Taihu based on a field investigation and lab analysis to show the spatial distribution of CDOM. Absorption spectral of CDOM was measured from 240 to 800 nm. Concentrations of DOC ranged from 10.48 to 19.72 mg/L with an average of 13.20 ±2.78 mg/L. CDOM absorption coefficients at 280 nm and 355 nm were in the range 18. 73-31. 91 m^-1 ( average 23.19±4.36mm^-1) and 4. 63-7. 14 mm^-1 (average 5. 76±0. 90 mm^-1)，respectively. The values of the DOC-specific absorption coefficient at 355 nm ranged from 0. 34 to 0. 57 L/( mg · m). Fluorescence emission at 450 nm, excited at 355 nm, had a mean value of 1. 69 ±0. 77 nmm^-1. A significant lake zone difference is found in DOC concentration, CDOM absorption coefficient and fluorescence. This regional distribution pattern was in agreement with the location of sources of yellow substance ： highest concentrations close to river mouth under the influence of river inflow, lower values in outlet of Meiliang bay. CDOM fluorescence and absorption coefficient were significantly and positively correlated. The results show a good correlation between CDOM absorption and DOC coefficients during 280-500 nm short wavelength intervals. The coefficient of variation between CDOM absorption and DOC concentration decreased with the increase of wavelength from 280 to 500 nm. The linear regression equations between fluorescence, DOC concentration and absorption coefficients at 355 nm are: F_n ( 355 ) = 0.692( ±0. 135)a(355)-2.291 (±0.786)，a(355) =0.233(±0.061)DOC +2.690(±0.816)，respectively. The exponential slope coefficient ranged from 11.0 to 14.9 um^-1 with a mean value 13. 86 ± 0. 91um^-1， 15. 8-20. 7 um^-1 with a mean value 18. 54 ± 1. 11 um^-1 and 9.9-13. 9 um^-1 with a mean value 12. 93 ± 0. 92um^-1 over the 280-500 nm, 280-360 nm and 360-440 nm intervals. A significant negative linear correlation was found between spectral slope coefficient and DOC specific absorption coefficient, but a significant positive linear correlation for spectral slope coefficient and the ratio of a( 250)/a( 365 ). Larger specific absorption coefficient corresponds to smaller a (250)/a (365) and spectral slope coefficient, which shows higher ratio humic in CDOM.

Photodegradation of DOC in a shallow prairie wetland: evidence from seasonal changes in DOC optical properties and chemical characteristics

Wetlands across the Canadian prairies are typically shallow ( 10 mg l−1). Studies have shown that DOC in such shallow wetlands is not as reliable an indicator of ultraviolet radiation (UVR) attenuation as it is in clearwater. Changes in DOC character and composition as a result of sunlight exposure might provide a reasonable explanation for this observation. To test this, we investigated seasonal changes in DOC optical and chemical properties in a shallow prairie wetland over a 2-year period. Although DOC concentration increased at least two-fold from spring until fall, DOC specific absorption (at 350 nm) and fluorescence decreased by 30 and 32%, respectively, for the same period. In both years, seasonal decreases in DOC molecular weight and size (from measurements of tangential filtration and mass electrospray mass spectrometry) were reflected in concomitant increases in spectral slope. 13C NMR analysis of DOC isolated on XAD-8 resins revealed a 49% decrease in aromatic moieties when spring values were compared to those in the fall. As well, δ13C signatures of this isolated DOC became heavier seasonally. In a short term photodegradation experiment (6 days) we noted a 47% decline in DOC specific absorption coefficients at 350 nm and a 15% increase in spectral slope when water exposed to the total light spectrum was compared to that of a dark control. Taken together, all of these observations were consistent with the occurrence of seasonal DOC photodegradation in shallow prairie wetlands and underlined the importance of this process in shaping DOC character and composition in these hydrologically dynamic systems. Our data also indicates that constant mixing and shallow depths in these wetlands were factors which enhanced DOC photodegradation. Although the high DOC concentrations of prairie wetlands should theoretically offer protection for their biota, seasonal photodegradation of DOC means that these systems may not be as protected as their high DOC concentrations suggest.

Spectral Absorption and Fluorescence of Chromophoric Dissolved Organic Matter in Shallow Lakes in the Middle and Lower Reaches of the Yangtze River

ABSTRACT The optical properties of chromophoric dissolved organic matter (CDOM) in shallow lakes in the middle and lower reaches of the Yangtze River were investigated. Concentrations of dissolved organic carbon (DOC) ranged from 2.7 to 10.1 mg/L with an average of 5.65±1.70 mg/L. All spectra of CDOM had a common shape with maximal absorption in shorter wavelength and minimal absorption in longer wavelength. CDOM absorption coefficients at 280 nm and 355 nm were in the range 3.52–24.22 m−1 (average 8.69±4.76 m−1) and 0.53–8.08 m−1 (average 2.03±1.65 m−1), respectively. The values of the DOC-specific absorption coefficient at 355 nm ranged from 0.09 to 0.80 L(mg org.C)−1m−1. The exponential slope coefficient ranged from 0.015 to 0.023 nm−1 with a mean value 0.020±0.002 nm−1 over the 280–400 nm interval. Fluorescence emission at 450 nm, excited at 355 nm, had a mean value of 0.86 nm−1. CDOM fluorescence and absorption were significantly and positively correlated. The coefficient of variation between CDOM absorption and DOC concentration decreased with the increase of wavelength from 280 to 800nm. The CDOM absorption measurements are of use to study regional distribution of CDOM, contribution of CDOM to light attenuation, and development of regional models for the optical properties of CDOM which allow higher precision in remote sensing applications of water quality monitoring.

A Preliminary Study of Chromophoric Dissolved Organic Matter (CDOM) in Lake Taihu, a Shallow Subtropical Lake in China

The optical properties and spatial distribution of chromophoric dissolved organic matter (CDOM) in Meiliang Bay of Lake Taihu were evaluated and compared to the results in literature. Concentrations of dissolved organic carbon (DOC) ranged from 8.75 to 20.19 mg L-1 with an average of (13.10 +/- 3.51) mg L-1. CDOM absorption coefficients a(lambda) at 280 nm, 355 nm, and 440 nm were in the range 11.28...33.46 m(-1) (average (20.95 +/- 5.52) m(-1)), 2.42...7.90 m(-1) (average (4.92 +/- 1.29) m(-1)), and 0.65...2.44 m(-1) (average (1.46 +/- 0.44) m(-1)), respectively. In general, CDOM absorption coefficient and DOC concentration were found to decrease away from the river inflow to Meiliang Bay towards the lake center. The values of the DOC-specific absorption coefficients a*(lambda), given as absorption coefficient related to mass concentration of organic carbon (C) ranged from 0.28 to 0.47 L mg(-1) m(-1) at 355 nm. The determination coefficients between CDOM absorption and DOC concentration decreased with the increase of wavelength from 280 to 550 nm. The linear regression relationship between CDOM absorption at 280 nm and DOC concentration was following: a(280 nm) = 1.507 L mg(-1) m(-1) (.) DOC + 1.215 m(-1). The spectral slope S values were dependent on the wavelength range used in the regression. The estimated S values decreased with increasing wavelength range used. A significant negative linear relationship was found between CDOM absorption coefficients, DOC-specific absorption coefficients and estimated S values especially in longer wavelength range. The linear regression relationship between DOC-specific absorption coefficients at 440 nm and estimated S values during the wavelength range from 280 to 500 nm was following: a*(440 nm) = (-0.021 mu m (.) S + 0.424) L mg(-1) m(-1).

Changes in UV penetration associated with marine intrusions and freshwater discharge in a shallow coastal lagoon of the Southern Atlantic Ocean

We studied the changes in UV penetration associated with the dynamics of a shallow (mean depth = 0.6 m) coastal lagoon of South America that communicates periodically with the Atlantic Ocean. Two characteristic situations, i.e., freshwater dominance and salt-wedge intrusion were considered. Nine stations were sampled along the main axis of the lagoon, the main tributary, and the adjacent coast. The attenuation in the UV-B, UV-A and PAR wavebands were related to changes in the concentration of dissolved organic carbon (DOC), chlorophyll a (chl a), absorption (a d) of the chromophoric dissolved organic matter (CDOM), fluorescence of CDOM (Fd), organic (OSS) and inorganic (ISS) suspended solids. The area most influenced by the marine intrusion showed the lowest DOC concentration (1.8 mg l -1 ) and the highest UV penetration. In this area, the depth corre- sponding to 10% of the irradiance below the surface (Z10 %) accounted for 66 and 100% of the water column for the UV-B and UV-A wavebands, respectively (diffuse attenuation coefficient, Kd = 7.3 and 2.1 m -1 ). The other zones of the lagoon (DOC = 5.7 to 9.3 mg l -1 ) presented low UV-B penetration (Kd = 29 to 64 m -1 ), and the Z10% for UV-A accounted for 30 to 64% of the water column (Kd = 7 to 14 m -1 ). Under both hydrological situations, the Z10 % for PAR reached the bottom in most parts of the lagoon (Kd = 1 to 5 m -1 ). A significant reduction in DOC-specific absorption (a d*) during the marine intrusion indicated a change in the absorption characteristics of the CDOM. The variability in Kd for UV-B radi- ation was mainly explained by the concentration of dissolved and particulate organic substances, while the Kd values for UV-A and PAR were best predicted by the concentration of DOC and chl a.