Insights on the Optical Properties of Estuarine DOM - Hydrological and Biological Influences (original) (raw)
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Estuarine, Coastal and Shelf Science, 2020
Dissolved organic matter (DOM) is an important source of carbon in aquatic ecosystems, and colored DOM (CDOM), which is smaller than 0.2 μm and interacts with ultraviolet (UV) and visible light, affects the spectral quality and quantity of light in water. In this study, the spatial and temporal variations of CDOM with changes in environmental conditions were investigated from March 2014 to May 2017 in the coastal waters and two estuaries (Zuari and Mandovi) of Goa, western India, and the major sources and sinks controlling the optical properties of these waters were identified. The CDOM absorption in the estuaries was two times higher than that of the coastal waters. It was also determined that the CDOM absorption at 412 nm (a g 412) in the coastal and estuarine waters significantly varied between seasons. The a g 412 was found to be higher in the coastal waters during the spring inter-monsoon (SIM) and fall inter-monsoon (FIM) than during the northeast monsoon (NEM). The high absorption during the SIM was of autochthonous origin, while terrigenous DOM was the primary contributor mainly during the FIM. The photobleaching of CDOM was highest during the SIM, resulting in the predominance of low-molecular-weight DOM in the coastal waters. This photobleaching of DOM also resulted in deeper UV light penetration, as indicated by the diffuse attenuation coefficient K d at 350 nm. In the Mandovi and Zuari estuaries, higher levels of CDOM were observed during the southwest monsoon (SWM) and SIM than the FIM and NEM. The terrigenous DOM contribution was higher in the estuaries during the SWM, while phytoplankton contributed to a higher level of CDOM during the SIM. CDOM exhibited non-conservative mixing behavior in the study region, as it decreased in estuaries with lower salinities and increased at salinities between 20 and 31. Considering the importance of CDOM in the carbon cycle, this study highlights the various sources and sinks of CDOM controlling the optical properties and biogeochemical processes of coastal and estuarine waters.
2000
The optical properties of chromophoric dissolved organic matter (CDOM) in Danish estuaries and coastal waters were investigated. A new method for estimating the spectral slope coefficient (S) was examined and found to give a closer fit for the measured absorption (92% reduction in sum of residuals) than the traditional method. The spectral pattern in residuals produced by the new technique enabled the identification of CDOM originating from areas of different land uses/types where S coefficients were similar. S values were found to behave conservatively with respect to salinity in all waters except for the off-shore North Sea region where CDOM from marine sources was suspected to have more influence. The specific absorption coefficient of CDOM did not vary significantly in Danish coastal waters. These waters are of particular interest in bio-optical studies as they allow the study of CDOM across a gradient from estuarine, brackish to near-oceanic environments. The results demonstrate that it is possible to develop regional models for the optical properties of CDOM which would then allow higher precision in remote sensing applications.
Marine Pollution Bulletin, 2019
The diurnal variations in water quality and optical properties of organic matter were studied in the Gautami-Godavari estuary during two contrasting seasons. Dissolved inorganic nitrogen (DIN) and silicate showed similar patterns with the tide during summer, whereas in winter contrasting trends were noticed. Three-folds higher N to P ratio was recorded in winter than in summer. The spectral slope ratio (S R) and specific ultra violet absorption coefficient (SUVA) peaked during summer (1.28 ± 0.09 and 3.95 ± 0.2) followed by winter (1.10 ± 0.18 and 1.91 ± 0.35). The parallel factor (PARAFAC) analysis of excitation emission matrix (EEM) fluorescence spectra was extracted three humic (C1, C2, C3) and one protein-like (tryptophan (C4)) fluorophore components. Humic like fluorophores inversely correlated with the tide in both the seasons, due to influence of seawater. In summer, the Chlorophyll a (Chl a) and dissolved organic carbon (DOC) showed positive correlations with humic like and C4 fluorophores, suggesting the insitu organic matter production. Dissolved organic matter (DOM) plays a crucial role in the biogeochemical processes of marine and aquatic environments (Benner 2002). Major composition of DOM is carbohydrates, phenols, amino acids and other structural compounds. A very small fraction of the DOM is coloured and exhibits absorption and fluorescence characteristics (Coble, 2007), known as chromophoric dissolved organic matter (CDOM). The CDOM in the coastal and marine environments are mainly terrestrial in origin due to degradation of upland vegetation runoff through the riverine systems and autochthonous physical and biogeochemical processes, including phytoplankton exudates, and leachates from the degraded organic matter through microbial and photochemical activity (Sieburth and Jensen, 1970; Tranvik et al., 1993; Coble, 2007; Shank et al., 2009). The nature and characterization of CDOM can be explained by its absorbance and fluorescence properties and in association with the derived parameters such as absorption coefficient, spectral slope and ratio, specific ultra violet absorption (SUVA), humic and protein like fluoropores (
Optical Proxies for Terrestrial Dissolved Organic Matter in Estuaries and Coastal Waters
Dissolved organic matter (DOM) absorbance and fluorescence were used as optical proxies to track terrestrial DOM fluxes through estuaries and coastal waters by comparing models developed for several coastal ecosystems. Key to using these optical properties is validating and calibrating them with chemical measurements, such as lignin-derived phenols—a proxy to quantify terrestrial DOM. Utilizing parallel factor analysis (PARAFAC), and comparing models statistically using the OpenFluor database (http://www.openfluor.org) we have found common, ubiquitous fluorescing components which correlate most strongly with lignin phenol concentrations in several estuarine and coastal environments. Optical proxies for lignin were computed for the following regions: Mackenzie River Estuary, Atchafalaya River Estuary (ARE), Charleston Harbor, Chesapeake Bay, and Neuse River Estuary (NRE) (all in North America). The slope of linear regression models relating CDOM absorption at 350 nm (a 350) to DOC and to lignin, varied 5–10-fold among systems. Where seasonal observations were available from a region, there were distinct seasonal differences in equation parameters for these optical proxies. The variability appeared to be due primarily to river flow into these estuaries and secondarily to biogeochemical cycling of DOM within them. Despite the variability, overall models using single linear regression were developed that related dissolved organic carbon (DOC) concentration to CDOM (DOC 2 = 40 ± 2 × a + ± 350 138 16; R = 0.77; N = 130) and lignin (8) to CDOM (= 2.03 ± 0.07 × a .47 2 − 0 ± = 8 350 0.59; R 0.87; N = 130). This wide variability suggested that local or regional optical models should be developed for predicting terrestrial DOM flux into coastal oceans and taken into account when upscaling to remote sensing observations and calibrations.
Marine Chemistry, 2004
One of the most important natural sunlight absorbing substances in water is the chromophoric dissolved organic material (CDOM). Its influence on optical properties has been studied for many years, but questions of how its structural and optical characteristics change in the environment still remain. CDOM water samples were collected from coastal waters of southwest Florida during three cruises in the year 2001: one in the dry season (June) and two in the rainy season (September and November). Analyses of molecular mass (MM) distribution and optical characteristics were done using the flow field-flow fractionation (FlFFF) separation technique with absorbance and fluorescence detectors. On the basis of the results, mixing, source variability, degradation, and excited state quenching processes are important determinants of CDOM composition and optical properties in this region. During the dry season, the MM distribution did not change significantly spatially. In September and November, higher MM compounds and an increase of CDOM fluorescence were observed, found inside the rivers and in near-coastal samples, consistent with the rivers being a significant CDOM source. Results suggest that chromophores were broken down to smaller MM compounds faster than fluorophores, indicating that the fluorescent moieties are more resistant to degrading/removal processes than the non-fluorescing compounds, even though the fluorophores were always centered at lower MM. On the other hand, the fluorescence quantum yield decreased faster than absorbance coefficients, suggesting that fluorophores were quenched by complexing with some metals. For all months, the chromophores' MM for offshore samples remained the same. The carbon concentration ratio between chromophores and fluorophores changed from lower to higher salinities, suggesting that the composition of the waters changed toward offshore. The differences in the optical characteristics, MM distributions, and carbon concentration observed suggest that the CDOM sources, physical, and photochemical degradation processes change seasonally.
Marine Chemistry, 2010
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.
2011
The aim of this study was to investigate the dynamics of the fluorescent dissolved organic matter (FDOM) in Paranaguá Estuarine System (PES) as to infer about the contribution of allochthonous FDOM to the estuarine waters in relation to tidal condition and seasons. Fluorescence spectroscopy was used for such purpose and DOM characterization through fluorescence emission was performed using excitation wavelengths of λex 350 nm and λex 450 nm, the two main fluorescence groups known to be present in natural DOM. Relations between emission wavelength (λem) and environmental variables, and the relevance of these variables to the different tides and seasons were identified by principal component analysis. The results showed that the first class of fluorophores (λex 350 nm) changed from the river (freshwater) towards the estuary, whilst the second class (λex 450 nm) has a more conservative nature and does not change as significantly as the first. Allochthonous DOM contribution to the estuarine system is intensified during the rainy season, especially in spring tides, whereas in the dry season the ratio of autochthonous DOM to total DOM in PES waters increased. We concluded that the variation of maximum λem of the first class of fluorophores (λex 350 nm) is mainly related to allochthonous contribution, whilst the maximum of emission for the second class of fluorophores (λex 450 nm) is dependent on the contribution of the different sources of organic matter (freshwater and marine water DOM contribution).
Marine Environmental Research, 2010
The light absorbing fraction of dissolved organic carbon (DOC), known as chromophoric dissolved organic matter (CDOM) showed wide seasonal variations in the temperate estuarine zone in front of the Po River mouth. DOC concentrations increased from winter through spring mainly as a seasonal response to increasing phytoplankton production and thermohaline stratification. The monthly dependence of the CDOM light absorption by salinity and chlorophyll a concentrations was explored. In 2003, neither DOC nor CDOM were linearly correlated with salinity, due to an exceptionally low Po river inflow. Though the CDOM absorbance coefficients showed a higher content of chromophoric dissolved organic matter in 2004 with respect to 2003, the spectroscopic features confirmed that the qualitative nature of CDOM was quite similar in both years.CDOM and DOC underwent a conservative mixing, only after relevant Po river freshets, and a change in optical features with an increase of the specific absorption coefficient was observed, suggesting a prevailing terrestrial origin of dissolved organic matter.