Dissolved Organic Matter In Surface Waters Research Articles

Dissolved organic matter characteristics within the Lake Superior watershed

Within the aquatic carbon cycle, dissolved organic carbon (DOC) plays key roles: as a link between terrestrial and aquatic systems, a major food source for biota, a player in photochemical reactions and a sunscreen/competitor for light for aquatic organisms. The molecular-level composition of the dissolved organic matter (DOM) is believed to determine the efficacy of DOM in these roles. While much detailed compositional information is available on selected DOM isolates from both freshwater and marine systems, relatively few studies have presented molecular information for multiple sites within the same system and few of these have focused upon large lakes. Therefore, we present an initial study of surface water DOM characteristics in a temperate large lake (Lake Superior, USA) and selected tributaries. UV–visible spectroscopy, electrospray mass spectrometry and Fourier transform infrared spectroscopy (FTIR) analyses of bulk DOM, along with FTIR and direct temperature-resolved mass spectrometry analyses of C 18 extracts, illustrate a shift from higher molecular weight organic matter enriched in protein and lignin at the tributary sites toward compositionally different material in the open lake. Differences in optical characteristics and DOC concentration between tributary and open lake sites are consistent with possible photodegradation of tributary DOM. Comparison of our FTIR data with those from a previous Chesapeake Bay study indicates that DOM in both the freshwater tributaries and the open lake contain a wider range of aromatic and carbohydrate components than that in the saltwater tributary and bay. This highlights the need for further cross system comparisons of natural organic matter composition.

Long‐term photochemical and microbial decomposition of wetland‐derived dissolved organic matter with alteration of 13C:12C mass ratio

We investigated the long‐term photochemical and microbial decomposition of biologically recalcitrant humic‐like dissolved organic matter (DOM) leached from a vascular wetland plant, the common rush (Juncus effusus). Although the leachate would have been characterized as biologically recalcitrant by short‐term (<14 d) bioassays, microbes decomposed 51% of its organic carbon in 898 d with a first‐order biological decomposition coefficient of 0.0008 d‐1 in darkness. Solar radiation exposure accelerated the decomposition of leachate. Under 459‐d exposure to surface solar radiation, up to 90% of organic carbon was mineralized. During the exposure, the photochemical reactions preferentially mineralized the 12C fraction of organic carbon and enriched the 13C of organic carbon by 6% in the residual leachate. Solar radiation also decomposed nearly completely (up to 99.7%) chromo‐ and fluorophores of DOM. A 439‐d bioassay following the solar radiation exposure resulted in up to 97.3% mineralization of organic carbon. Solar radiation together with microbial metabolism can completely mineralize at least some forms of wetland‐derived DOM in surface waters with sufficiently long residence times.

Chemical characterization of DOM in channels of a seasonal wetland

Although wetlands are known to be important sources of dissolved organic matter (DOM) within watersheds, production of DOM within wetlands is not well understood. In the Okavango Delta, a large wetland located in Botswana, large amounts of DOM produced in the wetland are transported in the river network and to the subsurface. The purpose of this study was to gain insight into environmental processing of DOM in wetland surface waters by examining chemical characteristics of plant litter leachates and fulvic acids isolated from two surface water sites in the Panhandle (PHFA) and Seasonal Swamp (SSFA) of the Okavango Delta. Spectroscopic properties measured over the course of leaching experiments indicated a greater abundance of plant-derived DOM over time. Results of elemental and 13CNMR analyses showed that aromaticities and C:N ratios of PHFA and SSFA and a Cyperus papyrus leachate fulvic acid (CPLFA) were in the range typical for fulvic acids derived from vascular plants. Fluorescence analyses of fulvic acids using parallel factor analysis (PARAFAC) further indicated the importance of plant litter sources in surface water DOM. Environmental processing of DOM in downstream surface waters by bacterial and photodegradation was suggested by higher N and S content for SSFA compared to CPLFA and by differences in δ15N, δ34S, δ13C and fluorescence signatures among the 3 fulvic acid samples. These chemical characterization results suggest that a progressive enrichment of DOM by plant-derived material occurs with distance downstream and that this DOM undergoes some environmental processing within the surface water system.

Influence of Dissolved Organic Matter and Fe(II) on the Abiotic Reduction of Pentachloronitrobenzene

Nitroaromatic pesticides (NAPs) are hydrophobic contaminants that can accumulate in sediments by the deposition of suspended solids from surface waters. Fe(II) and dissolved organic matter (DOM), present in suboxic and anoxic zones of freshwater sediments, can transform NAPs in natural systems. We studied the reduction of pentachloronitrobenzene (PCNB) to pentachloroaniline (PCA) in controlled studies using Fe(II) and surface water DOM isolates from Pony Lake, Antarctica, and Suwannee River, GA, in unfiltered and 0.45 microm filtered solutions. We observed rapid reduction of PCNB to PCA in the presence of Fe(II) and DOM (t(1/2) approximately = 30 min to 4 h) and very limited reduction in DOM-only systems. DOM in unfiltered systems inhibited iron colloid formation and potentially limited the formation of reactive Fe(ll)-iron colloid surface complexes, causing reductive transformation in Fe(II)-DOM media to be slower in some cases relative to Fe(ll)-only controls. Conversely, in 0.45 microm filtered solutions, PCNB reduction in Fe(III)-DOM media was faster than the Fe(II)-only controls, suggesting that DOM enhances the reductive capacity of Fe(ll) in the absence of iron colloids. This work shows that DOM may significantly affect the reactivity of Fe(ll) toward NAPs under suboxic and anoxic conditions in natural wetland sediments.

Hydroclimatic controls on dissolved organic matter (DOM) characteristics and implications for trace metal transport in Hwangryong River Watershed, Korea, during a summer monsoon period

AbstractFrequent heavy rainfalls during the East Asian summer monsoon drastically increase water flow and chemical loadings to surface waters. A solid understanding of hydroclimatic controls on watershed biogeochemical processes is crucial for water quality control during the monsoon period. We investigated spatio‐temporal variations in the concentrations and spectroscopic properties of dissolved organic matter (DOM) and the concentrations of trace metals in Hwangryong River, Korea, during a summer period from the relatively dry month of June through the following months with heavy rainfall. DOM and its spectroscopic properties differed spatially along the river, and also depended on storm and flow characteristics around each sampling time. At a headwater stream draining a forested watershed, the concentrations (measured as dissolved organic carbon (DOC)), aromaticity (measured as specific UV absorbance at 254 nm), and fulvic acid‐ and protein‐like fluorescence of DOM were higher in stormflow than in baseflow waters. DOC concentrations and fluorescence intensities increased along the downstream rural and urban sites, in which DOC and fluorescence were not higher in stormflow waters, except for the ‘first flush’ at the urban site. The response of DOM in reservoir waters to monsoon rainfalls differed from that of stream and river waters, as illustrated by storm‐induced increases in DOM aromaticity and fulvic‐like fluorescence, and no significant changes in protein‐like fluorescence. The results suggest that surface water DOM and its spectroscopic properties differentially respond to changes in hydroclimatic conditions, depending on watershed characteristics and the influence of anthropogenic organic matter loadings. DOC concentrations and intensities of spectroscopic parameters were positively correlated with some of the measured trace metals (As, Co, and Fe). Further research will be needed to obtain a better understanding of climate effects on the interaction between DOM and trace metals. Copyright © 2006 John Wiley & Sons, Ltd.

Spatial and diurnal dynamics of dissolved organic matter (DOM) fluorescence and H2O2 and the photochemical oxygen demand of surface water DOM across the subtropical Atlantic Ocean

Diurnal dynamics of dissolved organic matter (DOM) fluorescence and hydrogen peroxide (H2O2) concentrations were followed in the upper 100 m of the water column at five stations across the subtropical Atlantic Ocean in July and August 1996. The 10% levels of surface solar radiation for the ultraviolet (UV) B range (at 305‐ and 320‐nm wavelengths) were at 16 and 23 m in depth and for the UVA range (at 340‐ and 380‐nm wavelengths) were at 35 and 63 m in depth, respectively. The DOM fluorescence decreased over the course of the day, whereas H2O2 concentrations increased, especially in the diurnally stratified surface water layers extending to 10‐50‐m depth. In situ H2O2 net production varied between 5.5 nmol L−1 h−1 at 5‐m depth and 1 nmol L−1 h−1 at 40‐m depth, resulting in an H2O2 net production of ~38 µmol m−2 d−1 in the upper 50 m of the water column. Photochemical oxygen (O2) demand of water collected at 10‐m depth in the early morning and exposed to surface solar radiation varied between 0.9 and 2.8 µmol O2 L−1 d−1 and was found to be consistently higher (by a 1.3‐8.3‐fold measure) than bacterial respiration (measured in 0.8 µm‐filtered seawater in the dark). UVB radiation was responsible for 0‐30% of the photochemical O2 demand. A simple one‐dimensional physical model was combined with a photochemical/biological model in order to describe the photochemical production of H2O2 at different depth layers over the course of the day and to determine the contribution of physical versus biological processes in terms of the loss of H2O2 from the surface layers in the late afternoon. The model reflects well the observed diurnal H2O2 dynamics. It further provides evidence that mainly biological breakdown determines the loss of H2O2 in the upper 50 m of the water column during the day; however, in the late afternoon, vertical mixing is important in transporting H2O2 from the uppermost 5‐m layer to the 10‐20‐m layers.

Molecular masses and chromophoric properties of dissolved organic ligands for copper(II) in oceanic water

Abstract The distribution of molecular masses of organic ligands for copper(II) in oceanic water was investigated. The bulk dissolved organic matter (DOM) was fractionated by ultrafiltration and organic ligands were extracted from the resultant fractions by using immobilized metal ion affinity chromatography (IMAC). Contributions of total organic ligands were 2.0–4.4% of the bulk DOM in surface waters, as determined by the UV absorbance. In the distribution of molecular masses of organic ligands, relative contribution of the fraction with low molecular masses ( 10,000 Da fraction. The distribution of molecular masses of organic ligands shifted to higher molecular masses, as compared with that of the bulk DOM. The fluorescence intensities of organic ligands were shown to be associated with carboxyl contents, based on peak excitation/emission wavelengths and the pH-dependence of fluorescence. Two ligand classes with different conditional stability constants (log KCuL′≈7 and 9) were determined from fluorescence quenching of ligand fractions during copper(II) titration. Organic ligands in low molecular mass fractions were relatively weak and strong ligands occurred in higher molecular mass fractions. It is suggested that the weaker ligand sites would consist of two or more carboxyl groups (log KHL′=4), whereas carboxyl groups (log KH2L′=2), which are protonated at lower pH, and primary amine may additionally contribute to the formation of more stable copper(II) complexes of the stronger ligand.

Detection of domestic wastes in Kurose river using synchronous fluorescence spectroscopy

This study was conducted to determine the applicability of synchronous fluorescence spectroscopy (SFS) in differentiating natural organic matter (NOM) from dissolved organic matter (DOM) derived from domestic wastes and in detecting their presence in Kurose River. Sewage effluent, gray water, water extracts from leaf mold and top soil, respectively, algal culture media, and water samples from different points of the river were analyzed. A laboratory mixing experiment was conducted by combining the leaf mold water extract (LM-NOM) and sewage effluent at different ratios. The SFS of the samples were run at offset value of Δ λ=40 nm (Δ λ= λ em− λ ex). The corrected synchronous fluorescence spectra of LM-NOM and the sewage effluent showed different spectral features. A very intense peak at about 531 nm was observed in the sewage effluent spectrum which was not found in the LM-NOM sample. The mixing of fluorescing organic material in LM-NOM and sewage effluent resulted in the simple addition of their respective spectra. This allows the detection of pollutants in surface waters by the appearance of characteristic peaks of the added wastewater. The corrected spectra of the river water sample from a relatively unpolluted point of Kurose River exhibited a simple spectrum with a single peak while the spectra of those collected downstream showed peaks similar to those found in the sewage effluent and gray water spectra. These results demonstrate the applicability of SFS in detecting the presence of different DOM in surface waters.

Role of photoreactions in the formation of biologically labile compounds from dissolved organic matter

Dissolved organic matter (DOM) can be degraded by sunlight into a variety of photoproducts that stimulate the growth and activity of microorganisms in aquatic environments. All biologically labile photoproducts identified to date fall into one of four categories: (1) low‐molecular‐weight (MW) organic compounds (carbonyl compounds with MW of <200); (2) carbon gases (primarily CO); (3) unidentified bleached organic matter; and (4) nitrogen‐ and phosphorus‐rich compounds (including NH4+ and PO43−). A number of laboratory studies using bacterial bioassay approaches have shown that the photochemical breakdown of DOM can stimulate biomass production or activity by 1.5‐ to 6‐fold. Results of photochemical studies, extrapolated to estimate formation rates of biologically available photoproducts from DOM in surface waters, also predict important biological roles for these compounds. In a continental shelf system, for example, full exposure of surface seawater to sunlight for one summer day can produce DOM photodegradation products equivalent to >20% of the bacterial carbon demand. Likewise, 30% of the bacterial nitrogen demand can be met by photodegradation of the nitrogen components of DOM, a process likely to be of particular importance in nitrogen‐limited systems. When considered on a depth‐integrated basis around the globe, at least 1.0 × 1015 g C and 0.15 × 1015 g N are estimated to be available annually for utilization by planktonic microorganisms through the conversion of light‐absorbing fractions of DOM to more biologically labile compounds. By comparison, direct photochemical mineralization of DOM is estimated to convert 12−16 × 1015 g C to CO2 annually.

Synchronous fluorescence spectroscopy of dissolved organic matter in surface waters: Application to airborne remote sensing

Synchronous fluorescence spectroscopy of dissolved organic matter (DOM) in surface waters is investigated as a method of inferring DOM composition. The synchronous fluorescence technique, which has previously been shown to provide structural information for polycyclic hydrocarbons, including crude oils, has been extended to DOM as a method for “fingerprinting” humic substance (HS) types. Fluorescence emission and synchronous fluorescence data were collected for 21 lake water samples. The lake sample synchronous spectra, which are compared with synchronous spectra of lignin and data derived from the literature of other model compounds, provide new insights into the structure of surface water DOM. Further support for this approach is provided by a comparison between the multiple linear regression result found when relating measured pH to fluorescence emission data (R 2 = 0.75) and that found when relating the measured pH to synchronous fluorescence data (R 2 = 0.90). Adapting the synchronous fluorescence spectral technique to a remote sensing context is discussed in terms of using the method for choosing optimal excitation wavelengths for a multiple-wavelength lidar.

Anomalous temperature dependence of resistivity in heavily doped germanium : Y. Otsuka, S. Ikehata, S. Kobayashi and W. Sasaki. Solid-St. Commun.20, 441 (1976)

A high-recovery technique of dissolved organic matter (DOM) isolation – reverse osmosis coupled with electrodialysis (RO/ED) – was used to isolate DOM from the North Atlantic Senegal-Mauritanian upwelling area surface water (5 m), North Atlantic oxygen minimum water (415 m) and deep water (3000 m), North Pacific subtropical gyre surface water (5 m), and North Pacific intermediate water (674 m) and deep water (3500 m). Samples were characterized by ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry with electrospray ionization (ESI FTICR-MS). RO/ED isolated DOM samples share a significant number of common formulas accounting for 54–79% of formulas in each sample. Total dissolved carbohydrate (TCHO) concentrations in RO/ED isolated DOM were specifically measured using a colorimetric method, and were found to have higher contribution to DOC than estimated by FTICR-MS data. Percentages of TCHO-C in DOC are in the range of 3.7–19.6% in all samples, with the North Pacific deep (3500 m) water having the lowest % and the North Atlantic upwelling core surface water having the highest %. Principal component analysis (PCA) using the relative magnitudes of MS peaks facilitated identification of specific peaks that are enriched in different samples. Peaks enriched in surface samples have higher H/C values than peaks enriched in deep samples, in both the North Atlantic DOM and the North Pacific DOM. This enrichment pattern is likely due to the selective photo-degradation of aromatic compounds and the bio-production of aliphatic and carbohydrate-like compounds in surface waters, and the selective bio-degradation of aliphatic and carbohydrate-like compounds with increasing depth. In further support of a photo-degraded signature for DOM in surface waters, photo-resistant and photo-produced molecular formulas were present in the highest numbers in the surface North Pacific subtropical gyre DOM. Peaks enriched in the North Pacific intermediate and deep DOM have significantly higher O/C values than the North Atlantic oxygen minimum layer and deep DOM, for both CHO formula compounds and CHON formula compounds. This difference in O/C values observed for the deep Pacific vs. Atlantic suggests oxidation of DOM, possibly via microbial activity during the ageing of DOM or the preferential remineralization of DOM from sinking particles at depth in the Pacific.