SCOPE 42 - Biogeochemistry of Major World Rivers

9.1 INTRODUCTION

Organic matter in natural waters covers a continuous size spectrum ranging from free monomers via macromolecules and colloids to aggregates and large particles (e.g. Thurman 1985). For practical reasons, this reservoir is usually subdivided into two major pools by operational definition: organic matter that upon filtration of a water sample is retained on a 0.45 µm filter is termed POM (particulate organic matter), whereas that passing on to the filtrate is termed DOM (dissolved organic matter). Analogously, the terms POC (PON POP) and DOC (DON DOP) apply when considering the organic matter's carbon (nitrogen, phosphorus) only. In this chapter we are concerned with properties, geochemical significance and global fluxes of DOC in rivers. Although considerable data exist on the quantity of DOC in rivers, so far no more than about one-quarter has been chemically characterized as carbon in carbohydrates, amino acids, hydrocarbons, fatty acids and phenolic compounds. The uncharacterized fraction has been found to be rather stable and is variously termed as fulvic acid (Oden 1919), yellow organic acid (Shapiro 1957), Gelbstoff (Kalle 1966), aquatic humus (Gjessing 1976) and 'unknown photoreactive chromophores' (Zafiriou 1985). This fraction has a plant and soil source and the carbon in it has a chance to reach the open sea in a reduced state. It thus provides a link between the large reactive pools of organic carbon on land and in the sea. Its ultimate fate in the ocean is oxidation, the pathways and rates of which are still a matter of reséarch. Until they are established, the source/sink characteristics of riverine DOC export from land to the sea with regard to the atmospheric CO₂ pool on time scales of anthropogenic perturbation cannot be quantified.

9.2 METHODS OF MEASUREMENT

Determination of DOC involves the removal of inorganic carbon from the sample, oxidation of the organic carbon to carbon dioxide and quantitative determination of the resulting CO₂. DOC can be oxidized to CO₂ by wet chemical oxidation methods (e.g. persulfate oxidation), by high temperature combustion of the liquid or dried sample in the presence of an oxidizing or surface catalyst or photochemically by UV-irradiation, with or without the presence of an oxidizing agent (Beattie et al.1961; Menzel and Vaccaro 1964; Strickland and Parsons 1968; Armstrong and Tibbitts 1968; Ehrhardt 1969; Collins and Williams 1977; Wangersky and Zika 1978; Gershey et al.1979). Carbon dioxide can be quantified by conductometry, coulometry or-most frequently used¾by infrared absorption. Although it is in principle not possible to verify truly complete oxidation because the chemical nature of the DOC is only partially known, the good agreement between the different methods as reported, e.g. by Cauwet (1985) is supporting evidence that it is possible to quantify DOC in rivers as long as fresh water samples are analysed. From a global budgeting point of view we assume that the reproducibility between methods is as good as the reproducibility of sampling and that of handling the sample in the field and in the laboratory. The samples collected in the framework of the SCOPE/UNEP River Project were analysed for DOC with a Carlo Erba Total Carbon Monitor Model 400 analyser (Michaelis and Ittekkot 1982) and a UV-thin film DOC-analyser as described in Muller and Bandaranayake (1983). The Carlo Erba M400 combusts the injected liquid sample at 900°C in the presence of CuO as a catalyst. CO₂ is converted to methane, which is quantified by a flame ionization detector. In strictly fresh waters both methods agree within ±0.5 mg C/l. However, trace addition of salt to a fresh water sample decreases significantly the UV-analyser's recovery of DOC (Spitzy unpublished).

9.3 MEANS, RANGES AND FLUXES OF DOC IN RIVERS

Concentrations of DOC in rivers range from less than 1 mg/l in alpine streams to more than 20 mg/l in some tropical or polluted rivers and rivers draining swamps and wetlands (Malcolm and Durum 1976; Brinson 1976; Naiman and Siebert 1978; Mulholland and Kuenzler 1979). Average DOC:POC ratios can range from 1 to 20 in rivers of North America (Malcolm and Durum 1976; Naiman and Sedell 1979) to less than 0.5 in tropical rivers (Richey et al. 1980; Wissmar et al. 1981). In terms of concentration, seasonal DOC-variations within rivers are usually within an order of magnitude. Since most rivers show a 'flushing effect', i.e. increasing DOC with increasing discharge, the respective fluxes have a more pronounced variability. The flushing effect has been verified in many temperate zone studies (e.g. Malcolm and Durum 1976; Mantoura and Woodward 1983; Spitzy 1985; Cauwet and Meybeck 1987; Moore 1989) as well as in tropical and subtropical rivers (e.g. Depetris and Paolini, this volume; Richey et al., this volume). A dominant soil and plant organic matter source of riverine DOC is thereby indicated. Climate has a pronounced effect on DOC levels in rivers. Partly based on the then available SCOPE data (Degens 1982) Thurman (1985) gave the following estimates of mean DOC concentrations in mg/l and ranges (in parentheses) for various climatic zones: small rivers in Arctic and alpine environments: 2 (1-5); taiga: 10 (8-25); cool temperate: 3 (2-8); warm temperate: 7 (3-15); arid:  3(2-10); wet tropical: 6 (2-15); rivers draining swamps and wetlands: 25 (5-60). Meybeck (1988) relates typical DOC concentrations to morphoclimatic zones and calculates the respective percent contributions to the global DOC export (see Table 9.1). An update of means, ranges and fluxes of DOC based on the SCOPE data (Degens 1982; Degens et al. 1983; Degens et al. 1985; Degens et al. 1987; Degens et al. 1988) is given in Table 9.2, including water runoff and area-specific DOC export from the watersheds. The rivers listed there represent 37.7% of a global discharge of 37400 km³/year. Extrapolation to 100% yields a total of 218 X 10¹² g C/year. The annual DOC loss per unit area of catchment correlates significantly with the catchment's runoff, as shown in Figure 9.1.

Table 9.1 DOC and water export from major morphoclimatic zones based on discharge and 'typical' DOC-concentration data given by Meybeck (1988)

Table 9.2 Water and DOC transport data for big rivers (numbers of rivers relate to Figure 9.1; d.w. mean = discharge weighted mean)

Figure 9.1 Area-specific annual DOC export by rivers given in Table 9.2 versus their respective runoff. River Nr.21 is a small, temperate river, draining a forested catchment of 20 km² (Spitzy 1985)

9.4 ORIGIN OF DOC IN RIVERS

Dissolved organic carbon inputs to rivers are generally subdivided into allochthonous carbon which is produced on land and autochthonous carbon which is produced in rivers, reservoirs and lakes. Determination of the origin of DOC is generally performed by measurement of stable carbon isotope ratios, measurement of specific bioindicators, or by the use of spectral pattern differences on isolates of DOC.

Stable carbon isotope ratios (d¹³C), have great value in assigning the origin of plant residues that are not significantly degraded because terrestrial plants and fresh water plankton generally have d¹³C values of -20 to -30 per thousand, and marine plankton, which is responsible for the majority of autochthonous carbon production in the ocean, has d¹³C values of -13 to - 22 per thousand (Galimov 1985). The biological processes that cause carbon isotopic fractionation are fairly well understood (Galimov 1985), but the effects on d¹³C fractionation caused by diagenetic modifications of plant residues are not well understood. Harvey et al. (1984) noted that photolysis of an unsaturated lipid to produce a DOC product changed its d¹³C value from -28.1 per thousand to -19.7 per thousand. This large change casts considerable doubt on the utility of carbon isotope ratios to determine the origin of DOC that has been altered by photolytic processes. It is also likely that biological degradation of plant residues significantly alters carbon isotope ratios of refractory DOC that remains after degradation because of selective degradation of plant fractions with different isotope signatures.

Bioindicators used to identify origin of DOC include carbohydrates, amino acids and tannin and lignin residues. Carbohydrates and amino acids are relatively low in the soil-derived base-flow of rivers but increase when relatively fresh organic material (e.g. plant debris) degrading in the flood-plains is flushed from there into the river at the rising limb of the hydrograph (Ittekkot et al. 1982; Safiullah et al. 1987; Depetris and Paolini, this volume, Chapter 5). Mycke (1985) used catalytic hydrogenation to release lignin phenols from Elbe River samples. Pempkowiak and Pocklington (1983) employed alkaline nitrobenzene oxidation to release phenolic aldehydes as indicators of lignin residues in the Vistula River. Ertel et al. (1986) used alkaline copper oxide oxidation to release lignin phenols from dissolved humic substances isolated from the Amazon River. The presence of lignin phenols is a definite indicator of an allochthonous source for DOC, but the estimation of percentage allochthonous versus autochthonous input to DOC based on lignin phenol content is uncertain because only 3% to 8% of the carbon in dissolved humic substances can be ascribed to recognizable lignin structural units (Ertel et al. 1986).

An approach presently being developed to assess character and origin of DOC is to examine carbon and proton distributions of DOC isolates through nuclear magnetic resonance (NMR) spectroscopy. This assessment is dependent upon the efficiency of the isolation procedures for DOC because different fractions give different NMR spectra (Leenheer 1985). However, within a given DOC fraction, such as the fulvic acid fraction which comprises approximately 50% of the DOC, differences in carbon and proton distribution can be used to ascribe terms to DOC.

Figure 9.2 compares the ¹³C-NMR spectra of the fulvic acid fraction from Big Soda Lake, Nevada, where DOC is produced primarily from phytoplankton (Cloern et al. 1983) with the fulvic acid fraction from the Suwannee River, Georgia, where the DOC is produced primarily from perennial terrestrial plants (Gunther and Casagrande 1984). The ¹³C-NMR spectra were obtained under conditions whereby the integral of the spectra gives quantitative carbon distributions (Thorn 1987). The proton NMR spectra of these two end- member fulvic acids are shown in Figure 9.3, and these spectra were also obtained under quantitative conditions (Noyes and Leenheer 1987).

Figure 9.2 Carbon-13 NMR spectra of (a) fulvic acid from Big Soda Lake, Nevada and (b) fulvic acid from Suwannee River, Georgia. Solution state spectra of sample (a) in deuterium oxide and sample (b) in ¹³C-depleted dimethylsulfoxide

Figure 9.3 Proton NMR spectra of (a) fulvic acid from Big Soda Lake, Nevada, and (b) fulvic acid from Suwannee River, Georgia. Solution state spectra of samples dissolved in deuterium oxide

The Big Soda Lake fulvic acid is characterized by large aliphatic carbon content (0-100 ppm, Figure 9.2), low aromatic carbon content (100-170 ppm) and negligible ketone content (190-222 ppm); whereas the Suwannee River fulvic acid has significant ketone content and large aromatic carbon content. The proton NMR spectra of Figure 9.3 show differences in both aliphatic hydrogen (0 to 3.5 ppm) and aromatic hydrogen (6.5 to 8.5 ppm). The aromatic hydrogen distribution of the Suwannee River fulvic acid is indicative of lignin and tannin residues as shown by the peak at 6.8 ppm which represents aromatic hydrogen adjacent to phenol and methoxy groups. This 6.8 ppm peak is absent in the Big Soda Lake fulvic acid proton NMR spectrum. The significant differences shown by the NMR spectra of these two end member fulvic acid samples indicate that allochthonous origin of DOC may be quantitatively differentiated from autochthonous origin.

The hydrology of major world rivers has a significant influence on the origin of DOC. Rivers which drain large lakes, such as the St Lawrence and Mackenzie Rivers, can be expected to have significant phytoplankton inputs to DOC. Rivers which do not have lakes or reservoirs, and have high sediment concentrations which suppress primary carbon production, contain DOC of allochthonous origin. Examples of such rivers are the Amazon and Changjiang Rivers. Rivers such as the Nile, Indus and Missouri have been altered by the construction of reservoirs which decreases sediment concentrations and increases autochthonous carbon contribution to DOC. The hydrogeology of a river can affect the origin of DOC as is the case of the Elbe River which flows across a lignite coal deposit. Lastly, groundwater inputs to river DOC, although generally low (Leenheer et al. 1974), can be a significant source in certain regions such as Amazônia (Klinge 1966).

9.5 TRANSFORMATIONS OF DOC

Biochemical processes that degrade both particulate organic carbon POC and DOC in rivers and lakes are summarized by Thurman (1985). Simple low molecular weight compounds, such as carbohydrates, amino acids and fatty acids, are decomposed by heterotrophic bacteria in a mlatter of hours, whereas high molecular weight dissolved organic substances that are biodegradable, but cannot pass cell membranes until they are broken down by extracellular enzymes or abiotic processes, decompose in a range of days to months (Saunders 1976). Straight-chain aliphatic hydrocarbon structures are readily broken down by microorganisms by the beta oxidation process, but branched and cyclic aliphatic structures, such as terpenoids and steroids, are refractory to aerobic degradation processes. Highly substituted aromatic ring structures, which are found in tannin and lignin residues, are also resistant to biological degradation. As a consequence, DOC in rivers is usually comprised of minor amounts of biodegradable plant, phytoplankton and bacterial residues, which are being rapidly recycled, and major amounts of bio- logical refractory residues comprised of substituted aromatic structures, and branched and cyclic aliphatic structures which are partially oxidized.

Thurman (1985) lists five geochemical processes that are known to transform dissolved organic compounds in water; they are sorption/partitioning, precipitation, volatilization, oxidation/reduction and complexation. Polar dissolved organic compounds sorb on to sediment surfaces via site-specific hydrogen bonding, ligand exchange, cation exchange, and anion exchange interactions, and polar dissolved organic compound partition into organic cations or organic matrices of sediment. Precipitation of DOC may occur when the pH of water decreases, salinity increases (salting out of humic acid in estuaries; Sholkovitz 1976), and polyvalent cation (Fe³⁺, A1³⁺, Ca²⁺, and Mg²⁺) content increases which causes precipitation of bridge-bonded complexes. Volatile organic compounds are usually less than 1% of the DOC in surface water, but their input into the atmosphere by volatilization may be an important process with regards to the organic trace gas content of the atmosphere. Dissolved organic matter in water is a reducing agent for both biotic and abiotic processes. Ferric iron is reduced to ferrous iron by dissolved humic substances; especially when catalyzed by photochemical processes, various manganese, vanadium, mercury and iodine species can also be reduced by dissolved humic substances. Lastly, DOC is well known for its ability to complex trace metals such as copper, cadmium, lead and zinc. If concentrations of these metals in solution are large such as at mine tailing sites, precipitation of the DOC-metal complex occurs.

Three additional geochemical processes are postulated to cause DOC transformations in surface waters; these processes are photolysis, abiotic hydrolysis and salting-in of DOC. Reaction rates and specific reactions of photolytic degradation of natural DOC input by rivers into the ocean have been documented (Kieber and Mopper 1987; Kieber et al.1989). They identified the alpha-keto acids glyoxylic and pyruvic acid as photolytic breakdown products of terrestrial DOC inputs to the ocean.

The presence of aromatic ketone functional groups in a fulvic acid derived from terrestrial plants was postulated by Leenheer et al. (1987a) to be evidence for the following photolytic reactions:

The first reaction has potential significance with regard to naturally occurring branched aliphatic-aromatic hydrocarbon precursors of DOC, such as certain terpenoid and flavonoid compounds. The photo-oxidation reaction not only produces an aryl-aliphatic ketone; but the R radical is likely to produce an alkane hydrocarbon which may volatilize from water to add to the atmosphere trace gas pool. The second reaction is a photo-Fries rearrangement which may be responsible for degradation of gallotannins in water.

The presence of the negative charge potential around most of the organic polyelectrolytes that comprise DOC in low ionic strength waters likely retards the abiotic hydrolysis rate of ester linkages in these polyelectrolytes. Line-weaver (1945) observed up to 400% de-esterification reaction rate increases at pH 9 when the negative charge of dissolved plant pectins in water was suppressed by ionic strength increases or by addition of calcium or magnesium to form neutral ion complexes. The suppression of ester hydrolysis in low ionic strength water was related to ionic repulsion of the hydroxide ion from the pectin polyelectrolyte. Various hydrolyzable tannin components of DOC have ester linkages that are likely affected by ionic strength and polyvalent cation content. River environments where abiotic hydrolysis rates of DOC (and POC) may increase are upper estuaries (suppression of polyelectrolyte charge by ionic strength increase) and at the confluence of blackwater rivers with rivers containing significant dissolved calcium and magnesium concentrations (suppression of polyelectrolyte charge by neutral ion complexes).

Another geochemical process which may be caused by ionic strength increases is salting-in of amphoteric components of POC to form DOC. Proteinaceous constituents of POC and DOC are amphoteric, and their minimum solubility is in low ionic-strength waters at their isoelectric point which is often near neutral pH values. As ionic strength increases, low- solubility protein zwitterions unfold to become more soluble dipolar ion pairs with the salt anions and cations (Cohn and Edsall 1942). This salting-in process should occur in the upper estuarine zone and may be partially responsible for the loss of POC reported by Eisma et al. (1985) in the Ems and Gironde estuaries.

9.6 BIOGEOCHEMICAL SIGNIFICANCE OF DOC

The allochthonous fraction of riverine DOC has predominantly a plant and a soil source and this carbon has a chance to reach the open sea in a reduced state. It thus provides a link between the large reactive pools of organic carbon on land and in the sea. With regard to the role of riverine DOC within the anthropogenically perturbed carbon cycle two aspects are of note. First, radiocarbon age determination on riverine DOC fractions shows that the turnover of the parent carbon pool occurs on time scales below one hundred years (Hedges et al. 1986). Second, anthropogenic disturbances of river catchments such as deforestation have been shown to increase the flux of DOC for at least ten years (Moore 1989).

In addition to the significance of DOC in rivers to carbon cycle research, DOC affects contaminant availability and transport and nutrient availability and transport. Until recently, many contaminants were thought to exist in equilibrium between the dissolved and sediment phases in water. However, recent research (Gschwend and Wu 1985) has shown that contaminants also sorb to nonsettling microparticles (colloids) and organic macromolecules (DOC) so that three phases (shown below) must be considered for realistic contaminant transport modelling.

On one hand, the colloid phase can be considered an operational artifact to incomplete phase separation of sediment; however, the colloid phase has considerable significance for contaminant transport in rivers because: (1) it moves conservatively with the dissolved phase and may transport nonconservative contaminants; (2) binding of contaminants by colloid and sediment phases generally diminishes their toxic properties to organisms; and (3) changes in water chemistry and physics may cause the colloid phase to aggregate into the sediment phase or disaggregate into the dissolved phase with consequent redistribution of sorbed contaminants. Wershaw et al.(1986) have proposed that humic acids in water exist as a mixed, micelle-like aggregate in water which can be regarded as a colloid organic phase into which non-polar organic contaminants partition. Similarly, Marinsky and Ephraim (1986) have found that humic and fulvic acids have ion-charge and metal-binding properties that are best described by regarding these organic polyelectrolytes as a separate phase in water.

Dissolved organic carbon in river water has significance both as a primary nutrient and as a micronutrient for various organisms. Obviously, low molecular weight amino acids and carbohydrates dissolved in water are primary nutrients for bacteria, whereas dissolved humic substances are generally regarded as biologically refractory. However, that portion of DOC regarded as refractory in a certain riverine environment may become labile in a different riverine environment as the various geochemical and biological processes discussed previously degrade DOC. DOC in dilute concentrations in river water is physically less available to micro-organisms than POC because microorganisms are generally attached to sediment particles.

Nitrogen and phosphorus are contained as nutrients within DOC. Fulvic acids from rivers contain about 1% nitrogen and humic acids 2% nitrogen (Thurman 1985); whereas only trace amounts of phosphorus (0.2% in river fulvic acid) are present.

The influence of humic substances on biological systems has been considered largely in terms of beneficial effects. Prakash and MacGregor (1983) state that these beneficial effects have included: (a) increases in growth, root initiation, yield, nutrient uptake, chlorophyll synthesis, seed germination, etc.; (b) stimulation of the various physiological and biochemical processes associated with cellular metabolism; and (c) attenuation of toxicity of heavy metals and other toxic compounds. The causes for many of these beneficial effects are not specifically known, and many causes are generally related to transport of nutrients (or toxins) through cell membranes by low molecular weight components of DOC. The recent finding that DOC in groundwater at neutral pH dissolves quartz by complexation with silica (Bennett and Siegel 1987) has implications with regard to the role of DOC in the uptake of silica by phytoplankton in surface waters.

Various ecological aspects of dissolved humic susbtances in aquatic environments are reviewed by McKnight and Feder (1987). Acidic blackwater streams, rich in humic substances, absorb light so as to limit photosynthesis by phytoplankton; there is also evidence that phytoplankton growth in these streams is also limited by complexation of essential nutrients by DOC which limits nutrient availability (Prakash and MacGregor 1983). Dissolved humic substances play an important role in buffering the pH of natural waters. The range of pK_a of the majority of the acid groups in DOC is from 3 to 5; thus maximum buffering capacity is near a pH of 4. This buffering capacity of DOC is of significance for watersheds affected by stream acidification processes, such as acidic precipitation or acid mine drainage.

Lastly, DOC may be significant in its effects on human health (McKnight and Feder 1987). Dissolved organic compounds that occur naturally in river waters are not known to be toxic; but breakdown products or fragments of humic substances produced by chemical or biological processes may be assimilated by humans and have an effect on health. Various halogenated hydrocarbons are produced by the reaction of DOC with halogens used for disinfection treatments (Rook 1974), and these halogenated products have been found to produce mutagenic effects in various test organisms. Endemic goiter has recently been related to the presence of humic substances in the environment (Cooksey et al.1985), various polyphenolic breakdown products of humic substances are believed to inhibit uptake of iodine by the thyroid gland. Potentially, the most important effect of DOC on human health is its ability to transport nutrients and toxins into or out of the digestive tract. However, this 'carrier molecule' role of DOC and humic substance in general has not been authenticated by research at this time.

9.7 RESEARCH NEEDS

The primary research need for studies of DOC in rivers are quantitative methods of isolation of DOC from water. Most studies of DOC in water have been on the humic and fulvic acid fraction which is readily isolated (Mantoura and Riley 1975), but humic substance only accounts for about 50% of the DOC in river water. Comprehensive methods for isolation of DOC based on adsorption chromatography (Leenheer 1981) and distillation (Leenheer et al.1987b), exist, but these methods are too labor-intensive to be widely applied. A possible comprehensive approach for isolation of DOC for study would be to combine preparative ultrafiltration with adsorption chromatography. Adsorption chromatography would be used to isolate hydrophobic solutes (including humic substances), and preparative ultrafiltration could isolate hydrophilic solutes greater than 500 daltons molecular mass, which is the minimum pore size membrane that can be used. It is likely that most of the hydrophilic fraction of DOC in river water is greater than 500 daltons because (a) lower molecular mass hydrophilic solutes are rapidly biodegraded and (b) high molecular mass hydrophilic solutes are more soluble than hydrophobic solutes of comparable size.

Once DOC is isolated from water for study, chemical analysis of DOC structural components could be performed to gain a better understanding of diagenetic processes that transform plant components to DOC. The extreme molecular complexity and mixture heterogeneity of DOC is the reason why most components of DOC, especially the humic fraction, have not been analysed at the molecular level. The problem could be approached in a similar manner that the natural-product chemists take in determining molecular structures of trace components: (a) extraction and multiple preparative chromatography of multigram quantities of DOC isolates; (b) compound class structural determinations by ¹³C and ¹H-NMR; and (c) high-resolution liquid chromatography mass spectrometry or tandem mass spectrometry of compound classes using gentle molecular ionization techniques which do not fragment labile DOC molecules. Complementary, the dynamic nature of DOC can be revealed from studies of changing DOC characteristics along spatial or temporal physico-chemical gradients within or between rivers (e.g. McKnight et al.1988).

A research need for geochemical studies of DOC in water is better interpretation and use of carbon isotope data. The various biotic and abiotic processes that transform plant residues into DOC need to be studied with respect to the fractionation of stable carbon isotopes that occurs during the degradation process. The newly developed accelerator technique for determining ¹⁴C age of DOC can be applied on small quantities of DOC isolated from various environments to understand better carbon-cycling time scales in various hydrologic environments.

Lastly, abiotic processes that produce and degrade DOC have been generally overlooked in the past because of a scientific bias that biotic processes are primarily responsible for organic matter decay. Photolytic oxidation is now receiving increasing emphasis because of the rapid rate of photo-oxidation, and because of its importance with regard to the fate of terrestrial DOC in the ocean. Abiotic processes that affect solubility (salting- in, salting-out, hydrolysis) and molecular weight are also receiving more study because these factors affect the degradability of DOC.

Much progress is being made in addressing research needs for DOC in water. Most of all, dedication is required to devote scientific careers to the study of one of the most challenging problems in organic geochemistry¾analysis of DOC in major world rivers.

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