SCOPE 42 - Biogeochemistry of Major World Rivers, Chapter 9, Dissolved Organic Carb... Page 1 of 19
apparently this is a 1991 thing- not
sure how to cite- go to http://
www.icsu-scope.org for info?
SCOPE 42 - Biogeochemistry of Major World Rivers
9 Dissolved Organic Carbon in Rivers
ALEJANDRO SPITZY
SCOPE/UNEP International Carbon Unit, University of Hamburg, Max- Planck Institute of Meteorology,
Hamburg, Federal Republic of Germany
and
JERRY LEENHEER
Geological Survey, Denver, USA
9.1 INTRODUCTION
9.2 METHODS OF MEASUREMENT
9.3 MEANS, RANGES AND FLUXES OF DOC IN RIVERS
9.4 ORIGIN OF DOC IN RIVERS
9.5 TRANSFORMATIONS OF DOC
9.6 BIOGEOCHEMICAL SIGNIFICANCE OF DOC
9.7 RESEARCH NEEDS
REFERENCES
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
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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 usedby 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
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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 (560). 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 areaspecific 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)
Morphoclimatic zone
Typical DOC
conc.*
(mg/l)
Water discharge
(km3/year)
DOC export
(% of total) (106 t/year)
(% of total)
Tundra
2
1 122
3
2 .2
1
Taiga
7
4 376
11 .7
30 .6
13
Temperate
Wet tropic
Dry tropic
Semi-arid
4
8
3
1
10 285
19 186
2 169
262
27 .5
51 .3
5 .8
0 .7
41 .1
153 .5
6 .5
0 .3
17 .6
65 .5
2 .8
0 .1
243 .2
100
Total
37 400
100
*mean of typical DOC concentrations: 4.2
discharge weighted mean global DOC concentration: 6.3.
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)
DOC
River
Discharge Area
(km3/year)
(103
km2)
Runoff
concentration (mg/l) flux
(mm/year) range d.w. mean
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(106
t/year)
(t/km2/year) Reference
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1 Amazon
5520
6300
876
3-5
3.6
20
3.2
2 Orinoco
1135
950
996
2-5
2.87
3.22
6.1
3 Parana
480
2800
l7l
-20
6.1 (ENSO:
10.2)
2.8
(ENSO: 1
7.5)
4 Uruguay
158
350
451
2-8
3.2
0.5
1.4
5 Mississippi 439
3267
154
8
3.5
1.1
6 St.
Lawrence
413
1150
359
3.7
1.6
1.4
7 Yukon
210
840
250
8.8
1.9
3.0
8 Columbia
135
670
202
2.7
0.4
0.7
9
883
Yangtsekiang
1950
453
5-23 13.4
11.8
6.1
10
609
Brahmaputra
580
1050
1-6
3.2
1.9
3.3
11 Ganges
366
975
375
1-9
4.6
1.7
1.7
12 Indus
211
950
222
2-22 16.1
0.75
0.8
13 Huanghe 44
745
59
5-25 12.3
0.54
0.72
14 Zaire
1237
4000
309
3-10 8.5
10.5
2.6
15 Niger
152
1125
171
2-6
2.9
0.55
0.5
16 Gambia
4.6
42
110
1-4
2.4
0.011
0.27
17 Lena
533
2430
219
9.5 (TOC)
5.1
2.1
18 Ob
419
2550
164
8.8 (TOC)
3.7
1.5
3-5
http://www.icsu-scope.org/downloadpubs/scope42/chapter09.html
Richey et
al. 1985
Paolini et
al. 1987
Depetris
and Paolini.
this
volume,
Chapter 5
Manosa,
unpubl.
Leenheer
1982
Pocklington
and Tan
1983
Leenheer
1982
Leenheer
1982
Gan
Weibin et
al. 1983
Safiullah et
al. 1987
Safiullah et
al.1987
Arain 1987
Gan
Weibin et
al. 1983
Martins and
Probst, this
volume,
Chapter 6
Martins
1982
Lesack et
al. 1985
Telang et
al. this
volume,
Chapter 4
Telang et
al. this
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19 Yenisei
562
2580
218
20 Mckenzie 249
1810
138
Sum
3-6
7.4 (TOC)
4.1
1.6
5.2
1.3
0.7
volume,
Chapter 4
Telang et
al. this
volume,
Chapter 4
Telang et
al. this
volume,
Chapter 4
13795.6
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.
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Stable carbon isotope ratios (13C), have great value in assigning the origin of plant residues that are
not significantly degraded because terrestrial plants and fresh water plankton generally have 13C
values of -20 to -30 per thousand, and marine plankton, which is responsible for the majority of
autochthonous carbon production in the ocean, has 13C 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 13C 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 13C 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 floodplains 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).
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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
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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
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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:
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The first reaction has potential significance with regard to naturally occurring branched aliphaticaromatic 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.
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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.
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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 pKa 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
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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 photooxidation, 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
geochemistryanalysis of DOC in major world rivers.
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