River DOC concentrations

Fig. 4 Predicted versus observed summer Anoxic Factor (AF) in (a, b) Foix Reservoir (Spain), (c, d) San Reservoir (Spain), (e, f) Brownlee Reservoir (USA), and (g, h) Pueblo Reservoir (USA). The results have been arranged to place the systems along a gradient of relative human impact (Foix Reservoir at the top, Pueblo Reservoir at the bottom). Predictions are based on linear models using different independent variables (in brackets) Inflow = streamflow entering the reservoir during the period DOCjjiflow = mean summer river DOC concentration measured upstream the reservoir CljjjAow = mean summer river CU concentration measured upstream the reservoir and Chlepi = mean summer chlorophyll-a concentration measured in the epilimnion of the reservoir. The symbol after a variable denotes a nonsignificant effect at the 95% level. Solid lines represent the perfect fit, and were added for reference. Modified from Marce et al. [48]...

There have been a number of regional analyses showing that stream and river DOC concentrations appear to be highly influenced by the flowpath of water across the landscape. Streams draining landscapes dominated by water flowpaths at the land surface in contact with organic-rich soil horizons... 

TABLE II Regional Analyses of Stream and River DOC Concentrations and ... 

Figure 16.4 Observed [ 02] in water samples from some Swiss rivers (R) and lakes (L) as a function of the dissolved organic carbon (DOC) concentrations of these waters. The results apply for noontime light intensity on a clear summer day at 47.5°N (data from Haag and Hoigne, 1986).

DOM is derived from autochthonous sources such as phytoplankton and photosynthetic bacteria (16) at Big Soda Lake near Fallon, Nevada. This lake is alkaline (pH 9.7) and chemically stratified. It contains DOC concentrations as high as 60 mg/L and dissolved salt concentrations as high as 88,000 mg/ L (17). The DOM in this lake is colorless. The fulvic acid fraction was isolated by adsorption chromatography (Amberlite XAD-8 resin) (18) and by zeo-trophic distillation of water from N,N-dimethylformamide (19). Average molecular model synthesis was achieved in a manner similar to that used for fulvic acid from the Suwannee River. The characterization data are presented in Table I and the structural model is presented in Structure 2. 

Sampling Site DOC Concentration (mg/L) DOC Load (metric tons/day) River Discharge0 (m3/s)... 

FIGURE 1 DOM pie diagram based on distribution of DOC in a typical river with a DOC concentration of 5 mg C/L (adapted from Thurman, 1985). Fulvic acids typically constitute the largest percentage of DOC, followed by low-molecular-weight organic acids. 

The concentrations of DOC in major rivers typically range from 250 to 750 pM, and concentrations in the surface ocean range from 60 to 90 J.M (Table I). Most of the river data compiled in Table I are from the Amazon River system (Hedges et al., 1994, 2000), the Parana River system (Depetris and Kempe, 1993), and the Mississippi River (Benner and Opsahl, 2001). The seawater data are from surface water samples collected in the Pacific and Atlantic Oceans (see Table I for references). Total hydrolyzable neutral sugars (glucose, galactose, mannose, xylose, fucose, rhamnose, and arabinose) account for about 1-2% of river DOC and 2-6% of ocean DOC, indicating... 

TABLE I Typical DOC Concentrations in Rivers of Different Biomes from High Latitudes to Low Latitudes... 

FIGURE 3 Relationships between annual runoff and watershed export of DOC in streams and rivers reported in the literature. The respective lines extend only over the range of runoff values included in the dataset. The slopes of each line are approximately equivalent to the mean annual DOC concentration for that group (in parentheses). Sources for each relationship are as follows streams with wetlands, temperate (Mulholland, 1997) streams with wetlands, N. Carolina (Mulholland and Kuenzler, 1979) large rivers, global (Spitzy and Leenheer, 1991) large rivers, N. America (Mulholland and Watts, 1992) streams, tropical (McDowell and Asbury, 1994) streams, N. America (Mulholland, 1997). 

Stable carbon isotope ratios have also been used to determine the sources of lake DOC. Baron et al. (1991) used 13C analysis to show that autochthonous sources dominated the DOC of an alpine lake during most periods while allochthonous sources dominated the DOC in a subalpine lake. The high DOC concentrations observed during spring snowmelt and early summer in both lakes, however, were mostly derived from allochthonous sources. As with streams and rivers, synoptic regional studies of 13C and 14C would provide important new information on broad spatial patterns and controls on the sources of DOC in lakes. 

In contrast to rivers and lakes, autochthonous processes appear to be the overwhelmingly dominant source of DOC in the oceans. In a survey of shallow and deep waters of the Atlantic and Pacific Oceans, Opsahl and Benner (1997) used concentrations of lignin in DOM to determine that terrigenous matter accounted for only 0.7-2.4% of the DOC present. Terrigenous DOC concentrations were about 2.6 times higher in the Atlantic than in the Pacific Ocean, presumably because of the 3.6 times greater riverine water discharge to the Atlantic. 

A recent study of rainwater DOC suggested that rain also may be a significant source of DOC to the open ocean. The DOC concentration of marine rain was reported to be 0.3 mg L 1 and the global rainwater DOC flux to the oceans was estimated to be 0.43 x 1012 gC yr 1, about the same magnitude as the river flux of DOC (Willey et al., 2000). 

Figure 12.7. Plot of DOC concentration versus elevaUon for rivers of the Bolivian Amazon basin. From Guyot and Wasson, 1994.

The most widely used nominal pore size for ultrafiltration is 1 nm, which is estimated to retain compounds with MWs >1000 Da. The 1 nm pore-sized membrane isolates 20% of the total DOC in surface and deep ocean waters and up to 55% of the DOC in coastal and estuarine environments (Benner et ai, 1997 Carlson et ah, 1985 Guo and Santschi, 1996). Ultrafiltration membranes with a smaller pore size are rare and do not show reproducible retention characteristics filters with a larger pore size retain only a small fraction of total DOC and they are not widely used. In general, the actual MW retained and the isolation of reproducible quantities of DOC by ultrafiltration depends strongly on the membrane (e.g. construction material, manufacturer), sample type (e.g. river, coastal, open ocean), total DOC concentration, concentration factor, extent of desalting and operating conditions (Buesseler et al, 1996 Guo and Santschi, 1996 Guo et ai, 2000). Losses to the ultrafiltration membrane can also be significant (Guo et al., 2000) and depend primarily on the physiochemical characteristics of the particular molecule. 

Table 1 Database oftrace element concentration in the dissolved load (<0.2 xm) of rivers. All concentrations inppb (p-gL )except forRa(fgL and Os (pgL ). DOC, TSS, TDS are Dissolved Organic Carbon (mgL ), total suspended solid (mgL ), and total dissolved solutes (mgL ). Water discharge and surface are in s and 10 Km, respectively. 

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