Ocean humic substances from

Humic substances account for 40-70% of the DOC in rivers and 5-25% of the DOC in the ocean (Table I). It is important to note that recoveries of adsorbed humic substances from XAD resins are not quantitative, so the chemical characteristics of the recovered humic substances are not necessarily representative of all the humic substances retained by the resin. Tangential-flow ultrafiltration retains 45-80% of the DOC in rivers and 25-40% of the DOC in the surface ocean (Table I). Essentially all of the DOC retained during ultrafiltration is recovered for chemical characterization. In general, ultrafiltration recovers a larger fraction of the DOM from these systems. These methods also isolate DOM based on different mechanisms. Adsorption onto XAD resins at low pH chemically fractionates the DOM and isolates the more hydrophobic components, whereas ultrafiltration principally separates components of DOM on the basis of size and shape. 

There are major differences in the chemical compositions of DOM isolated by XAD resins and ultrafiltration (Table I). In rivers and in the ocean, humic substances (XAD isolation) are depleted in N relative to UDOM. The C/N ratios of UDOM are more representative of bulk DOM than those of humic substances. Most of the functional groups identified by NMR are found in more than one class of compounds, so in most cases specific functional groups are not assigned to a particular group of biochemicals. However, in some circumstances it is possible to estimate the fraction of carbon associated with a biochemical class, such as carbohydrates. Carbohydrates are the most abundant polyalcohols in nature, and the ratio (4-5 1) of areas associated with NMR peaks at specific chemical shifts [e.g., 72 ppm (C—O) -102 ppm (O—C—O)] indicates that carbohydrates are their primary source (see Table I for references). In general, humic substances are depleted in carbohydrates (C—O and O—C—O) and enriched in aromatic and unsaturated C (C=C) relative to UDOM (Table I). As mentioned earlier, humic substances are relatively hydrophobic components of DOM, and it is consistent that they are depleted in N and carbohydrates and enriched in aromatic components. The UDOM fraction includes more hydrophilic components that are relatively enriched in N and carbohydrates. Humic substances from the ocean are enriched in aliphatic C (C—C) relative to UDOM, and this could reflect the more hydrophobic nature of the humic substances. 

Bussmann, I. (1999). Bacterial utilization of humic substances from the Arctic Ocean. Aquat. Microb. Ecol. 19, 37-45. 

The focus of this chapter is on the geochemistry of stream humic substances and should provide the reader with an appreciation of the dynamics, importance, and uniqueness of streams within the hydrologic system. Streams should not be envisioned only as arteries connecting lake, ground water, and soil environments (which are considered in previous chapters) with estuaries and oceans (which are presented in following chapters), nor as integrators of humic substances from upgradient, but rather they should be viewed as a different and unique aquatic environment where stream humic substances also have a dififerent and unique character. 

On the basis of few data from a limited range of geographical areas, it appears that humic substances in estuarine zones exhibit many attributes of the transitional nature of the environment. Aquatic humic substances show concentrations and chemical characteristics intermediate between those found in river and ocean waters, indicating relatively little in situ production, consumption, or chemical change. Sedimentary humic substance concentrations are somewhat higher than are usually found in the ocean, reflecting the high primary productivity and shallow water depths, but are chemically similar to either riverine or oceanic endmembers. The actual nature of estuarine humic substances is poorly known, but this problem is no worse than for humic substances from most environments. 

Further work on differentiating marine and coastal runoff humic substances was hindered by the lack of suitable isolation techniques for the marine material since concentrations in the open sea rarely exceed 0.25 mg/ L. Sorption of marine humic substances from seawater onto solid phases is now a standard technique and can be used to extract gram quantities of marine humic substances for chemical and physical studies (see Aiken, Chapter 14). Sieburth and Jensen (1968) first used rolled nylon stockings as an adsorbant but the method suffered from contamination. Kerr and Quinn (1975) used a specially treated charcoal and obtained quantitative recovery of the dissolved colored substances in seawater. Riley and Taylor (1969) introduced the use of cross-linked polystyrene resins, specifically Amberlite XAD-2. This polymer is now the most widely used for open-ocean work (Stuermer and Harvey, 1974 Bada et al., 1982 Harvey et al., 1983) and in estuaries (Mantoura and Riley, 1975). These isolation methods have made available sufficient quantities of seawater humic substances for detailed chemical studies. 

Based on comparison of data from UV, fluorescence, and NMR spectroscopy, and from carbon isotope determination for humic substances isolated from coastal and open ocean environments, the authors have concluded the following (1) other than its metal complexation and redox functions, the only resemblance between humic substances from open ocean (marine) and terrestrial environments is that they are both colored organic acids soluble in water, and (2) marine humic substances are formed in situ and only in the coastal zone is there an admixture of terrestrially derived humic substances from rivers. However, this second conclusion has not yet been reconciled with the observations discussed by Mayer in Chapter 8 that riverine humic... 

The possibility of formation of humic substances from a melanoidin (melanin-like) type in the ocean water was underlined by Kalle (1962), Duursma... 

Figure 3 summarizes the inputs and cycling of DOM in the sunfit upper layer of the surface ocean, the photic zone. Possible sources of DOM in the ocean include both river inputs and in situ production by phytoplankton. DOM in the marine environment was originally regarded as a complex mixture of both terrestrial material and marine-derived humic substances from phytoplankton... 

Both of these methods have been used for DOM isolation from major rivers and the surface ocean, and the general characteristics of these fractions of DOM are presented in Table I. The major C functional groups of humic substances and ultrafiltered DOM (UDOM) have been characterized by solid-state, cross-polarization magic angle spinning 13C nuclear magnetic resonance (NMR) spectroscopy. The samples of humic substances that were characterized by NMR spectroscopy were collected from the Amazon River... 

There are also structural differences between humic substances or UDOM collected from rivers and oceans (Table I). Humic substances and UDOM from rivers are enriched in aromatic components compared with their counterparts from the ocean. Terrestrial vegetation is relatively rich in aromatic components, such as lignins and tannins, and this is reflected in the greater aromatic nature of DOM in rivers. These biopolymers are relatively resistant to microbial degradation and are important components of river DOM. Humic substances and UDOM from the ocean are enriched in carbohydrates compared with their counterparts from rivers. This is consistent with observations of higher C-normalized yields of neutral sugars in bulk DOM from the ocean compared with rivers (Table I). 

Black C, produced by wild fires and humic substances (HS), the natural by products of SOM decomposition in soil and water systems, are certainly the classes of organic compounds that most closely approximate this recalcitrant behavior. HS occur widely, being found in large amounts not only in the soil and sediments but also in lakes, rivers, ground waters, and even the open ocean (Stevenson, 1994). Besides these relatively refractory substances, more labile compounds can persist in soil for a much longer time than would be predicted from their inherent recalcitrance to decomposition. SOM stabilization (Figure 5.2) is generally considered to occur by three main mechanisms (i) physical protection, (ii) chemical stabilization, and (iii) biochemical stabilization (Six et al., 2002). 

Only occasionally has the N content of solid phase extracts been reported. At a site in the Atlantic Ocean the carbon to nitrogen ratio (C N) of XAD 8 and XAD 2 extracts fell in the range of 40-57 (57 0.9 and 41.1 3.3, respectively DrufFel et ai, 1992). In contrast, at the same site XAD 4, when used as the second resin in series with XAD 8 or XAD 2, extracted compounds with lower C N ratios - 19—24 (21.0 2.4). These values are only slighdy higher than ratios reported for total DOM (see below). McKnight and Aiken (1998) reported a C N value of 37 for DOM extracted by XAD 8 at one site in the Pacific Ocean at other sites in the N. Pacific Ocean XAD 2 was found to extract DOM with a C N ratio between 32 and 46.5 (Druffel et al, 1992 Meyers-Schulte and Hedges, 1986). Bronk (2002, Table III) compiled various literature values and arrived at an average C N ratio of 32.8 19.5 for total humic substances isolated from a variety of aqueous environments (see McCarthy and Bronk, this volume). 

Proton and C-NMR data compare well with each other and suggest that surface ocean HMWDOM has a H C ratio of approximately 1.8—1.9 (Aluwihare, 1999 Benner et al, 1992) and an 0 C ratio between 1 and 1.1. These H C and 0 C ratios are very close to those of a pure carbohydrate with a general hexose structure (e.g., C6H12O6). In comparison, humic substances isolated from seawater have an H C ratio between 1.2 (direct elemental analyses) and 1.4 (based on NMR estimates) and are therefore, relatively C-rich (Hedges et al, 1992). The H C and 0 C composition of phytoplankton as estimated by NMR spectroscopy is approximately 1.7 and 0.3, respectively (Hedges et al, 2002). In comparison to phytoplankton... 

In the Arctic and Antarctic Ocean amino acids were also found in humic substances isolated from DOM by XAD-2 resins (Hubberten et ai, 1995). The concentration of THAA in humic substances was between 233—246 nM, with aU hydrolysable amino acids in the deep ocean and 60% of amino acids in the surface ocean residing in this fraction. Glycine was by far the most abundant amino acid detected in the humic fraction. These authors concluded that amino acids in the XAD-2 extracts represent a refractory protein background that is present throughout the ocean. The dominance of this refractory protein background in the surface and deep ocean could explain the relatively stable amino acid distribution observed by Yamashita and Tanoue (2003) at their open ocean sites. 

Often the study of humic substances in estuaries has been undertaken because estuaries are more easily sampled than the open ocean, and not because of the unique aspects of estuaries per se. It is only in the past few years that systematic studies of humic materials, traversing the salinity gradient, have been carried out. Techniques used in most studies have been those borrowed from the classical fields of soil humus studies it seems likely that in time the techniques will evolve in response to the unique chemical processes that humic materials undergo in the estuarine zone. Quantitatively considered, the literature on humic materials in estuaries lacks the extent of the geographical or topical coverage of the soils literature. This paucity of data causes many conclusions drawn so far to be quite tentative and in need of corroboration. 

Nevertheless, it seems reasonable to infer that aquatic humic substance concentrations in estuaries are intermediate between those of rivers and those of the open ocean, as would be expected from a mixture of river water and seawater. The concentrations of humic acids at intermediate salinities do not correspond, in a linear fashion, to the relative proportions of river water and seawater (Fig. 2), primarily because of a removal from solution of a portion of the riverine contribution. This behavior is in contrast to that of total dissolved organic carbon (DOC) which generally does show linear mixing lines when plotted versus salinity (Sholkovitz et al., 1978 Moore et al., 1979 Laane, 1980 Fox, 1983a Mantoura and Woodward, 1983). 

Allusion has been made above to changes that occur in the humic substances introduced to estuaries by the riverine source. This section reviews the chemistry of these changes, and considers their effect on the delivery of riverine humic substances to the oceans. Because riverine humic substances derive from zones of low ionic strength, the rapid increase in both the types and concentration of dissolved salts upon estuarine mixing should have important effects on their ion-exchange properties, their conformations in solution, and their solubility. 

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