CHAPTER 16 • Oceanic DOC Measurements
301
the Pacific surface water along 140° W is 60 flM C at the equator and 80 flM C at 10° N
and 10° S, but in deep water it is almost constant at about 36 flM C.
The new data also necessitated a re-evaluation of the extent of'characterized' (meaning of known molecular structure) vs. 'uncharacterized' organic matter by a similar
multiple. The implications of the new study were that this 'new' (i.e. newly discovered)
material consisted of biologically derived macromolecules which were biologically
reactive, particularly to bacteria (Kirchman et al. 1991; Kepkay and Wells 1992; Coffin
et al. 1993) but resistant to chemical and photochemical oxidation (Sugimura and
Suzuki 1988; Suzuki and Tanoue 1991). However, other studies pointed out that the bulk
of the oceanic DOM comprises small molecules, some of which are relativley unavailable to microorganisms (Amon and Benner 1994), and that the carbon that is missed
by wet chemical oxidation methods is in the low-, rather than high-molecular mass
fraction (Ogawa and Ogura 1992). Measurements on 14C showed that the DOC in deep
water is very old (Bauer et al. 1992). The application of ultrafiltration techniques to
the study of oceanic samples has allowed further characterization of DOC. Spanning
the size range from 0.4-0.2 flm to 1 nm, ultrafiltered dissolved organic matter (UDOM)
is composed of macromolecular components and true colloids. Depth-weighted results of the size distribution of organic matter in sea water indicated that about 75%
of marine organic carbon was low-molecular-weight DOC «1 nm), -24% was highmolecular weight DOC (1-100 nm) and -1% was particulate organic carbon (>100 nm)
(Benner et al.I997). The re-evaluated distribution of carbon in surface water was therefore shifted to a greater relative abundance of larger fractions, suggesting a diagenetic
sequence from macromolecular material to small refractory molecules. Surface UDOM
is greater than deep UDOM by a factor of 2-3. Elemental as well as molecular-level
aldose and amino acid compositions were recently determined (McCarthy et al. 1996).
UDOM has a characteristic organic composition, clearly distinct from other marine
materials such as fresh plankton, sinking particles or humic substances. It is rich in
galactose and deoH sugars, and ubiquitous regardless of depth or location. In contrast the amino acid content of UDOM is a relatively minor component.
Various methods for the measurement of DOC, as well as the various problems common to all the techniques and those that are method specific, have been reviewed by
Wangersky (1993). This paper is intended to bring the reader up to date with the developments on the DOC measurements and to provide a survey of new concepts in
such analytical procedures.
16.2
DOC Measurement Techniques
The sheer complexity of DOM rules out any kind of rigorous analytical approach designed to individually quantify all of the contributory organic chemicals. Instead, the
strategy that has generally been adopted is to convert the organic carbon into CO 2 ,
OC may be classified according to the particle size. Since the distribution of molecular components of sea water covers a continuous size spectrum, ranging from small
molecules to colloids and larger particles, operational criteria have been developed
for the classification of OM. Accordingly, filtration may be performed using nominal
pore size of 0.45 flm, to get the so-called dissolved organic carbon (DOC). In the filtrate most of the colloids are recovered. The colloidal fraction contributes to 10-30%
301
the Pacific surface water along 140° W is 60 flM C at the equator and 80 flM C at 10° N
and 10° S, but in deep water it is almost constant at about 36 flM C.
The new data also necessitated a re-evaluation of the extent of'characterized' (meaning of known molecular structure) vs. 'uncharacterized' organic matter by a similar
multiple. The implications of the new study were that this 'new' (i.e. newly discovered)
material consisted of biologically derived macromolecules which were biologically
reactive, particularly to bacteria (Kirchman et al. 1991; Kepkay and Wells 1992; Coffin
et al. 1993) but resistant to chemical and photochemical oxidation (Sugimura and
Suzuki 1988; Suzuki and Tanoue 1991). However, other studies pointed out that the bulk
of the oceanic DOM comprises small molecules, some of which are relativley unavailable to microorganisms (Amon and Benner 1994), and that the carbon that is missed
by wet chemical oxidation methods is in the low-, rather than high-molecular mass
fraction (Ogawa and Ogura 1992). Measurements on 14C showed that the DOC in deep
water is very old (Bauer et al. 1992). The application of ultrafiltration techniques to
the study of oceanic samples has allowed further characterization of DOC. Spanning
the size range from 0.4-0.2 flm to 1 nm, ultrafiltered dissolved organic matter (UDOM)
is composed of macromolecular components and true colloids. Depth-weighted results of the size distribution of organic matter in sea water indicated that about 75%
of marine organic carbon was low-molecular-weight DOC «1 nm), -24% was highmolecular weight DOC (1-100 nm) and -1% was particulate organic carbon (>100 nm)
(Benner et al.I997). The re-evaluated distribution of carbon in surface water was therefore shifted to a greater relative abundance of larger fractions, suggesting a diagenetic
sequence from macromolecular material to small refractory molecules. Surface UDOM
is greater than deep UDOM by a factor of 2-3. Elemental as well as molecular-level
aldose and amino acid compositions were recently determined (McCarthy et al. 1996).
UDOM has a characteristic organic composition, clearly distinct from other marine
materials such as fresh plankton, sinking particles or humic substances. It is rich in
galactose and deoH sugars, and ubiquitous regardless of depth or location. In contrast the amino acid content of UDOM is a relatively minor component.
Various methods for the measurement of DOC, as well as the various problems common to all the techniques and those that are method specific, have been reviewed by
Wangersky (1993). This paper is intended to bring the reader up to date with the developments on the DOC measurements and to provide a survey of new concepts in
such analytical procedures.
16.2
DOC Measurement Techniques
The sheer complexity of DOM rules out any kind of rigorous analytical approach designed to individually quantify all of the contributory organic chemicals. Instead, the
strategy that has generally been adopted is to convert the organic carbon into CO 2 ,
OC may be classified according to the particle size. Since the distribution of molecular components of sea water covers a continuous size spectrum, ranging from small
molecules to colloids and larger particles, operational criteria have been developed
for the classification of OM. Accordingly, filtration may be performed using nominal
pore size of 0.45 flm, to get the so-called dissolved organic carbon (DOC). In the filtrate most of the colloids are recovered. The colloidal fraction contributes to 10-30%