Chapter 16
Oceanic DOC Measurements
C. Minero . E. Pelizzetti . M.R. Preston
16.1
Introduction
Dissolved organic carbon (DOC) is by far the largest of the organic carbon reservoirs
in the oceans with a carbon load that for many years was accepted to be approximately
200 Gt C (Hedges 1987). It is made up of a very complex mixture of chemicals, which
are generally known collectively as 'dissolved organic matter' or DOM, and which derive either directly from biological processes or from the enzymatic or bacterial degradation of defunct organisms. The heterogeneous nature of the composition of DOM
is also reflected in its molecular size distribution which ranges from that of the simplest organic molecules up to complex macromolecules whose properties lie at an indistinct boundary between truly dissolved and colloidal or particulate matter (Gough
and Mantoura 1991).
This DOM has a major influence in determining the nature of a wide variety of
physical, geochemical and biological processes in the oceans (Gagosian and Steurmer
1977; Jackson 1988). For example, the accumulation of DOM at the air-sea interface alters
the surface tension of the water and may modify the transfer of wind energy, air-sea
gas exchange rates and the incursion of light (Mopper et al. 1991). The adsorption of
DOM onto the surfaces of inorganic particles radically modifies their physicochemical properties with profound consequences for other adsorption reactions (e.g. involving trace metals) and interactions with the biota (Stumm 1992). Degradation of
DOM, normally by bacteria, may alter both dissolved oxygen concentrations and redox potentials with significant consequences for other biogeochemical processes.
The first studies of DOM in the oceans occurred in the 1930S (Krough 1934; Datzko
1939) and to some extent difficulties over the selection of the most appropriate analytical procedures were evident from the outset. Early efforts using dichromate or
chromic acid were abandoned fairly soon, but discrepancies emerged between dry
combustion methods (used by e.g. Skopintsev et al. 1966,1968 and Starikova 1970;
MacKinnon 1978) and wet chemical!UV oxidation methods (used by others, e.g. Menzel
1970; Banoub and Williams 1972; Gershey et al.1979).
As a general rule, dry combustion methods gave dissolved organic carbon (DOC)
values that were roughly twice those of wet chemical methods. The debate about the
merits of these various figures has been reviewed by, for example, Wangersky (1993)
and was conducted along the competing charges of incomplete reaction (levelled at
adherents of the wet chemical oxidation procedure) and contamination of the reaction (levelled at the dry combustion enthusiasts). By the end of the 1970S, and well into
the 1980s, the view of the proponents of wet chemical/UV oxidation methods had
largely prevailed. As a result some sort of consensus that marine DOC concentration
Oceanic DOC Measurements
C. Minero . E. Pelizzetti . M.R. Preston
16.1
Introduction
Dissolved organic carbon (DOC) is by far the largest of the organic carbon reservoirs
in the oceans with a carbon load that for many years was accepted to be approximately
200 Gt C (Hedges 1987). It is made up of a very complex mixture of chemicals, which
are generally known collectively as 'dissolved organic matter' or DOM, and which derive either directly from biological processes or from the enzymatic or bacterial degradation of defunct organisms. The heterogeneous nature of the composition of DOM
is also reflected in its molecular size distribution which ranges from that of the simplest organic molecules up to complex macromolecules whose properties lie at an indistinct boundary between truly dissolved and colloidal or particulate matter (Gough
and Mantoura 1991).
This DOM has a major influence in determining the nature of a wide variety of
physical, geochemical and biological processes in the oceans (Gagosian and Steurmer
1977; Jackson 1988). For example, the accumulation of DOM at the air-sea interface alters
the surface tension of the water and may modify the transfer of wind energy, air-sea
gas exchange rates and the incursion of light (Mopper et al. 1991). The adsorption of
DOM onto the surfaces of inorganic particles radically modifies their physicochemical properties with profound consequences for other adsorption reactions (e.g. involving trace metals) and interactions with the biota (Stumm 1992). Degradation of
DOM, normally by bacteria, may alter both dissolved oxygen concentrations and redox potentials with significant consequences for other biogeochemical processes.
The first studies of DOM in the oceans occurred in the 1930S (Krough 1934; Datzko
1939) and to some extent difficulties over the selection of the most appropriate analytical procedures were evident from the outset. Early efforts using dichromate or
chromic acid were abandoned fairly soon, but discrepancies emerged between dry
combustion methods (used by e.g. Skopintsev et al. 1966,1968 and Starikova 1970;
MacKinnon 1978) and wet chemical!UV oxidation methods (used by others, e.g. Menzel
1970; Banoub and Williams 1972; Gershey et al.1979).
As a general rule, dry combustion methods gave dissolved organic carbon (DOC)
values that were roughly twice those of wet chemical methods. The debate about the
merits of these various figures has been reviewed by, for example, Wangersky (1993)
and was conducted along the competing charges of incomplete reaction (levelled at
adherents of the wet chemical oxidation procedure) and contamination of the reaction (levelled at the dry combustion enthusiasts). By the end of the 1970S, and well into
the 1980s, the view of the proponents of wet chemical/UV oxidation methods had
largely prevailed. As a result some sort of consensus that marine DOC concentration