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Chemical Oceanography, 4th Edition
chemical reactions that take place in the sea. They provide a nutritional and energy base
for micro- and macroorganisms, have a major impact on the speciation of many metals
by such processes as complexation and adsorption, and are precursors of fossil fuels such
as petroleum and oil shale. These organic compounds can also provide growth promoters and inhibitors that may control phytoplankton succession. Much research activity
has been directed toward identifying the specific components of organic matter in the
seas and toward untangling the complex web of biogeochemical relationships between
dissolved and particulate compounds associated with microbial processes (amino acids,
carbohydrates, fatty acids, etc.).
The source of most of the organic compounds in seawater is primary production in
the euphotic zone (Table 9.1). Part of the organic matter from photosynthesis is partially
metabolized to satisfy the energy requirements of phytoplankton. The other fraction is
assimilated by zooplankton and released into the water column by the autolysis of dead
cells or is excreted by organisms. Bacteria eventually break down most of the organic compounds to carbon dioxide. In coastal waters, a significant proportion of the organic matter can also come from the land via rivers and coastal sediments. The various sources of
organic compounds in the oceans are discussed in the next section.
9.4.1 Sources of Organic Matter
Primary production by marine phytoplankton has been estimated to be responsible for the
production of approximately 2 × 10 16 gC yr –1 . Much of this organic matter is consumed by
organisms, but a significant amount is decomposed at the surface, while 1 to 12% of the
annual production (0.2 to 2.4 × 10 15 gC yr –1 ) remains as a refractory fraction, quite resistant
to biological and chemical transformations. The residence time of this residual DOC in
the sea has been estimated by Williams and Druffel (1987) to be 1310 years for surface and
6240 years for deep dissolved organic matter (DOM). Most of this residual organic carbon
consists of humic substances.
The other sources of input of organic matter to seawater, in addition to primary productivity by marine plankton, include terrestrial input by rivers and by the atmosphere,
excretion by marine organisms, and the resuspension of organic matter from marine
sediments. The direct addition of organic matter from oil spills is an additional source.
9.4.1.1 Terrestrial Input by Rivers
The typical concentration of DOC in estuaries (Figure 9.34) shows that rivers can be a
source of DOC to the oceans. Based on levels of POC and DOC of major river systems,
it has been estimated by Meybeck (1987) and others that the amount of total organic carbon (TOC) transported to the seas by river systems amounts to about 33 × 10 12 mol C
yr –1 . Approximately half of this carbon comes from POC and the other half from DOC.
Terrestrially derived carbon is often referred to as allochthonous carbon (which means
that it is derived from outside the system being considered—the oceans). In freshwater
systems, the two major sources of organic carbon are plant- and soil- derived carbon.
Rainwater that penetrates soils can leach organic soil and plant debris, as well as inorganic
constituents, into dissolved and suspended phases.
Numerous studies on the leaching of plant organic matter into streams have shown that
within 24 h as much as 40% of the organic matter of fresh plant litter can be solubilized by
water. At least half of the DOC of the leachates from plant debris consists of carbohydrates.
The remaining DOC consists of a mixture of substances that are either contributors or
Chemical Oceanography, 4th Edition
chemical reactions that take place in the sea. They provide a nutritional and energy base
for micro- and macroorganisms, have a major impact on the speciation of many metals
by such processes as complexation and adsorption, and are precursors of fossil fuels such
as petroleum and oil shale. These organic compounds can also provide growth promoters and inhibitors that may control phytoplankton succession. Much research activity
has been directed toward identifying the specific components of organic matter in the
seas and toward untangling the complex web of biogeochemical relationships between
dissolved and particulate compounds associated with microbial processes (amino acids,
carbohydrates, fatty acids, etc.).
The source of most of the organic compounds in seawater is primary production in
the euphotic zone (Table 9.1). Part of the organic matter from photosynthesis is partially
metabolized to satisfy the energy requirements of phytoplankton. The other fraction is
assimilated by zooplankton and released into the water column by the autolysis of dead
cells or is excreted by organisms. Bacteria eventually break down most of the organic compounds to carbon dioxide. In coastal waters, a significant proportion of the organic matter can also come from the land via rivers and coastal sediments. The various sources of
organic compounds in the oceans are discussed in the next section.
9.4.1 Sources of Organic Matter
Primary production by marine phytoplankton has been estimated to be responsible for the
production of approximately 2 × 10 16 gC yr –1 . Much of this organic matter is consumed by
organisms, but a significant amount is decomposed at the surface, while 1 to 12% of the
annual production (0.2 to 2.4 × 10 15 gC yr –1 ) remains as a refractory fraction, quite resistant
to biological and chemical transformations. The residence time of this residual DOC in
the sea has been estimated by Williams and Druffel (1987) to be 1310 years for surface and
6240 years for deep dissolved organic matter (DOM). Most of this residual organic carbon
consists of humic substances.
The other sources of input of organic matter to seawater, in addition to primary productivity by marine plankton, include terrestrial input by rivers and by the atmosphere,
excretion by marine organisms, and the resuspension of organic matter from marine
sediments. The direct addition of organic matter from oil spills is an additional source.
9.4.1.1 Terrestrial Input by Rivers
The typical concentration of DOC in estuaries (Figure 9.34) shows that rivers can be a
source of DOC to the oceans. Based on levels of POC and DOC of major river systems,
it has been estimated by Meybeck (1987) and others that the amount of total organic carbon (TOC) transported to the seas by river systems amounts to about 33 × 10 12 mol C
yr –1 . Approximately half of this carbon comes from POC and the other half from DOC.
Terrestrially derived carbon is often referred to as allochthonous carbon (which means
that it is derived from outside the system being considered—the oceans). In freshwater
systems, the two major sources of organic carbon are plant- and soil- derived carbon.
Rainwater that penetrates soils can leach organic soil and plant debris, as well as inorganic
constituents, into dissolved and suspended phases.
Numerous studies on the leaching of plant organic matter into streams have shown that
within 24 h as much as 40% of the organic matter of fresh plant litter can be solubilized by
water. At least half of the DOC of the leachates from plant debris consists of carbohydrates.
The remaining DOC consists of a mixture of substances that are either contributors or