CHAPTER 5 . Particulate Organic Matter Composition and Fluxes in the Sea
135
decomposition before they are recovered. This treatment can result in an overestimated
flux due to the collection of "swimmers:' that is, zooplankton or fish that swim into
the trap and die from contact with the poison (Lee et al.1988). In shallow traps placed
in coastal areas, much of the material collected can be swimmers. Technological
changes in sediment traps address this problem (Peterson et al.1993; Buesseler et al. 2000).
The preferential removal of certain components, both organic and inorganic, from
sinking particles leads to major changes in their composition. Decomposition and
transformation rates vary depending on the molecular structure of individual compounds and their availability as substrates for heterotrophic metabolism.
5.4
Compositional Changes During Degradation
5.4.1
Initial Composition
Just as primary productivity affects the amount of material sinking in the ocean, compounds produced by plankton influence the composition of organic matter in sinking
particles. Some of the organic matter produced in the euphotic zone does sink deep
in the water column, eventually reaching the sediment, and the composition of this
particulate organic matter can vary significantly depending on it original source. The
fact that some of the original composition is preserved is the basis of using "molecular biomarkers" as indicators of the source of sedimentary organic matter. Variation
in the complex biochemical composition of living organisms provides a variety of
natural biomarkers. The great number and chemical diversity of biomarkers among
the biochemicals typical of living organisms provides a "fingerprint" that can be used
to determine sources of organic matter (Lee and Wakeham 1989; Wakeham and Lee
1989). Such fingerprints are intrinsic characteristics of organic matter; as concentrations relative to total carbon or as individual compound concentration ratios, they can
be applied without many of the problems inherent in estimates of sediment trap fluxes.
For example, just the presence of certain fatty acids and sterols in particles from such
biologically diverse oceanic regimes as the highly-productive Peru upwelling area and
the oligotrophic North Central Pacific reflects the different plankton communities
found in these areas (Wakeham and Lee 1989). In addition, specific lipids can clearly
distinguish marine from terrestrial sources; marine hydrocarbons and fatty acids typically have shorter carbon chain-lengths than their terrestrial counterparts. Compositional differences among carbohydrates and amino acids can also reflect biological
sources, although these compound classes are usually not as source-specific as lipids
and pigments. Carbohydrates and amino acids also make up a substantial amount of
the organic matter in marine organisms. Some organisms like siliceous diatoms are
reportedly characterized by elevated percentages of glycine and fucose, while organisms with carbonate tests like coccolithophores often exhibit unusually high concentrations of aspartic acid and arabinose (Mitterer 1968; Hecky et al. 1973; Ittekkot et al.
1984a,b). Although both plankton and bacteria are characterized by high relative abundances ofribose and fucose (Cowie and Hedges 1984), O-methyl sugars and uronic
acids may provide a means of discriminating between these two important sources of
organic matter (Mopper and Larsson 1978). In addition, muramic acid and certain
135
decomposition before they are recovered. This treatment can result in an overestimated
flux due to the collection of "swimmers:' that is, zooplankton or fish that swim into
the trap and die from contact with the poison (Lee et al.1988). In shallow traps placed
in coastal areas, much of the material collected can be swimmers. Technological
changes in sediment traps address this problem (Peterson et al.1993; Buesseler et al. 2000).
The preferential removal of certain components, both organic and inorganic, from
sinking particles leads to major changes in their composition. Decomposition and
transformation rates vary depending on the molecular structure of individual compounds and their availability as substrates for heterotrophic metabolism.
5.4
Compositional Changes During Degradation
5.4.1
Initial Composition
Just as primary productivity affects the amount of material sinking in the ocean, compounds produced by plankton influence the composition of organic matter in sinking
particles. Some of the organic matter produced in the euphotic zone does sink deep
in the water column, eventually reaching the sediment, and the composition of this
particulate organic matter can vary significantly depending on it original source. The
fact that some of the original composition is preserved is the basis of using "molecular biomarkers" as indicators of the source of sedimentary organic matter. Variation
in the complex biochemical composition of living organisms provides a variety of
natural biomarkers. The great number and chemical diversity of biomarkers among
the biochemicals typical of living organisms provides a "fingerprint" that can be used
to determine sources of organic matter (Lee and Wakeham 1989; Wakeham and Lee
1989). Such fingerprints are intrinsic characteristics of organic matter; as concentrations relative to total carbon or as individual compound concentration ratios, they can
be applied without many of the problems inherent in estimates of sediment trap fluxes.
For example, just the presence of certain fatty acids and sterols in particles from such
biologically diverse oceanic regimes as the highly-productive Peru upwelling area and
the oligotrophic North Central Pacific reflects the different plankton communities
found in these areas (Wakeham and Lee 1989). In addition, specific lipids can clearly
distinguish marine from terrestrial sources; marine hydrocarbons and fatty acids typically have shorter carbon chain-lengths than their terrestrial counterparts. Compositional differences among carbohydrates and amino acids can also reflect biological
sources, although these compound classes are usually not as source-specific as lipids
and pigments. Carbohydrates and amino acids also make up a substantial amount of
the organic matter in marine organisms. Some organisms like siliceous diatoms are
reportedly characterized by elevated percentages of glycine and fucose, while organisms with carbonate tests like coccolithophores often exhibit unusually high concentrations of aspartic acid and arabinose (Mitterer 1968; Hecky et al. 1973; Ittekkot et al.
1984a,b). Although both plankton and bacteria are characterized by high relative abundances ofribose and fucose (Cowie and Hedges 1984), O-methyl sugars and uronic
acids may provide a means of discriminating between these two important sources of
organic matter (Mopper and Larsson 1978). In addition, muramic acid and certain