CHAPTER 5 . Organic Matter Preservation: Lipid Behavior from Plankton to Sediments
131
optically as part of the EqPac program. Estimates of lipid production by plankton (see
Wakeham et al.1997b for details) range from 30-290 mg lipid m- 2 d- 1 and show a latitudinal dependence corresponding to spatial variations in primary production (3001800 mg oe m- 2 d- 1 ; Barber et al. 1996). Fluxes of lipids decreased quickly in the water column, resulting in delivery rates of lipids to sediments that were 5-6 orders of
magnitude reduced (0.00004-0.0032 mg lipid m -2 d- 1 ).
Overall, two zones appear most responsible for this substantial decrease in flux. The
upper several hundred meters of the water column (the epipelagic zone) is the region
where zooplankton grazing and bacterial decomposition are greatest and utilize most
organic matter. And, the benthic boundary layer, especially the water-sediment interface, is another biological "hotspot" where benthic macro- and microfauna consume
much of that small fraction of organic material that rains down onto the seafloor. The
relatively small decrease in poe and lipid fluxes between shallow moored traps at
1 000 m below the seasurface and deep moored traps at 1000 m above the seafloor
indicates that decomposition in the ocean's interior is quantitatively relatively minor
and particles may transit the deep ocean with minimal degradation.
Simultaneous with the decrease in lipid flux during passage through the water column is a reduction in the proportion of organic carbon that lipids comprise (Fig. 5.2).
Lipids represented from up to 25% of poe in EqPac net-plankton but only"" 0.1% of
poe in subsurface sediments. This means that the proportion of organic carbon that
was lipid decreased by up to 250-fold, in marked contrast to amino acids whose contribution to poe decreased by "" 4-fold and carbohydrates that actually increased by
about 25% (Wakeham at el. 1997a). To a certain extent, the reactivity of the marine lipids that comprise the bulk of the particulate organic matter is due to their high degree of unsaturation, but not all marine lipids are so reactive as will be shown below.
Vascular plant lipids that are relatively minor components in particles but that become
an increasingly important part of sedimentary organic material often are highly saturated and as a result may be considerably more stable. The relative stability of carbohydrates may be related to that fact that one of their functions is as structural components that may help protect them from degradation (Hernes et al. 1996).
5.4
Seasonal Variations in Lipid Flux
Temporal variations in primary productivity in surface waters, resulting from physical oceanographic forcing functions, can drive strong temporal variability in the flux
of material to the deep sea. The Arabian Sea study measured time- and depth-dependent fluxes of organic materials as a function of variations of coastal upwelling in response to the monsoons off the coast of Oman (Lee et al. 1998). A strong seasonal effect on poe and lipid flux was observed at three depths in the Arabian Sea (Fig. 5.3).
In the shallowest trap, deployed at about 500 m, fluxes varied by 5-fold over the annual cycle. Maximal fluxes were measured from July through September, lagging the
south-west monsoon by about a month (Weller et al.1998) and presumably due to large
diatom blooms responding to enhanced upwelling. Secondary flux maxima occurred
during December and January following the weaker north-west monsoon. The seasonal variation in flux in the upper water column is significantly greater than corresponding temporal variations in primary productivity (1100 ±200 mg poe m -2 d- 1 ; Lee
131
optically as part of the EqPac program. Estimates of lipid production by plankton (see
Wakeham et al.1997b for details) range from 30-290 mg lipid m- 2 d- 1 and show a latitudinal dependence corresponding to spatial variations in primary production (3001800 mg oe m- 2 d- 1 ; Barber et al. 1996). Fluxes of lipids decreased quickly in the water column, resulting in delivery rates of lipids to sediments that were 5-6 orders of
magnitude reduced (0.00004-0.0032 mg lipid m -2 d- 1 ).
Overall, two zones appear most responsible for this substantial decrease in flux. The
upper several hundred meters of the water column (the epipelagic zone) is the region
where zooplankton grazing and bacterial decomposition are greatest and utilize most
organic matter. And, the benthic boundary layer, especially the water-sediment interface, is another biological "hotspot" where benthic macro- and microfauna consume
much of that small fraction of organic material that rains down onto the seafloor. The
relatively small decrease in poe and lipid fluxes between shallow moored traps at
1 000 m below the seasurface and deep moored traps at 1000 m above the seafloor
indicates that decomposition in the ocean's interior is quantitatively relatively minor
and particles may transit the deep ocean with minimal degradation.
Simultaneous with the decrease in lipid flux during passage through the water column is a reduction in the proportion of organic carbon that lipids comprise (Fig. 5.2).
Lipids represented from up to 25% of poe in EqPac net-plankton but only"" 0.1% of
poe in subsurface sediments. This means that the proportion of organic carbon that
was lipid decreased by up to 250-fold, in marked contrast to amino acids whose contribution to poe decreased by "" 4-fold and carbohydrates that actually increased by
about 25% (Wakeham at el. 1997a). To a certain extent, the reactivity of the marine lipids that comprise the bulk of the particulate organic matter is due to their high degree of unsaturation, but not all marine lipids are so reactive as will be shown below.
Vascular plant lipids that are relatively minor components in particles but that become
an increasingly important part of sedimentary organic material often are highly saturated and as a result may be considerably more stable. The relative stability of carbohydrates may be related to that fact that one of their functions is as structural components that may help protect them from degradation (Hernes et al. 1996).
5.4
Seasonal Variations in Lipid Flux
Temporal variations in primary productivity in surface waters, resulting from physical oceanographic forcing functions, can drive strong temporal variability in the flux
of material to the deep sea. The Arabian Sea study measured time- and depth-dependent fluxes of organic materials as a function of variations of coastal upwelling in response to the monsoons off the coast of Oman (Lee et al. 1998). A strong seasonal effect on poe and lipid flux was observed at three depths in the Arabian Sea (Fig. 5.3).
In the shallowest trap, deployed at about 500 m, fluxes varied by 5-fold over the annual cycle. Maximal fluxes were measured from July through September, lagging the
south-west monsoon by about a month (Weller et al.1998) and presumably due to large
diatom blooms responding to enhanced upwelling. Secondary flux maxima occurred
during December and January following the weaker north-west monsoon. The seasonal variation in flux in the upper water column is significantly greater than corresponding temporal variations in primary productivity (1100 ±200 mg poe m -2 d- 1 ; Lee