The Descent Through The Water Column

The physical and biological properties of the aggregates determine their transport behavior in the water column. The excess density over that of the surrounding water controls the speed with which aggregates descend to the sea floor. For particles with Reynolds numbers <1, Stokes' law can be applied to determine the settling velocity:

d2 (Ap)g 18v where d is the particle diameter, Ap is the excess density of the particle over seawater, g is the gravitational constant and v is the kinematic viscosity of the fluid. The kinematic viscosity is strongly temperature dependent and has an enormous influence on the behavior of particles of low Reynolds numbers. v nearly doubles from warm surface waters (0.01 cm2 sec-1 at 20°C) to cold bottom waters (0.018 cm2 sec-1 at 1°C). As Stokes law was originally applied to rigid, impermeable spherical particles of known density, it is difficult to apply to nonspherical aggregated particles which virtually represent most particulate material in the ocean. However, Stokes can then be used to back-calculate the particle density of the aggregates as discussed by.14 During the last two decades empirical particle-size/settling-velocity relationships have been developed for different oceanic regimes (Figure 11.1). The data reveal that organic rich aggregates from surface waters at continental margins show much lower settling velocities than those of similar size but enriched in ballast. This lithogenic ballast is added to the organic aggregates during resuspension events.

The surface mixed layer at the top of the ocean varies in thickness from tens to hundreds of meters and aggregate concentrations inside this layer are related to the processes of production, destruction, and sinking. Peak concentrations are often located at the base of the surface mixed layer, which can extend up to a few hundred meters during winter. This layer is subject to rapid changes in heat, turbulence, nutrients, and depth of mixing. The peak concentrations at the base of the surface mixed

FIGURE 11.1 Particle settling velocities/particle diameter relationships of aggregates from ocean surface waters (A), intermediate and bottom nepheloid layers (B), and from the benthic boundary layer (C, D, E) determined by Alldredge and Gottschalk4 for A, McCave14 for B, Thomsen et al.23 for C (Celtic margin), Thomsen et al.24 for D (Iberian margin), and Sternberg

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