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A flow system was developed which enables studies of hydrodynamics and mass transfer in freely sinking aggregates. The aggregates stabilized their positions in the water phase at an upward flow velocity which balanced and opposed the sinking velocity of the individual aggregate. The flow field was shown to b e larninar at flow velocities ranging from 40 to 200 m d-', which cover typical sinking velocities of aggregates in the ocean. A viscous boundary layer with steep gradients of flow velocity was measured around sinking aggregates by flow visualization techniques. Velocity gradients in the viscous boundary layer along the sides of the aggregate parallel to the flow could by explained by creeping flow, while a non-turbulent wake was present at the rear (downstream) of the aggregate. The oxygen distribution inside a ca 3.5 mm large diatom aggregate and in the surrounding water was mapped in 2 dimensions from 360 nucroelectrode measurements. The data showed the existence of a diffusive boundary layer surrounding the aggregate, which indicates that the radial transport of solutes occurred by molecular diffusion. The diffusive boundary layer thickness was 0.38 mm upstream, 0.85 mm along the sides, and 1.0 mm downstream The oxygen microenvironment of aggregates changed dramatically when they were sitting on a solid surface compared with when they were sinking.
Ploug et al. (Fri,) studied this question.