Planktic movement and laminar flow

Let us take the condition of laminar flow. McNown and Malaika (1950) showed good adherence to the Stokes formulation whilst Re < 0.1 and that the error was <10% for Re < 0.5. For comparison, Walsby and Reynolds (1980) applied published data for phytoplankton to solve Eq. (2.13) for various phytoplankton, approximating pw as 103 kg m-3 and n as 10-3 kg m-1 s-1 in each case. For the large marine centric diatom Coscin-odiscus wailseii (d ~150 x 10-6 m), and substituting ws = 0.1 x 10-3 ms-1 for us (from Smayda, 1970), Eq. (2.13) was balanced by Re = 0.015. Similarly, using measurements from Reynolds (1973a) for a freshwater centric diatom, Stephan-odiscus rotula (d ~50 x 10-6 m, ws = 25 x 10-6 m s-1), Re = 0.00125. The deduction that the movements of most phytoplankton comfortably conform to the laminar flow condition is, however, challenged by very large plankters. According to Smayda's (1970) data, the sinking of the extraordinary Ethmodiscus rex, one of the largest known diatoms (d ~ 1 mm, ws = 6 x 10-3 ms-1), generates an Re ~ 6. Working with a size range of colonies of the Cyanobac-terium Microcystis, Reynolds (1987a) showed that the Stokes equation (2.14) predicted velocities well in colonies of known densities up to (d =) 200 x 10-6 m in diameter, but in larger colonies (d up to 4 mm), velocities became significantly overpredicted especially when Re > 1.

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