Phytoplankton sinking and floating

The buoyant properties of non-motile plankters, having rigid walls but lacking flagella or cilia, moving through a column of water, in response to gravity (g = 9.8081 ms-2), are subject to the same forces that govern the settlement of inert particles in viscous fluids, which were quantified over a century and a half ago (Stokes, 1851). As the body moves, it displaces some of the fluid. Provided the movement of the displaced fluid over the particle is laminar, thus causing no turbulent drag, then its velocity (ws) is related to its size (diameter, d) and the difference between its density from that of the water (pw). For a spherical particle of uniform density (pc), ws = gd2(pc - Pw)(18 n) 1

The power required to maintain this momentum, ~0.5 W kg-1, is also quite trivial when compared

This is the well-known Stokes equation. Note that for a buoyant particle (pc < pw), Eq. 2.14 has a negative solution, representing a rate of flotation upwards. An empirical verification by McNown and Malaika (1950), who measured the sinking rates of machined metal shapes in viscous oils, is also frequently cited in the literature on phytoplankton. The Stokes equation is implicitly taken as a valid base for predicting the sinking behaviour of phytoplankton but it is necessary also to test all its assumptions and components if we are to grasp the many mechanisms

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