I x I03

~ 1.03 x I03

Absolute viscosity n (kgm-1 s-1)

1.8 x I0-5

1 x I0-3

~ I.I x I0-3

the motion of natural water bodies. Empirical description of entrainment relies on the analogous relativity between the intrinsic sinking velocity of the plankter (ws) and the turbulent velocity of the motion (u*) This immediately introduces an anomaly, which must be addressed at once. The idea is that the smaller is ws relative to u* then the more complete is the entrainment of the particle in the motion. This is another way of saying that the best way to ensure entrain-ment is to minimise the rate of sinking. Isn't this just the idea that was so summarily dismissed in the first paragraph? No, for the comment was directed to the redundant, notional context of a slow settlement of plankters through a static water column. What is really needed, if full entrainment is the goal, is a slow rate of settlement relative to the water immediately adjacent to the organism and its instantaneous trajectory and velocity.

The approach adopted in developing this chapter is to first consider the nature, scale and variability of water movements and the estimation of u*. Then the question of settling velocities, buoyant velocities and swimming rates (ws) is reviewed, before the consequences on spatial and temporal distributions are considered at the end.

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