physical environment of plankton is determined not by the intensity of mechanical energy introduced but by the rate of its dissipation and the sizes of the smallest eddies that it can sustain. Simply, the greater is the rate of dissipation, the finer is the structural grain.

In much the same way, we can deduce that the size of the smallest eddies (im) in a structure is independent of the forcing but depends only on the rate of energy dissipation (work) per unit mass (E, in J kg-1 s-1, which cancels to m2 s-3; see glossary of units, symbols and abbreviations) and the kinematic viscosity (v, in m2 s-1):

Solutions of Eq. (2.12) range from the order of millimetres in mixed layers, extending to metres in stratified layers (Spigel and Imberger, 1987). According to the data on well-mixed systems compiled by Reynolds (1994a), some of which are reproduced here as Table 2.2, the smallest eddy sizes calculated to be experienced in oceans and deeper lakes are hardly smaller than 1.3 mm. In rivers and shallow lakes, the smallest eddies may be are only half as large for the same input of kinetic energy. Tidal mixing of estuaries and coastal embayments powers some of the fastest rates of dissipation, and here the smallest eddies may be in the range 200-400 ¡m.

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