Introduction

The aims of this chapter are to develop an appreciation of the adaptive requirements of phytoplankton for pelagic life and to demonstrate the consequences of its embedding in the movements of the suspending water mass. The exploration begins by dismissing the simplistic notion that the essential requirement of plankton is to prevent or minimise the rate of sinking, in the sense that this will prolong its residence in the upper part of the water column. This would be a clear nonsense, were there no counteractive mechanism to ensure that organisms start out at the top of the water in the first instance. Moreover, slow sinking from the upper layers is of illusory respite if the downward passage to depths beyond the adequacy of penetrating light, whether that is 50 cm or 50 m beneath the water surface, is inevitable, unless there is some mechanism for the organism's return. Manifestly, it is not enough just to reduce the rate of irreversible sinking to qualify as a phytoplankter.

Prolonged residence in the upper insolated layers of the open water of lakes and seas (the photic zone) is, without doubt, a primary requirement of the individual phytoplankter, if it is to synthesise sufficient organic carbon to build the tissue of the next generation. The survival of the genetic stock and the seed population capable of providing the base of subsequent generations may also depend upon the survival of a relatively small number of extant individuals.