The aims of this chapter are to develop an appreciation of the adaptive requirements of phyto-plankton 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. It is not a condition either for the individual or for the persistence of the clone that residence is continuous, only that individuals of any given generation spend sufficient of their life in the photic zone to make the net autotrophic gains in syn-thesised carbon, over the burden of respiration, to be able to sustain the next cell replication. The point here is that the essential adaptation is to maximise the exposure to adequate light, by any appropriate mechanism.
The mechanisms for this are not self-evident, unless the behaviour of the water itself is taken into account. For this is the feature that the classical explanation of phytoplankton adaptations rather omits - that, at every scale, the water is never a passive component. Under the influence of its warming and cooling, of the influence of gravity, the pull of the Moon, of the work of wind and even of the rotation of the Earth, water is in motion. Some of these inputs are continuously variable, and their various interactions with the internal viscous forces contribute to a spectrum of motion that is characteristically variable, in both time and space.
Thus, there is an explicit, inescapable and variable velocity component to the medium. Moreover, the movement is, almost always, turbulent, so that flow tends to be in billowing eddies rather than along direct trajectories. Such movements are capable of alternately enhancing or counteracting the intrinsic velocity of the vertical tendency of the settling plankter and, within the finite bounds of the water mass, may force its lateral displacement or even push it upwards.
These possibilities are the basis of the principle of entrainment of phytoplankton in
Table 2.1 Comparison of the physical properties of air, pure water and sea water
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