The correct place to begin any exposition of a major component in biospheric functioning is with precise definitions and crisp discrimination. This should be a relatively simple exercise but for the need to satisfy a consensus of understanding and usage. Particularly among the biological sciences, scientific knowledge is evolving rapidly and, as it does so, it often modifies and outgrows the constraints of the previously acceptable terminology. I recognised this problem for plankton science in an earlier monograph (Reynolds, 1984a). Since then, the difficulty has worsened and it impinges on many sections of the present book. The best means of dealing with it is to accept the issue as a symptom of the good health and dynamism of the science and to avoid constraining future philosophical development by a redundant terminological framework.
The need for definitions is not subverted, however, but it transforms to an insistence that those that are ventured are provisional and, thus, open to challenge and change. To be able to reveal something also of the historical context of the usage is to give some indication of the limitations of the terminology and of the areas of conjecture impinging upon it.
So it is with 'plankton'. The general understanding of this term is that it refers to the collective of organisms that are adapted to spend part or all of their lives in apparent suspension in the open water of the sea, of lakes, ponds and rivers. The italicised words are crucial to the concept and are not necessarily contested. Thus, 'plankton' excludes other suspensoids that are either non-living, such as clay particles and precipitated chemicals, or are fragments or cadavers derived from biogenic sources. Despite the existence of the now largely redundant subdivision tychoplank-ton (see Box 1.1), 'plankton' normally comprises those living organisms that are only fortuitously and temporarily present, imported from adjacent habitats but which neither grew in this habitat nor are suitably adapted to survive in the truly open water, ostensibly independent of shore and bottom. Such locations support distinct suites of surface-adhering organisms with their own distinctive survival adaptations.
'Suspension' has been more problematic, having quite rigid physical qualifications of density and movement relative to water. As will be rehearsed in Chapter 2, only rarely can plankton be isopycnic (having the same density) with the medium and will have a tendency to float upwards or sink downwards relative to it. The rate of movement is also size dependent, so that 'apparent suspension' is most consistently achieved by organisms of small (<1 mm) size. Crucially, this feature is mirrored in the fact that the intrinsic movements of small organisms are frequently too feeble to overcome the velocity and direction of much of the spectrum of water movements. The inability to control horizontal position or to swim against significant currents in open waters separates 'plankton' from the 'nekton' of active swimmers, which include adult fish, large cephalopods, aquatic reptiles, birds and mammals.
Some definitions used in the literaure
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