The diversification of phytoplankton

Current estimates suggest that between 4000 and 5000 legitimate species of marine phytoplankton have been described (Sournia et al. 1991; Tett and Barton, 1995). I have not seen a comparable estimate for the number of species in inland waters, beyond the extrapolation I made (Reynolds, 1996a) that the number is unlikely to be substantially smaller. In both lists, there is not just a large number of mutually distinct taxa of photosynthetic microorganisms but there is a wide variety of shape, size and phylogenetic affinity. As has also been pointed out before (Reynolds, 1994a), the morphological range is comparable to the one spanning forest trees and the herbs that grow at their base. The phyletic divergence of the representatives is yet wider. It would be surprising if the species of the phytoplankton were uniform in their requirements, dynamics and susceptibilities to loss processes. Once again, there is a strong case for attempting to categorise the phytoplankton both on the phylogeny of organisms and on the functional basis of their roles in aquatic ecosystems. Both objectives are adopted for the writing of this volume. Whereas the former is addressed only in the present chapter, the latter quest occupies most of the rest of the book. However, it is not giving away too much to anticipate that systematics provides an important foundation for species-specific physiology and which is itself part-related to morphology. Accordingly, great attention is paid here to the differentiation of individualistic properties of representative species of phytoplankton.

However, there is value in being able simultaneously to distinguish among functional categories (trees from herbs!). The scaling system and nomenclature proposed by Sieburth et al. (1978) has been widely adopted in phytoplankton ecology to distinguish functional separations within the phytoplankton. It has also eclipsed the use of such terms as /.i-algae and ultraplankton to separate the lower size range of planktic organisms from those (netplankton) large enough to be retained by the meshes of a standard phytoplankton net. The scheme of prefixes has been applied to size categories of zooplankton, with equal success. The size-based categories are set out in Box 1.2.

At the level of phyla, the classification of the phytoplankton is based on long-standing criteria, distinguished by microscopists and biochemists over the last 150 years or so, from which there is little dissent. In contrast, subdivision within classes, orders etc., and the tracing of intraphyletic relationships, affinities within and among families, even the validity of supposedly well-characterised species, has become subject to massive reappraisal. The new factor that has come into play is the powerful armoury of the molecular biologists, including the methods for reading gene sequences and for the statistical matching of these to measure the closeness to other species.

Of course, the potential outcome is a much more robust, genetically verified family tree of authentic species of phytoplankton. This may be some years away. For the present, it seems pointless to reproduce a detailed classification of the phytoplankton that will soon be made redundant. Even the evolutionary connectivities among the phyla and their relationship to the geochem-ical development of the planetary structures are undergoing deep re-evaluation (Delwiche, 2000; Falkowski, 2002). For these reasons, the

Box 1.2

The classification of phytoplankton according to

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