The chapter provides an introduction to phytoplankton. The phytoplankton is defined as a collective of photosynthetic microorganisms, adapted to live partly or continuously in the open of the seas, of lakes (including reservoirs), ponds and river waters, where they contribute part or most of the organic carbon available to pelagic food webs. Although their taxonomy is currently undergoing major revision and even the phylo-genies are questioned, it is difficult to be categorical about the species representation and phyletic make-up of phytoplankton. It is reasonable to point to the description of some 4000 to 5000 species from the sea and, probably, a similar order of species from inland waters. The species belong to what appear to be 14 legitimate phyla, coming from both bacterial and eukaryotic pro-tist domains. In both the marine and the freshwater phytoplankton, there is a wide diversity of size, morphology, colony formation. Though generally microscopic, phytoplankton covers a range of organism sizes comparable to that spanning forest trees and the herbs that grow at their bases.

The early history of phytoplankton studies is recapped. Although a knowledge of some of the organisms goes back to the invention of the microscope, and many genera were well known to nineteenth-century microscopists, their role in supporting the aquatic food webs of open water, culminating in commercially exploitable fish populations, was not realised until the 1870s. The early work by Muller, Haeckel and Hensen

(who invented the name 'plankton') is briefly described. Some of the terms used in plankton science are noted with their meanings, while those that appear still to be conceptually useful are singled out for retention.

Despite variation of several orders of magnitude in the sizes of plankters, there is a powerful trend towards conservatism of the surface-to-volume ratio, which is achieved through distortion and departure from the spherical form among the larger species. This aids exchange of gases, nutrients and other solutes across the cell surface and it also has some role in prolongation of suspension. In an apparently diametrically opposite trend, some algae form mucilaginous coenobia that have very low surface-to-volume ratios. When it is combined with some other power of motility, the streamlining effect allows the colony to move relatively quickly through water and to move to a more favourable position in the water column.

The construction and composition of plankton are critically reviewed. Apart from a variety of scales, exoskeleta, plastid type and pigment composition, the ultrastructural components and architecture of the living protoplasm are comparable among the phytoplankton. Similarly, the elemental make-up of the protoplast is similar among all groups of phytoplankton, ideally occurring in approximately stable relative proportions. Discounting the ash from the mineral-reinforced walls, carbon accounts for about 50% of the dry mass, nitrogen about 8-9% and phophorus between 1% and 1.5%. Relative to phosphorus, these amounts correspond to a probabilistic atomic ratio of 106 C: 16 N: 1 P, close to the so-called Redfield ratio for particulate matter in the ocean. It is also similar to the composition of most living protoplasm. The amounts are related also to hydrogen, oxygen, silicon, sulphur and iron. Up to 12 other elements are regularly present in phytoplankton in trace proportions. Departures from the ratio are rarely systematic, merely indicative of one of the highly variable components falling to the minimum cell quota.

The amount of chlorophyll a is also highly variable according to growth conditions but nevertheless tends to average about 1% of the ash-free dry mass of the cell and to represent about 2% of the elemental carbon. A carbon:chlorophyll value of 50: 1 is considered typical but it may vary routinely between about 70 : 1 (cells in high light) to 30: 1 or lower (in cells exposed to consistently low light).

Despite the extreme diversity of phylogeny, morphology and size, both the marine and the freshwater phytoplankton are characterised by a striking and statistically predictable blend of elemental constituents. This proves very helpful in quantifying production and attrition processes contributing to the dynamics of natural, functioning assemblages of plankton.

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