PLANKTON, STATUS AND ROLE OF C. S. Reynolds Freshwater Biological Association and NERC Institute of Freshwater Ecology I. The Structure of Planktonic Communities reproduce on organic carbon sources, taken in dis- II. Habitat Constraints in the Plankton solved or particle form. III. Form, Function, and Selection in the metazoan Literally, a multicelled animal. Phytoplankton mixotrophy The ability of a normally autotrophic or- IV. Form, Function, and Selection in the ganism to switch, circumstantially, to phagotrophy, Zooplankton or to support an otherwise meager food supply by V. Function in the Bacterioplankton resorting to the ingestion and assimilation of bacteria VI. Temporal Patterns in the Organization and or their products. Diversity of Planktonic Communities pelagic The (open-water) part of the aquatic environ- VII. Mechanisms Promoting and Maintaining ment that is far from the shore and the bottom bed. Diversity in the Plankton phagotrophy A type of heterotrophy that involves the VIII. Conclusions and Implications consumption of protists, plants, or animals as food. photoautotrophy A type of autotrophy in which organ- isms gather light energy in order to reduce carbon dioxide to organic carbon; characteristic of green GLOSSARY plants, most algae, and some prokaryotes. prokaryote Organizational state of cells lacking a mem- autotrophy The ability of organisms to grow and repro- brane-bound nucleus and certain other organelles. duce independently of external sources of organic Bacteria, including the Cyanobacteria, are typically carbon compounds. prokaryotic. Ͻ Ȑ eukaryote An organizational state of cellular organisms picophytoplankton The smallest ( 2 m) size class in which the genome of the cell is stored in chromo- of photoautotrophic plankton. somes enclosed in a membrane-bound nucleus; all protists (algae and protozoa), fungi, plants, and ani- mals are eukaryotes. euphotic The top layer of a water body through which ‘‘PLANKTON’’ IS A COLLECTIVE TERM for organisms sufficient light penetrates to support net photosyn- adapted specifically for a life passed mainly in suspen- thetic gain and the growth of photosynthesizing or- sion in the open waters (the pelagic zone) of the sea ganisms. Rarely more than 100 m in depth, the eu- and of such inland waters as lakes, reservoirs, and rivers. photic layer can be as little as 1 m in turbid waters. Planktonic organisms include protists (allegedly sim- heterotrophy The ability of organisms to grow and ple, unicellular, or colony-forming algal primary pro- Encyclopedia of Biodiversity, Volume 4 Copyright 2001 by Academic Press. All rights of reproduction in any form reserved. 569 570 PLANKTON, STATUS AND ROLE OF ducers and their protozoan consumers), microorgan- and its solvent properties, which maintain nutrients isms, and certain types of small metazoan animals, all and metabolic gases in readily assimilable state. sharing a common liability to passive entrainment in In truth, however, the planktonic ways of life have water currents, generated by tide, wind, convection, evolved to accommodate several problems and draw- gravity, and the rotation of the earth. The inherent backs associated with living in open water. Dominant physical variability of open-water habitats typically fa- among these is the issue of turbulence. Water molecules vors absolutely short life histories; rapid changes in experience strong mutual attraction, which makes the dominant species composition, in response to fluctuat- liquid relatively viscous when compared to other fluids. ing environmental conditions, contribute to the mainte- Seeing waves break on the shore, or watching ‘‘white’’ nance of high biological diversity in individual habitats water plunging through riverine rapids, we may be casu- and to the survival of a high species richness among ally impressed by the fluidity of water flow but, without planktonic assemblages in general. the driving energy, calm is rapidly reestablished as vis- cosity overcomes the residual motion at the molecular level. What happens is that the introduced mechanical energy is dissipated through a cascade of propagating I. THE STRUCTURE OF eddies, of diminishing size and velocity, until molecular PLANKTONIC COMMUNITIES attraction imparts order over chaos and the molecular movement is overwhelmed. This behavior is now mea- The functional definition of plankton, ventured at the surable and it has been mathematically described (see, introduction to this chapter, has superseded the origi- for instance, Mann and Lazier, 1991). What is of partic- nal, nineteenth-century allusions to plankton ‘‘floating’’ ular interest here is that, depending upon the intensity in water. Nevertheless, even this is still unsatisfactory, of persistent wind- or gravitational forcing, viscosity for its implication that the suspension is either complete overcomes inertia within the range 0.2 to 3 mm (see or continuous is strictly erroneous. However, genuinely Reynolds, 1997b, for examples). This means that the planktonic organisms—which include the plantlike, immediate environment experienced by organisms chlorophyll-containing primary producers of the phyto- smaller than this (i.e., most of the phytoplankton, bac- plankton, the heterotrophic, decomposer microorgan- terioplankton, and the smaller components of the zoo- isms of the bacterioplankton, and the more animal-like plankton) is wholly viscous: far from being fluid, the consumers of the zooplankton—are too small (often forces acting on the microorganism are comparable to Ͻ20 mm) for their own intrinsic movements to be able those experienced by a human immersed in treacle or to overcome, often or at all, the dispersive effects of unset cement. The organisms do not experience turbu- water movement. Thus ‘‘embedded’’ within the tireless lence, neither are their delicate morphologies threat- and unconstrained motion of open water, planktonic ened with physical damage, but they remain entrained organisms broadly go wherever the flow takes them. In in the turbulent field and continue to be randomized this way, the ecology of plankton is inextricably related throughout its spatial extent. Larger zooplankton (say to the physical properties of the medium, the extent Ͼ0.2 mm), though still too feeble to resist entrainment and limits of its motion, and the environmental condi- consistently, are sufficiently tough and flexible to toler- tions set within these bounds (Reynolds, 1997b). This ate the millimeter range of turbulence and to exploit it situation contrasts with that of most fish and other effectively in food gathering (Rothschild and Os- larger (Ͼ20 mm) animals of open water—the ‘‘nek- borne, 1988). ton’’—whose swimming strength is usually able to over- Beyond the selective constraints imposed by the come normal movement of the water. physical properties of pelagic, open-water environ- The older literature also promulgated a view that ments, it is also necessary to recognize that, with respect suspension in water was necessarily beneficial, suppos- to the obligate material components of living cells, the ing water to be something of an ideal habitat. Living aqueous concentrations of some of these (especially in water does confer some notable positive advantages carbon, nitrogen, phosphorus, iron, and fifteen or so over terrestrial or aerial habitats. These include the micronutrient elements) are often so dilute that their mechanical (‘‘Archimedean’’) support water provides, availabilities place a severe constraint on the assembly as a consequence of its much greater density in compari- of planktonic biomass. Moreover, despite its alleged son with air; its slow temperature fluctuations, as a transparency, the absorbance of solar energy by pure consequence of its much higher specific heat than air; water (see, for instance, Kirk, 1994) is such that, at PLANKTON, STATUS AND ROLE OF 571 depths of Ͼ100 m, it is always as dark as night. Biologi- nia et al., 1991). The total number recorded from inland cal productivity in lakes is often severely constrained waters is not certainly known, but it is estimated that by rarefied resources or by deficiencies in processing there are quite 4000 of these as well (Reynolds, 1996). energy, or by both. Far from being an ideal environ- Few genera and still fewer species are common to both ment, the pelagic is a rather unpromising medium for fresh and salt waters. Even if a fairly conservative view successful exploitation by living communities. of their classification is adopted, the species are drawn Yet, within this general constraint, there is a remark- from at least six distinct protist phyla and at least two able richness of individual species inhabiting the plank- major prokaryote subdivisions (see Table I). The Purple ton of the world’s lakes and seas. Not all have even been (Chromatiaceae) and Green Sulfur Bacteria (Chlorobi- adequately described and separated. The extraordinary aceae) are represented in specialized, anoxic habitats. diversity and phyletic representation of planktonic or- Of the planktonic genera of Cyanoprokaryotes (for- ganisms may only be hinted at in the following subsec- merly classed as Cyanophyceae, or ‘‘blue green algae,’’ tions. As a preface to any such review of the planktonic and now most commonly referred to as ‘‘Cyanobacte- biota, however, it is necessary to emphasize that the ria’’) most occur in lakes, though several are also com- familiar separation, first of plants and animals and then mon in the low-salinity (Ͻ11 parts per thousand) areas their subdivision among phyletic divisions,