The concept of 'equilibrium', as a balance between contrasting forces, may resemble to be not appropriate to describe structure and dynamics of phytoplankton communities, where species show extremely high population growth rates and high temporal and spatial variability. However, the small body size of phytoplankton individuals, which strongly affects individual energetics and population potential to growth, actually enable populations to adapt rapidly to varying environmental conditions, increasing population resilience and facilitating the achievement of equilibrium conditions. In fact, in terms of biomass, phytoplankton communities emphasize patterns of equilibrium with the environmental limiting factors both in space and in time. Most of the debate on the occurrence of some kind of equilibrium in phyto-plankton communities deals with the taxonomic composition and richness of phytoplankton communities, being synthesized by the 'plankton paradox' concept. Hutchinson introduced the term 'plankton paradox' to describe the coexistence of so many phytoplankton species under limiting and relatively isotropic conditions, suggesting that whenever rates of competitive exclusion and of environmental changes are very close, a 'dynamic equilibrium' with supersaturated communities can be observed. The 'plankton paradox' is still unsolved; however, factors determining competitive abilities of phytoplankton species, potentially leading to 'stable equilibrium conditions', as well as factors contributing to the maintenance of supersaturated communities under 'dynamic equilibrium' conditions, have been emphasized. They include the intrinsic variability of phytoplankton populations and the occurrence of external factors of disturbance contributing to increase the patchiness in the apparently isotropic and unstructured water column.

See also: Marine Models; Stability; Stability versus Complexity.

Harris GP (1986) Phytoplankton Ecology: Structure, Function and Fluctuation. London: Chapman and Hall.

Huisman J and Weissing FJ (1999) Biodiversity of plankton by species oscillations and chaos. Nature 402: 407-410.

Hutchinson GE (1961) The paradox of the plankton. American Naturalist 95: 137-145.

Robinson C, Poulton AJ, Holligan PM, et al. (2006) The Atlantic Meridional Transect (AMT) programme: A contextual view 19952005. Deep-Sea Research II53: 1485-1515.

Tilman D (1982) Resource Competition and Community Structure. Princeton: Princeton University Press.

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