Life-forms of phytoplankton as survival alternatives in an unstable environment

RESUME Ramon Margalef Departamento de Ecologia, Universidad de Barcelona, Barcelona, 7, Spain. Received 4/4/78, in revised form 20/5/78, accepted 28/5/78. The different life-forms observed in phytoplankton are functionally interpreted as adaptations to survival in an unstable and turbulent environment. In comparison with terrestrial and benthic plants, the primary producers in phytoplankton are small, of rapid turnover and expendable; this is the result of evolution under conditions of automatic and passive exploitation, where grazing may be only secondary. Any atom is more likely to travel downwards when in a particle than in solution, and the most likely final situation in a completely stagnant environment would be one of segregation between light (on top) and nutrients (in depth). Stability and local diversification on different scales may be interpreted in the same manner. The combination of sedimentation with turbulence or variance in the components of velocity is believed to be the most important factor in the biology of phytoplankton. Consequent! y, the best predictor of primary production and of dominant life-forms in phytoplankton is the available externat energy, on which advection and turbulence depend. This factor overrules more detailed models using light and nutrients as most relevant parameters, and based on laboratory experiments. Energy controls transportation, although the motility of organisms also contributes to the vertical organization of the ecosystem. The amount of energy exchanged and degraded per unit of surface also finds expression in the horizontal dimension of the structures that may be studied as approximatively closed systems with reference to the cycle of matter. Comparative analysis of different life-forms and their relation to the properties of the environment leads to an ordination along a main sequence, from fertile and turbulent water to exhausted and stratified environments. Populations selected under such diverging conditions range typically from diatoms to dinofiagellates. This correspondence is explained with the help of the model of Riley et al. ( 1949), which may be further developed in different ways. Fresh waters considerably expand the niche space of phytoplankton over the area of specially enriched water, and in such conditions cells enveloped in mucilage are frequent in a life-form not altogether absent in marine phytoplankton. Interpretation of such covers in terms of selection is not completely satisfactory, but mucilaginous envelopes of this type retard absorption and act as a feedback mechanism of population control. lt is possible to balance in a single expression environmental factors and properties of the organisms, in such a way that probability of survival might be inferred from sorne performance index computed on such variables. Oceanol. Acta, 1978, 1, 4, 493-509 Les types biologiques du phytoplancton, consideres comme des alternatives de survte dans un milieu instable Les caracteristiques des differents types biologiques reconnaissables dans le phytoplancton sont interpre~ees fonctionnellement comme des adaptations a la survie dans un milieu essentiellement instable, turbulent. Contrairement aux producteurs primaires terrestres-et benthiques, les algues du plancton sont petites, a renouvellement rapide,