Evolution of the oceanic plankton

Abstract

In its evolution through geologic time, the oceanic plankton has reflected many changes in the physical environment while both causing and being affected by evolutionary change in the associated biota. Temporal variations in the vertical and latitudinal habitat partitioning also affected geochemical balances in the oceans and accumulation rates of biogenic sediments, and perhaps the atmospheric oxygen pressure. Advantages of the oceanic plankton habitat that led to its occupation by a varied sequence of organisms include its extensive geographic area, the possibility for rapid dispersal by currents, the availability of resources (ranging from light, carbon dioxide, and dissolved nutrients for the autotrophic organisms to the various lower trophic levels required by the heterotrophs), and the better oxygenation of the upper water layers. Convergent morphology of the plankton represents adaptations for suspension in the water column, for movement within this water mass to increase nutrient uptake, and for protection against grazing or predation. Necessary attributes for cosmopolitan species include eurythermy and euryhalinity. The patchiness of plankton diversity results from small-scale variations in water masses, the low level of interorganism contact or competition, and the possibility of rapid exploitation of favorable conditions by species present in the local area. Parallel evolution, radiation, and extinction of the various components of the oceanic plankton are similar in pattern to an ecologic succession — early evolution and conditions following major periods of extinction being homologous with the physically controlled or immature modern ecosystem associated with high stress levels and characterized by high net but low gross productivity, low diversity but great intraspecific variability, and small biomass. Evolutionary radiation is associated with more stable physical conditions and, like the seral progression, of an ecosystem, results in high diversity but less intraspecific variability. Increased biological interaction leads to higher gross but lower net productivity, hence more efficient energy and resource utilization. The drop in gross oceanic productivity at times of major extinction is associated with low diversity and decreased rate of accumulation of biogenic ooze, hence the apparent worldwide disconformities, as at the Cretaceous-Tertiary boundary, even in the deep sea.