SOME COMMENTS ON PICOPLANKTON AND ITS IMPORTANCE IN THE PELAGIC ECOSYSTEM

Picoplankton is an integral component of a microbial community, termed the ultraplankton, which seems to be ubiquitous in all seas and lakes. The picophytoplankton and heterotrophic bacterioplankton produce the bulk of the biomass in the community and their consumption by predators with matching growth rates results in a highly dynamic and self-contained system with efficient recycling of mineral nutrients and dissolved organic matter. Given adequate radiant energy the population levels of the component organisms are largely determined by the internal dynamics of the community rather than by external factors. Only when concentrations of mineral nutrients rise above those set by dynamics of the ultraplankton can larger phytoplankton compete effectively and multiply. There is a sharp distinction in form and dynamics between ultra- and microplankton. The metabolism of the ultraplankton commonly dominates in pelagic ecosystems, although it seems to contribute little organic matter to higher trophic levels, and must be taken into account in the determination of primary productivity.