Phytoplankton is a polyphyletic group of photosynthetic,oxygen producing protoctists and bacteria adapted topersist in suspension and liable to passive movement by wind and current (Reynolds,2006)[1].They inhabit almost all the aquatic ecosystems of the biosphere and show an enormously wide variability of shape,size,evolutionary and phylogenctic position,as well as energy and nutrient demands.Their amazing diversity in an apparently homogeneous medium,as the pelagic environment can be perceived at a first sight,caused concern a few decades ago and puzzled aquatic biologists for years.Actually, the contemporary occurrence of many species competing for the same few resources (apparently) violated the Competitive Exclusion Principles (Hutchinson,1961)[2].