Phytoplankton Very small, microscopic, floating and drifting plant; usually unicellular, free or colonial; multicellular at some stage of life cycle. Depending on the size, type of life cycle and ecosystem they are classified as limnoplankton, rheoplankton and haloplankton. Overgrowth of phytoplankton makes pond water colour different: Mycrocystic spp. make water deep blue-green colour; Gonium, Pandorina, Eudorina and Euglena spp make deep green colour. Surface watercolour of some closed water bodies becomes red-rusty or tea colour because of Euglena. Spirogyra, Pithophora spp. or Hydrodictyon reticulatum are found submerged in water bodies. Aphanothece, Rivularia and Gloetrichia form gray sticky bolus; Chaetophora spp. green bolus and Botryococcus braunii form sticky colony and can cover the closed water bodies. In Bangladesh most of the phytoplanktons belong to Bacillariophyceae. The abundance of other algal families are usually dependent on Ph, food material, nitrate phosphate etc. desmids grow in acidic water; blue green algae, euglenoids and chlorococcoids prefer high alkaline and biologically polluted water. In Bangladesh freshwater phytoplanktons belong to classes Chlorophyceae, Euglenophyceae, Dynophyceae, Chloromonadophyceae, Cryptophyceae and Cyanophyceae. In the coastal areas of Bangladesh species like Asterionella japonica blooms and the fishermen get itching from its secretion when they come in contact with such diatoms. Phytoplanktons are the primary producers in both fresh and marine water and principal source of food for fish and other organisms. Besides, they maintain the oxygen supply through photosynthesis. Humans consume Spirulina, a kind of phytoplankton as food and medicine. [AKM Nurul Islam]