By the word plankton we mean aquatic organisms that cannot actively sweem against the current and passively floats or drifts. Thus phytoplanktons are minute free-floating plants that drift in water, and may be found in either shallow or deep aquatic system. They are the primary biological producers in the aquatic system from which energy is transferred to higher trophic levels.
Nearly all marine plants, whether unicellular or multicellular, even those attached to substrata or free floating including some species of blue- green algae and green algae,pass some part of their life history in floating condition as phytoplankton. However, those organisms, which always remain planktonic through out the life cycle, are (1) Diatoms, (2) Dinoflagellates, (3), Coccolithophores.
Phytoplankton (and also the zooplankton) is classified according to their size. Megaplankton are all those organisms that are above 20cm. Macroplankton 2 to 20 cm in size, Mesoplankton between 0.2 to 20mm, Microplankton 20 to 200 �m (�m = micrometer = 0.001mm). Other categories are nanoplankton (2 to 20 �m), picoplankton (usually bacteria) 0.2 to 2.�m. The last two groups cannot be captured by plankton nets because of their extremely small size and usually obtained by centrifuging the seawater and other modern methods. Diatoms and dinoflagellates are microplanktons, but Coccolithophores are nanoplanktons.
Diatoms (Class Bacillariophyceae)
They have silicious cell wall called frustules, consists of two valves. The upper valve (Epitheca) closes on the lower valve (Hypotheca) like the lid of a pillbox. They have beautiful structured body, which either shows radial (discoid or triangular) or linear or bilateral symmetry. The common diatoms which are found in the Sundarbans are Skeletonema, Rhizosolenia, Coscinodiscus, Chaetoceros.
Dinoflagellates (Class Dinophyceae)
They are unicellular and autotrophic and contain two flagella. One of these lies partly in a groove called sulcus situated longitudinally and the other is situated in a transverse groove called cingulum or girdle.
Some dinoflagellates also capable of producing toxins (Gonialax, Gymnodinium) and due to several environmental factors they become extremely abundant in waters (2 to 8 million cells per 1000cc) causing bloom that has been termed as Red Tide. It may cause massive localized mortality seen in fish and invertebrates.
Other dinoflagellates, such as the common Noctiluca, are also highly bioluminescent and if present in large numbers, can actually light up the wakes of the boats and the breaking waves on a beach.
Dinoflagellates we commonly encounter in the Sundarbans are Ceratium, Gonialax, Protoperidinium, Noctiluca.
Coccolithophores (Class Prymnesiophyceae)
They are the smallest category of phytoplanktons (Nanoplankton). Some of them have flagella, while others are devoid of them. Tiny, calcified circular plates or shields of various designs shield their soft bodies. They are important components of filter feeding animals.
The coccoliths we commonly encounter in the Sundarbans are Isochrysis, Coccolithus.