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Phytoplankton

These are the primary producers of the oceans and may be the base of many food webs. Phyto is the Greek word for plant and as the name suggests constitutes the planktonic plant-like organisms that are free moving through the water. These will be algal organisms with groups like cyanobacteria, diatoms and dinoflagellates.

Coscinodiscus diatom showing the chloroplasts insideCoscinodiscus diatom showing the chloroplasts inside (magnified approx 200x)

A principle component of phytoplankton is the diatom, a form of alga. They are believed to be the most prolific and successful organism found in aquatic systems. The basic form is a single-celled organism with the outer wall made from a high level of silica. This is one of the most important nutrients needed for growth. They multiply by cell division and if there is sufficient silica in the water (see below) the cells will grow and then split again. Sometimes the cells do not separate and remain together so that chains of diatoms form. Those like Baccilaria remain attached and create great clumps of moving sections and are quite distinct under the microscope. Phytoplankton is very productive. Although at any one time the amount of primary production is limited (called standing crop) over a period like a year the net productivity can be higher than a meadow. This is down to the ability for diatoms to quickly multiply, less than half an hour to double. Only a few diatoms need survive and they can quickly reproduce.

Biddulphia diatom splitting in two Biddulphia diatom splitting in two.

Dinoflagellates are another group of algae that are an essential part of the phytoplankton. Most are unicellular and have one or more flagella. They are large, compared to most diatoms and again a very important food source.

As diatoms photosynthesise so they become more buoyant and may rise in the water column during the day. Their density also varies over a year.

See the species section on diatoms and dinoflagellates.

 


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