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A Quick Overview of Marine Plankton

Every month there will be differences in the variety of life in the plankton although some groups will always be present. Diatoms are single-celled algae constituting the phytoplankton, the main component carrying out photosynthesis. Likewise there will always be a few copepods and larval forms, the zooplankton. Copepods are generally cylindrical shapes often with long antennae and represent crustaceans that remain permanently in the plankton world. These permanent creatures form the holoplankton while the meroplankton is the name given to those organisms using this world for just part of their life cycle. For example, barnacles and periwinkles living on a rocky shore have nauplius and veliger larvae, respectively. 

Yet another way of categorising the life living in this environment is by size. Here is an approximate guide of sizes and examples:

Femtoplankton - Viruses up to 0.2 micron

Picoplankton - bacteria & cyanophytes 0.2-2 micron

Nanoplankton - some flagellates 2-20 micron

Microplankton - tintinnids & diatoms  20-200 micron

Mesoplankton - copepods, many larvae 0.2 -20mm

Macroplankton - arrow worms  2 - 20cm 

Megaplankton - large jellyfish up to 2 metres

Plankton sample with low magnification. The green circles and shapes are large diatoms while copepods form the bulk of the zooplankton here.

Ditylum diatom split stk2_edited.jpg

Phytoplankton - a small selection of diatoms

All of these are single-celled organisms or Protists

Click on photos

The marine worms, polychaetes, may live permanently in the plankton or as a larva

Crustaceans: Clockwise, large copepod (Calanus), water flea Podon, larva of a barnacle and larva of a porcelain crab

Molluscs are represented by larval forms in the Milford Haven plankton. Called veligers they have a large organ covered in cilia for movement 

Jellies: A tiny medusa, sea gooseberry or ctenophore

The larva of a sea anemone

To find out what these are click on the photo and it will take you to their section

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