Plankton are free flowing organisms that are found in seas, oceans or freshwater areas. They are plants, bacteria or animals which are unable to swim against the currents of the sea but can travel up through the depths.

The ocean can be thought of as a series of layers or zones which start from the surface down towards the sea bed. The surface layer is known as the pelagic zone which contrasts with the two deepest layers – the demersal and benthic zones.

The majority of plankton are found in the pelagic zone of the sea.

Groups of plankton

Plankton are discussed in terms of groups or sizes. There are three functional groups which include:

  • Zooplankton
  • Phytoplankton
  • Bacterioplankton


This group includes small animals such as jellyfish, arrow worms and crustaceans. These creatures feed upon other plankton and single cell organisms.


This refers to plant-like groups of plankton such as algae or dinoflagellates. They are found on the surface of water to enable access to light for photosynthesis.


This is the bacterial aspect of plankton. Many of these obtain energy by the consumption of organic deposits produced by other organisms or via photosynthesis.

Sizes of plankton

Plankton are more than often described by their size which also helps to determine its name. These sizes run from the largest to the smallest sized group.

They are:

  • Megaplankton: the largest sized group which includes jellyfish and cephalopods such as octopus and squid.
  • Macroplankton: this group includes zooplankton and larvae
  • Mesoplankton: the next sized group which contains small crustaceans, sea slugs and spiders.
  • Microplankton: this group contain tiny zooplankton such as ciliates and dinoflagellates.
  • Nanoplankton: contains smaller species of dinoflagellates known as silicoflagellates and small diatoms (algae).
  • Picoplankton: the next to smallest sized group. It contains bacteria and tiny micro-organisms known as protists.
  • Femtoplankton: the smallest sized group. This is comprised of viruses.

Plankton are found in lakes, rivers, seas and oceans and their quantity varies according to the seasons. This quantity also varies in a vertical or horizontal spread according to the amount of light available.

Their distribution is controlled by the amount of light and nutrients available.

Harmful plankton

This refers to a species of algae known as 'harmful algal blooms' or 'red tides' which are comprised of phytoplankton such as dinoflagellates.

This phytoplankton feed on most if not all the nutrients in water which causes an increase in bacteria, leading to a drop in oxygen levels. These low levels of oxygen make it difficult for many species of fish and other marine life to survive.

Several types of phytoplankton clog the gills of fish which then kills them. The plankton then feed on the tissues of these fish at this time which further weakens them. These fish usually die a couple of hours later.

Another by-product is toxins produced by this plankton such as sazytozin which is harmful to certain species of marine life, e.g. whales and seals. They can cause neurological damage to these species and are equally harmful if consumed by human beings as well.

Man-made pollution is considered to have contributed to this increase in red tides.

Research has been undertaken in Hawaii to determine the level of threat posed to its ecosystems from red tides. Plankton is obtained and observed on a regular basis to compare toxic and non-toxic levels.