Plankton are organisms that generally inhabit the surface area (pelagic zone) of oceans, seas or bodies of fresh water. Plankton can be animal, plant, archaean, or bacterial in nature, and they are a crucial food source for larger aquatic organisms like fish and whales.

The word plankton (singular plankter) comes from the Greek word "planktos" which means "wanderer" or "drifter." Plankton generally flow, or drift, with ocean currents, although some forms are capable of independent vertical movement. The study of plankton is called planktology.

Rather than using a taxonomic classification, biologists generally divide plankton in groups based upon either "trophic levels" or "size levels."

Trophic Groups
Biologists often divide plankton into ecological niches, or broad functional groups called "trophic levels." These niches separate plankton into producer, consumer and recycler groups.

1. Phytoplankton - from the Greek word "phyton", meaning plant. This is the "producer" group. Phytoplankton are autotrophic, prokaryotic or eukaryotic algae which live nearest to the water's surface where there is sufficient light to support photosynthesis. Types of phytoplankton include diatoms, cyanobacteria, dinoflagellates and coccolithophores.

2. Zooplankton - from the Greek word "zoon," meaning animal. This is the "consumer" group. Though many zooplanktonic species are microscopic in size, this group does include larger organisms such as jellyfish. They feed on other plankton and are the initial prey for almost all fish larvae. Some of the eggs and larvae of larger animals such as fish, crustaceans and annelids are included in the zooplankton category. There are two types of zooplankton, as follows:

Holoplankton - These plankton are "permanent" plankton and spend their entire life cycle as plankton. Examples include most algae, salps, copepods and some jellyfish.

Meroplankton - These organisms are "temporary" plankton, and are only planktonic for part of their lives, generally the larval stage. Examples include the larvae of sea urchins, starfish, crustaceans, marine worms, and most fish.


3. Bacterioplankton - This is the "recycler" group. Generally preyed upon by Zooplankton, Bacterioplankton are bacteria and archaea that act to remineralize, nitrify and denitrify organic material in the water. They often
obtain energy from the consumption of organic material (like minerals) produced by other organisms.


Size Groups
Plankton may also be described in terms of size, as follows: (mm = millimeter, 1 thousandth of a meter; µm = micrometer, or 1 millionth of a meter)

Megaplankton - Metazoans (multicellular animals) greater than 20mm in size. Examples are jellyfish, ctenophores, salps, and Cephalopda.

Macroplankton - Metazoans ranging in size from 2mm - 20mm. Examples are Pteropods, Chaetognaths, Euphausicacea (krill), Medusae, ctenophores,salps, pyrosomes, and Cephalopoda.

Mesoplankton - Metazoans ranging in size from 0.2mm - 2mm. Examples are copepods, Medusae, Cladocera, Ostracoda, Chaetognaths, Pteropds, Tunicata, and Heteropoda.

Microplankton - Large eukaryotic protists ranging in size from 20-200 µm. Examples are most phytoplankton, Protozoa, ciliates; Rotifera, and juvenile metazoans.

Nanoplankton - Small eukaryotic protists ranging in size from 2-20 µm. Examples are small diatoms, small flagellates, Pyrrophyta, Chrysophyta, Chlorphyta; and Xanthophyta.

Picoplankton - Small eukaryotic protists ranging in size from 0.2-2 µm. Examples are bacteria and Chrysophyta

Femtoplankton - Marine viruses ranging in size from < 0.2 µm.

The existence of nano-, pico- and femto-plankton has only been recognized since the 1980s when the development of more advanced technology made their discovery possible. It is currently believed that these groups make up the largest proportion of all plankton in both number and diversity.


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