Plankton are microscopic or macroscopic organisms of plant and animal origin found in the pelagic region of water bodies where sunlight penetration and nutrient availability are abundant. Plankton are defined based on the ecological niche they occupy rather than their phylogenetic or taxonomic classification. Plankton are at the bottom of the food chain and are a source of food for a lot of aquatic organisms. Plankton are found drifting at the surface of water bodies. In contrast to plankton are nekton, which are organisms that a capable of controlling their movement. Plankton are grouped under two categories holoplankton and meroplankton. Holoplankton are plankton that complete their life cycle as plankton e.g. copepods, salps and jellyfish. Meroplankton are those that are plankton for part of their lifecycle (usually larva stage). Plankton can either be of plant origin- phytoplankton, animal origin-zooplankton, algae or bacteria origin.
Zooplankton range in size from the microscopic copepods to the larger ones like jellyfish. Zooplankton are used extensively in aquaculture as the first food of larvae during the commencement of endogenous feeding. They are very rich in protein and serve as food for fish larvae, fry and fingerling. Zooplankton are found naturally on water bodies as part of the food chain. They could also be introduced into a fish pond or aquaria in aquaculture.
Phytoplankton, like every other green plant, are at the bottom of the food chain (primary producers) using available nutrients and sunlight in manufacturing their food. They are one-celled organisms found floating on exposed water bodies. They are also a source of food for fish fry, fingerling and juvenile. They are capable of replicating at a very fast rate and if uncontrolled, can result in water pollution through algae bloom and algae die-back. They can appear naturally in a natural water body or introduced through fertilization in aquaculture. The introduction of phytoplankton to a water body in itself is a form of aquaculture. Apart from being a food source for fish, phytoplankton serves the purpose of food for captive invertebrates in aquaria and also for bio-fuel. Scientists have found use for phytoplankton as being capable of reducing global warming by using up available atmospheric carbon dioxide for photosynthesis. Conversely, the increased population of phytoplankton in our water bodies will increase water temperature as a result of increased radiation absorption.
The available population of phytoplankton and plankton in general is highly dependent on the available nutrients, quantity of dependant organisms, the availability of other plankton and the rate of flow of the water body in question.