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Plankton. It isn’t a species or type of plant or animal. Plankton is the term used to describe all free floating organisms in open water (marine and fresh). Although we may usually think of plankton as being microscopic species, many can be seen by the naked eye (such as copepods) or can even be quite large organisms (like some species of jellyfish). Plankton float with the ocean’s current and will even fall to the extreme depths of the ocean (the benthic zone) in the form of “marine or reef snow”.
The water found at corals reefs, and in your home aquarium, should always have a healthy population of plankton. Plankton is important for a variety of reasons. To begin, it makes a terrific natural food source. Fish rely on plankton as a major food source as some point in their life cycle, most significantly in their larval stage. Additionally, many corals will capture zooplankton and bacterioplankton as food, despite their reliance on zooxanthellae (dinoflagellate algae) for nutrition via photosynthesis. In addition to their position as a food source, many forms of plankton contribute greatly to the breakdown of organic matter in the system. Many, such as amphipods, are scavengers, and will feed on the detritus and waste material (or alternatively, become the detritus after death) found in your small, closed system of a home aquarium. In this way, they contribute to the carbon cycle of the marine aquarium.
There are a variety of methods to both encourage and discourage the growth of plankton in your aquarium. The use of skimmers and filters will commonly remove free floating organic matter from the aquarium. This will remove plankton directly or remove their source of food. Some corals will thrive in your aquarium in the absence of a skimmer specifically because they will feed on the plankton that would normally be removed in the traditional filtration process.
The increased use of sumps as refugiums actually encourages the growth of plankton in your home system. Zooplankton and phytoplankton can live in the refugium without being over harvested by the fish, corals, and other invertebrates found in the display tank. On a regular basis, some of that plankton will be washed through the display tank and contribute food. As waste and detritus is washed into the sumps, the plankton will feed on it, breaking down the organic matter. Many aquarists intentionally add macroalgae to the sump to aid in sheltering the zooplankton.
There are a couple of ways to classify plankton. The first is by life cycle. Organisms which spend their entire lives as plankton are called holoplankton. Some examples of holoplankton are copepods, diatoms, and many algae. In contrast, meroplankton only spend a portion of their lives as plankton. For example, many species of fish, corals, and echinoderms spend their larval stage as plankton before maturation into a mobile organism.
The other way to classify plankton is by “trophic level”, their place in the food web as producer, consumer, or decomposer. For plankton, these groups are called phytoplankton, zooplankton, and bacterioplankton.
Phytoplankton is comprised of the algae and cyanobacteria (see also, bacterioplankton). These are producers, as they are primarily autotrophic, creating their own nutrients via photosynthesis. (Along deep sea vents, there are organisms that are autotrophic via “chemosynthesis”.) They can range from the unicellular forms (single celled algae) to multicellular forms (like seaweed). These are not vascular like plants. They lack roots, rhizomes, organs, and leaves (although they will simulate the appearance of leaves). Almost all algae are photosynthetic, and will produce oxygen as a byproduct. Some will also take up dissolved organic carbon from the aquarium water.
Zooplankton are the consumers of the food web, and need to eat other organisms for their nutrients. They may consume other plankton, like algae and the larvae of other organisms. Some examples of zooplankton are amphipods, copepods, and larvae of other marine organisms. Dinoflagellates, a form of algae, are often considered zooplankton and phytoplankton, depending on their species and whether they are autotrophic (producing their own nutrients).
Baterioplankton are prokaryotes (lacking membrane bound organelles), and are commonly the “decomposers” of the food chain. They will consume the organic byproducts (waste) of other organisms. As a result, they pose an important role in remineralisation, nitrogen fixation, nitrification, denitrification, and methanogenesis. Many aquarium test kits are geared towards testing for nitrite and nitrate in the water. Bacterioplankton have a major impact on those concentrations. Zooplankton will also feed on bacterioplankton.
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Fenner, R. M. (2008). The Conscientious Marine Aquarist: a common sense handbook for successful saltwater hobbyists. Neptune City, NJ: T.F.H. Publications, Inc.
Plankton. (2011). Retrieved on May 13, 2011 from http://en.wikipedia.org/wiki/Plankton