The term “plankton” comes from the Greek word planktos, meaning “drifter.” Not a single species or biological group, planktic species actually include a wide range of aquatic organisms – animals, plants, and bacteria – that drift in the world’s oceans, seas, rivers, and lakes. Some are capable of swimming to some degree, but not strongly enough to resist being carried by the water currents.
Plankton range in size from larger jellyfish and crustaceans to microscopic organisms; some (holoplankton) remain planktonic their entire lives, while others (meroplankton) are planktonic only in their larval stage. These species form a vital part of earth’s aquatic ecosystem, providing nourishment for a wide range of fish and cetacean species. Plankton also produce most of the atmospheric oxygen we breath.
Planktic species can be divided into three general groups:
Phytoplankton – plants living near the surface of bodies of water where they are exposed to enough sunlight to support photosynthesis. These are the species that produce oxygen. Zooplankton – small marine animals such as jellyfish and protozoans as well as larval forms of fish and crustaceans. Krill, a type of crustacean, is the main food source for a number of marine animals including many whale species. Bacterioplankton – microscopic organisms that remineralize organic material in the oceans. These species break down remains of other aquatic organisms, allowing their nutrients to be reused in the marine web of life. Some on the ocean’s floor feed off of the chemicals released by volcanic vents.
The abundance of plankton species depends on the availability of light and nutrients in the ecosystem. In turn, they provide the lower levels of the food chain that sustain virtually all marine life. The zooplankton species, which graze on phytoplankton, also contribute carbon to the food chain through respiration and the decay of their biomass. The detritus of dead plankton sinks to the ocean floor, creating the world’s largest carbon sink. In this way, plankton counterbalance the global warming effect of human activities, by reducing the amount of carbon dioxide in the atmosphere. However, increased plankton levels also depletes oxygen levels in the oceans, producing higher methane levels.
With increasing ocean surface temperatures, the phytoplankton levels have declined some 40% since the 1950s. In turn, lower levels of phytoplankton reduce the atmospheric oxygen levels and increase the levels of carbon dioxide, further feeding the global warming.
Even temporary reductions in phytoplankton, as occurs during El Nino years, seriously impacts marine mammal and seabird populations. Since most fish feed on zooplankton during their larval stages, reduced levels of plankton mean many fish larvae will not survive.
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