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Most of the ocean is filled with plankton: including tiny plants, animals, and bacteria that are carried by currents. These organisms are often so small they cannot be seen without a microscope (at least not in detail) but they have a large influence on ocean health, productivity, and in supporting larger organisms, including humans!
 | | Plankton has many different forms. a) Phytoplankton (photo credit: Russel Davidson) b) Microplankton including these cyanobacteria that gave the Red Sea it's name (photo credit: Annette Hynes) c) many different types of diatoms (photo credit: Wipeter) d) one of the many species of copepods (photo credit: Jan Michels) |
Phytoplankton
 | | Marine Food Web: Phytoplankton are the primary producers and zooplankton link them with larger carnivores. |
Phytoplankton consists of algae and other single-cell primary producers in the ocean. These organisms, much like plants in the terrestrial environment, convert carbon dioxide and sunlight into oxygen and sugar. They comprise such a large portion of the ocean, and therefore the Earth's surface, that they produce about half of all the oxygen in our atmosphere! This is quite an accomplishment for organisms that, individually, are often less than a millimeter in diameter. In addition to producing most of the oxygen that we surface-dwellers utilize, phytoplankton comprise the bottom of the ocean food chain. Just as terrestrial organisms rely on plants for food (energy), larger marine organisms rely on phytoplankton as their first energy source. Bacterial Plankton
Bacterial plankton are the great carbon composters of the oceans and seas. They can integrate dissolved organic material from the water into their bodies, thus making it available for zooplankton who eat the bacterial plankton. Because they tend to cluster around organic matter, they are often found in great colonies near coastlines. This makes them readily visible not only by the naked eye, but even by air. In fact, the Red Sea was named for cyanobacteria living in this body of water which were in such high quantities they colored the water red!
Zooplankton
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Copepod drawing
(credit: Giesbracht)
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Considered the animal portion of the plankton world, zooplankton come in many different shapes and sizes. They are classified as those that are able to swim (although they are still at the whim of currents), those that are sedentary, and those that are found inside other organisms.
One of the main roles zooplankton play is linking phytoplankton (our primary producers) with larger organisms, including fish and whales. One type of zooplankton, copepods, have been shown to help whale sharks by eating algae and bacteria off of their skin (Marine Conservation Society Seychelles, web blog 10/09/2008). Some dinoflagellites have developed a symbiotic relationship with corals, sea anemones, and giant clams. Instead of swimming, they live inside of these animals and are called zooxanthellae.
 | Left: Copepods collected from the surface of a whale shark (Photo credit: Katie Brooks) Right: Close up of the mantle of a giant clam. The dark spots are the zooxanthellae living within the clam (Photo credit: Reader's Digest Book of the Great Barrier Reef)
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Zooplankton are not always helpers in the ocean. Dinoflagellates produce toxins that can kill other marine life. When dinoflaggelite populations become too large, the amount of toxin in the water increases and can cause massive die-offs of species, which we refer to as Red Tide.  | |
Red Tide caused by increased dinoflaggelite populations in La Jolla, CA (photo credit: Kai Schumann)
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Aside from all the ways plankton affect larger organisms, they are interesting in their own right. Each plankton type has developed mechanisms to help it float in the water column or allow it to swim short distances. For instance, diatoms often have tiny hair-like appendages called "cilia" that help them move, whereas copepods use their antennae to propel them through the water. Another interesting fact about zooplankton is that many species have developed mechanisms whereby either the adults, eggs, or both can slow down their metabolism and live through periods when conditions are poor for survival. These animals can be collected from the sediment at the bottom of the ocean and used to understand ocean conditions and diversity at different periods in time. So you can see, for such tiny organisms, plankton pack a powerful punch in the overall scheme of life in our oceans and on land. | 
