The term 'plankton' refers to the group of organisms which float in the surface waters of the rivers, lakes and oceans. Derived from the Greek words phyto (plant) and plankton (made to wander or drift), I phytoplankton are microscopic plant organisms i that live in aquatic environments, both salty and fresh. Some phytoplankton are bacteria, some are protists, and most are single-celled plants. Among the common kinds are cyanobacteria, silica-encased diatoms, dinoflagellates, green algae, and chalk-coated coccolithophores. Like land plants, phytoplankton have chlorophyll to capture sunlight, and they use f photosynthesis to turn it into chemical energy. * They consume carbon dioxide, and release oxygen. All phytoplankton photosynthesize, but some get additional energy by consuming other organisms. These micro-algae are present throughout the lighted regions of all the seas and oceans including the Polar Regions. Their total biomass is many times greater than that of the total plants on land and they serve as the "pasture grounds" in the aquatic environment.
Based on their size, phytoplankton can be classified as
1. macroplankton (more than 1 mm),
2. microplankton (less than 1 mm, retained by nets of mesh size 0.06 mm),
3. nanoplankton (between 5 and 60 micrometers), and
4. ultraplankton (less than 5 micrometers).
Many phytoplankton species belong mainly to the nanoplankton and microplankton fractions.
Factors Affecting Phytoplanktons Biodiversity
Light
-Phytoplanktons are limited to the uppermost layers of the ocean where light intensity is sufficient for photosynthesis to occur.
-For most phytoplankton, the photosynthetic rate varies with light intensity.
Nutrients
-The major inorganic nutrients required by phytoplankton for growth and reproduction are nitrogen and phosphorus.
-Diatoms and silicoflageliates also require silicate (Si02) in significant amounts.
-Some phytoplankton can fix nitrogen and can grow in areas where nitrate concentrations are low.
-They also require trace amounts of iron which limits phytoplankton growth in large areas of the ocean because iron concentrations are very low.
-Other inorganic and organic nutrients may be required is small amounts.
-All of those nutrients are the limiting factors for phytoplankton productivity under most conditions.
Temperature
> Temperature acts along with other factors in influencing the variation of photosynthetic production.
> Generally, the rate of photosynthesis increases with ah increase in temperature, but diminishes sharply after a point is reached.
> Temperature, together with illumination, influences the seasonal variation of phytoplankton production in the temperate latitudes.
Salinity
> Besides light and temperature, salinity also is known to influence primary production.
> Many species of dinoflagellates reproduce actively at lower salinities.
Grazing by Zooplankton
> The grazing rate of zooplanktoii is one of the major factors influencing the size of the standing
crop of phytoplankton, and thereby the rate of production,
Distribution
> Marine phytoplankton are not uniformly distributed throughout the oceans of the world. The highest concentrations are found at high latitudes, with the exception of Upwelling areas on the continental shelves, While the tropics and subtropics have 10 to 100 times lower concentrations.
> In addition to nutrients, temperature, salinity and light availability; the high levels of exposure to solar UV-B radiation thai normally occur within the tropics and subtropics may play a role in phytoplankton distributions.
> Phytoplankton productivity is limited to the euphotic zone, the upper layer of the water column in which there is sufficient sunlight to support net productivity.
> The position of the organisms in the euphotic zone is influenced by the action of wind and waves.
Importance of phytoplankton
The food web
> Phytoplanktons are the foundation of the aquatic food web, the primary producers, feeding everything from microscopic animallike zooplankton to whales. Small fish and invertebrates also graze on the phytoplkanktons, and then those smaller animals are eaten by bigger ones.
> Phytoplankton can also be the harbingers of death or disease. Certain species of phytoplankton produce powerful biotoxins, making them responsible for so-called -red tides; - or harmful algal blooms. These toxic blooms can kill marine life and people who eat contaminated seafood.
> Phytoplankton cause mass mortality in other ways. In the aftermath of a massive bloom, dead phytoplankton sink to the ocean or lake floor. The bacteria that decompose the phytoplankton deplete the oxygen in the water, suffocating animal life; the result is a dead zone.
Phytoplankton - the Carbon Cycle and climate change
> Through photosynthesis, phytoplanktons consume carbon dioxide on a scale equivalent to forests and other land plants. Some of this carbon is carried to the deep ocean when phytoplankton die, and some is transferred to different layers of the ocean as phytoplankton are eaten by other creatures, which themselves reproduce, generate waste, and die.
> Worldwide, this "biological carbon pump" transfers/about 10 gigatonnes of carbon from the atmosphere to the deep ocean each year. Even small changes in the growth of phytoplankton may affect atmospheric carbon dioxide concentrations, which would feed back to global surface temperatures.
> Phytoplankton are responsible for most of the transfer of carbon dioxide from the atmosphere to the ocean. Carbon dioxide is consumed during photosynthesis, and the carbon is incorporated in the phytoplankton, just as carbon is stored in the wood and leaves of a tree. Most of the carbon is returned to near-surface waters when phytoplankton are eaten or decompose, but some falls into the ocean depths.
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