CONTENTS:
Introduction
Definations
Size Groups
Function Groups;
-Phytoplankton
-Zooplankton
-Bacterioplankton
Biogeochemical Significance of Plankton
Plankton Distribution
Role of Plankton in Aquatic Productivity
Reference
Plankton consist of any drifting organisms (animals, plants, archaea, or bacteria) that inhabit the pelagic zone of oceans, seas, or bodies of freshwater. Plankton are defined by their ecological niche rather than their genetic classification. They provide a crucial source of food to aquatic life.
DEFINITIONS
The name plankton is derived from the Greek word “planktos”, meaning “wanderer” or “drifter”. Although most of these organisms’ horizontal position is primarily determined by currents in the water bodies they inhabit; they cannot swim or move against currents, but some forms are capable of independent vertical movement in the water column and can swim up to several hundreds of meter vertically in a single day (a behaviour called diel vertical migration).
Based on the duration of their life cycle spend as being planktonic, plankton are in two forms:
Holoplankton- They are those organisms that spend their life cycle as part of the plankton e.g. most algae, copepods, salps, and some jelly fish.
Meroplankton- They are those organisms that are only planktonic for part of their lives (usually the larval stage), and then graduate to either the nekton or benthic (sea floor) existence e.g. sea urchins, starfish, crustaceans, marine worms and most fish.
SIZE GROUPS
Plankton are also often described in terms of size:
Picoplankton: smaller than 2mm; includes bacteria, prochlorophytes, and viruses.
Nanoplankton: between 2-20mm; includes diatoms, coccoliths, and silicoflagellates (pyrrophyta, chrysophyta, chlorophyta and xanthophyta)
Microplankton: between 20-200mm; includes most phytoplanktons; large diatoms, dinoflagellates and small zooplankton such as ciliates.
Macroplankton: between 200-2000mm; includes large zooplankton, copepods and invertebrate larvae.
Megaplankton: larger than 2000mm; includes fish larva and gelatinous zooplanktons such as jelly fish (ctenophores, salps, and pyrosomes).
FUNCTION GROUPS
Plankton are primarily divided into broad functional (or trophic level) groups as follows:
Phytoplankton- These are the autotrophic, prokaryotic or eukaryotic algae.
Zooplankton- They are the protozoans, metazoans or the eggs and larvae of lager aquatic animals
Bacterioplankton- These are the bacteria and archaea.
However, this scheme divides the plankton community into broad producer, consumer, and recycler groups. But, in the reality of it, the trophic level of some plankton is not straightforward; many species are mixotrophic depending upon their circumstances. For example, dinoflagellates are either photosynthetic bacteria or heterotrophic consumers. Even, the prokaryotic phytoplankton and the smallest ones among the zooplankton are bacterioplankton because they are also recyclers.
PHYTOPLANKTON
These are the autotrophic, prokaryotic or eukaryotic algae that live near the water surface where there is sufficient light to support photosynthetic activity. Phytoplankton fixed the inorganic carbon (as carbondioxide) utilized by marine and freshwater organisms into organic compound by their photosynthetic activity. And this makes the carbon available for higher trophic levels, thereby making them (phytoplankton) the primary producers; the base of the food web in oceans, lakes, rivers, and estuaries. They also absorb mineral elements such as nitrogen, phosphorous and iron and transformed these nutrients into organic matter using the light energy received by the chlorophyll they contain via the process of photosynthesis.
There are some major environmental factor influencing phytoplankton growths. They include temperature, light, nutrients availability. Phytoplankton growth is limited to the euphotic zone- the depth to which sunlight penetrates in the water. In the presence of nutrients such as nitrogen and phosphorous at desirable temperature, phytoplankton undergo rapid population growth making more food available to organism at higher trophic levels. But when this growth becomes very high i.e. phytoplankton bloom, this becomes harmful to the overall aquatic life. This is because most of the phytoplankton die and sink to the bottom where they decompose and this process depletes the dissolved oxygen of the water which is necessary for survival of other organisms in the water.
Among the more important groups of phytoplankton are the diatoms, cyanobacteria and dinoflagellates.
Diatoms
Diatoms are microscopic one-celled algae that have cell walls of silica and pectin. They may also occur as stalklike or branching colonies. Their cell walls consist of almost identical halves which are fixed together. Their cytoplasm contains the green pigment chlorophyll, but other pigment, especially the yellowish xanthophyll, gives the organism a golden-brown appearance. The reproduction is usually by cell division but periodical sexual reproduction by means of fertilization of haploid gametes may occur.
There are more than 8000 species of diatoms exist, mostly in freshwaters or on the surface layers of the oceans, where they are a major component of the plankton on which marine life depends.
Depending upon the classification system used, diatoms either make up Bacillariophyceae in the phylum Chrysophyta, or they make up the separate phylum Bacillariophyta.
Cyanobacteria (Blue-Green algae)
Cyanobacteria make up the phylum Cyanophyta in the kingdom Prokaryotae. This phylum contains about 150 genera and 2000 species. They are the blue-green algae. They are photosynthetic prokaryotes which are single-celled and lack enclosed nucleus and other specialized cell structures like mitochondria and chloroplasts. Cyanobacteria contain only chlorophyll “a” which is located in the chromatophores (not both chlorophyll “a” and “b” in the chloroplasts like green plants), where photosynthesis is carried out. The chromatophorebs are the infoldings of the plasma membrane. Cyanobacteria are distinguished from bacteria by the presence of internal membranes, called thylakoids (chromatophores). Cyanobacteria colour varies from blue-green to red or purple and is determined by the proportions of two secondary pigments which are; c-phycocyanin (blue) and c-phycoerythrin (red), which tends to mask the green chlorophyll present in the thylakoids.
Reproduction in cyanobacteria is by simple cell division, spore production or fragmentation of the filaments; forming singular cells, colonies, filaments or gelatinous masses. Although, most lack flagella and are non-motile, filamentous form such as Oscillatoria rotate in a screw-like manner, and the gelatinous forms glide along their slimy mucus.
Dinoflagellates
Dinoflagellates are unicellular aquatic organisms of phylum Dinoflagellata and they are always being covered with stiff cellulose plates that resemble armored helmets. Many species have unusual ornamentation, such as horns, spines or wings like. The armor is encircled by a narrow groove and there is a second groove that runs perpendicular to this narrow groove. Dinoflagellates are with two uneven flagella (for locomotion) that beat within these grooves, causing the dinoflagellates to spin like tops as they move through the water.
Several thousand species of dinoflagellates are known, most of which but not all contain chlorophyll and are photosynthetic. Most of the 130 genera and 2000 species in this phylum are planktonic and live in saltwater, although many are found in freshwater. Dinoflagellates show traits of both protozoans and algae because of their ability to photosynthesize and also absorb nutrients by being parasitic, or by ingesting organic particles. They are believed to second diatoms in contributing to primary production in aquatic ecosystem. Reproduction is by cell division.
However, certain species of the genera Gymmodinium and Gonyaulax produce a strong nerve toxin and responsible for the blooms called red tides. These blooms lead to the deaths of large number of fish and contamination of clams and mussels, which may be lethal to humans who eat them. Many species of dinoflagellates produce bioluminescence (i.e. emitting a pale-blue glow seen at night).
ZOOPLANKTON
Zooplanktons are planktonic free-floating animals of fresh and marine aquatic systems and are the major consumers of the organisms in the microbial food web. These organisms possess a wide range of feeding strategies. From nematocysts (sting cells) of cnidarians (e.g. jellyfish) to the complicated mouth path of copepods. Some are carnivorous (animal-eaters), some are herbivorous (plant-eaters), and some are omnivorous (eaters of plants and animals). The smallest zooplankton can be characterized as the recyclers of aquatic systems nutrients. The larger ones are food for forage fish species. These animals (zooplankton) can move by means of cilia, flagella, jointed appendages, jet propulsion, or tailed larva (as in tunicates to larval fish). Reproduction varies from asexual, to fission and fragmentation, to sexual reproduction; where some gametes are released into the water and fertilized, yet others retained and fertilized internally.
Zooplankton include many phylum; some live their entire life cycle in the water (holoplankton), whereas, some like the larval stages of fish and other benthic organisms (such as starfish) are only planktonic for part of their lives (meroplankton). The major zooplankton phyla include:
Protozoa- This group includes ciliates, dinoflagellates, foraminifera, and radiolarian.
Coelenterate (cnidaria) - They are typically known as jellyfish, the major group are Hydrozoa, Scyphozoa, and Anthozoa. The medusae of the hydrozoa group are the prominent members in zooplankton and the most common forms are aurelia, pelagia and siphonophores. They are gelatinous and are the major consumers of smaller zooplankton and some of the microbial food web.
Ctenophore- They are otherwise referred to as comb jellies, these posses eight “combs” rows of fused cilia. These animals consume phytoplankton and zooplankton and when they are abundant they can clear the water of food for other zooplankton.
Chaetognatha- They are the arrow worms and the common member of deep-water plankton. They are predacious carnivores that grasp their prey and paralyze them before ingesting them.
Annelida- This includes many species of marine polychaetes. Many of these organisms shed their gametes on the surface of water for sexual reproduction. Their larvae are abundant in the zooplankton community.
Mollusca- This includes gastropods larvae, pteropods and cephalopods (squid and octopus). They are consumers of larger zooplankton.
Echinodermata- This includes starfish, brittle stars and sea cucumbers. They are all meroplankton. Their larvae are a major presence in the zooplankton community.
Athropoda- This includes copepods, shrimps, crabs, lobsters and crustaceans. They are considered the major consumers of most of the organisms in the microbial loop. They can consume a wide range of food particles from nanoplankton to microplankton, as they mature. Copeopods are responsible for much of the carbon energy transferred from phytoplankton to larger zooplankton.
Chordates- They are the urochordates (tunicates), they include sea squirts and salps. Tunicates are now realized to be major consumers of phytoplankton and smaller zooplankton and can contribute to the entire food web dynamics as much as or even more than copepods.
BIOGEOCHEMICAL SIGNIFICANCE OF PLANKTONS
Aside from representing the bottom few levels of a food chain that lead up to commercially importance fisheries, plankton ecosystems play a role in the biogeochemical cycles of many important chemical elements. They play a significance role in the ocean’s carbon cycle. Phytoplankton fix carbon in sunlit surface waters via photosynthesis and through zooplankton grazing, this carbon enters the planktonic food web, where it is either respired to provide metabolic energy or accumulates as biomass or detritus. This tends to sink and in open ocean ecosystem away from the coasts, these lead to the transport of carbon from surface water to the deep. This process is known as the Biological pump, and is one of the reasons that oceans constitute the largest carbon sink on earth.
Also, plankton are often used as indicator and aquatic pollution because of their high sensitivity to water quality and environmental changes. They respond to low dissolved oxygen, nutrients levels and toxin contamination.
A good picture of current condition in any water body can be derived by studying the zooplankton indicator such as biomass abundance and species diversity.
PLANKTON DISTRIBUTION
Plankton are found in oceans, seas, lakes, and ponds. However the abundance and distribution of planktons varies horizontally, vertically, and seasonally.
(a) The primary cause of this variability is the availability of light. All plankton ecosystems are driven by the input of solar energy and this confines primary production to surface waters, and to geographical regions and seasons when light is abundant.
(b) The availability of nutrients is a secondary cause of the variability in the distribution of plankton. Large areas of the tropical and sub-tropical oceans have abundant light, but they experience relatively primary production due to poor availability of nutrients such as nitrates and phosphates. This is a result of large scale ocean circulation and stratification of the water column. Some regions of the oceans are unproductive despite significant concentration of macronutrients in them. Fields studies have found that the mineral nutrients are deficient in these regions, and that adding them can lead to the formation of blooms many kind of phytoplankton.
(c) Also, physical state of the water column and abundance of other plankton affect the variability of plankton distribution and abundance. Temperature changes may cause some plankters to migrate vertically to the cooler deeper water during the day light hour and then at night they migrate back to the surface (diel vertical migration). Also, for example, plankton such as jellyfish may migrate upward to the surface to locate food (protozoan) during the day light and downward to deeper water at night for bacteria.
However, all this above factors governing distribution of plankton may also responsible for diel vertical migration of plankton.
Plankton are found in the greatest abundance in surface water, they occur throughout the water column. Even at depth where no primary production occurs, zooplankton and bacterioplankton instead make use of organic material sinking from the more productive surface waters above.
ROLE OF PLANKTON IN AQUATIC PRODUCTIVITY
Plankton are the basis of biological productivity in the aquatic ecosystem. They are the major factor that biological processes in the aquatic system depend on. As the phytoplankton increase due to their ability to source energy (solar energy) from the sun to manufacture their food through the process of photosynthesis, the primary productivity for the food web in the aquatic system increases. High photosynthetic activity also contributes to increase in the water column pH by removing carbon dioxide and hydrogen ion from the water body thereby creating quality system for biological process in the aquatic system. At the same time, there will improvement in the addition of dissolved oxygen for aquatic animals, like fish. The phytoplankton also absorbs mineral elements such as nitrogen, phosphorous and iron, and transformed them into organic matter using the light energy received via photosynthesis thereby making more food available to organisms at the higher trophic levels. Consequently, increasing aquatic productivity. However, when the growth of these phytoplankton become very high, most die and sink to the bottom thereby increasing organic matter in the water column for increased microbial activities; leading to dissolved oxygen depletion in the system for aquatic life. This may also result in release of toxin to the water body. These reduce aquatic productivity.
Phytoplankton are consumed by zooplankton which are eaten by aquatic insects and small fishes and these small organisms serve as food for the larger animals in the water. Zooplankton are the sole prey items for almost all fish larvae as they switch to external feeding for nutrition and also adult fish and other large aquatic organisms. Even, they are the major source of nutrition for some larger like some whales and shark. The zooplanktons link the primary producers (phytoplankton) with the larger or higher trophic level organisms of aquatic world. Fish species rely on the density and distribution of zooplankton to coincide with first-feeding larvae for good survival of fish larvae and improve fish productivity and aquatic productivity at large.
REFERENCES
Barnes, R.S.K., and K.H. Mann. (1980): fundamentals of aquatic ecosystems. Oxford, U.K: Blackwell Scientific Publications.
Deidre, M.G. (2007): Plankton.http:www.waterencyclopedia.com/oc-po/plankton.html.
Martin, J.H., and S.E. Fitzwater. (1988): Iron-deficiency limits phytoplankton growth in the Northeast Pacific Subartic. Nature 331:341-343.
Valiala, Ivan. (1995): Marine Ecological Processes, 2nd ed. New York: Springer Verlag.
DEFINITIONS
The name plankton is derived from the Greek word “planktos”, meaning “wanderer” or “drifter”. Although most of these organisms’ horizontal position is primarily determined by currents in the water bodies they inhabit; they cannot swim or move against currents, but some forms are capable of independent vertical movement in the water column and can swim up to several hundreds of meter vertically in a single day (a behaviour called diel vertical migration).
Based on the duration of their life cycle spend as being planktonic, plankton are in two forms:
Holoplankton- They are those organisms that spend their life cycle as part of the plankton e.g. most algae, copepods, salps, and some jelly fish.
Meroplankton- They are those organisms that are only planktonic for part of their lives (usually the larval stage), and then graduate to either the nekton or benthic (sea floor) existence e.g. sea urchins, starfish, crustaceans, marine worms and most fish.
SIZE GROUPS
Plankton are also often described in terms of size:
Picoplankton: smaller than 2mm; includes bacteria, prochlorophytes, and viruses.
Nanoplankton: between 2-20mm; includes diatoms, coccoliths, and silicoflagellates (pyrrophyta, chrysophyta, chlorophyta and xanthophyta)
Microplankton: between 20-200mm; includes most phytoplanktons; large diatoms, dinoflagellates and small zooplankton such as ciliates.
Macroplankton: between 200-2000mm; includes large zooplankton, copepods and invertebrate larvae.
Megaplankton: larger than 2000mm; includes fish larva and gelatinous zooplanktons such as jelly fish (ctenophores, salps, and pyrosomes).
FUNCTION GROUPS
Plankton are primarily divided into broad functional (or trophic level) groups as follows:
Phytoplankton- These are the autotrophic, prokaryotic or eukaryotic algae.
Zooplankton- They are the protozoans, metazoans or the eggs and larvae of lager aquatic animals
Bacterioplankton- These are the bacteria and archaea.
However, this scheme divides the plankton community into broad producer, consumer, and recycler groups. But, in the reality of it, the trophic level of some plankton is not straightforward; many species are mixotrophic depending upon their circumstances. For example, dinoflagellates are either photosynthetic bacteria or heterotrophic consumers. Even, the prokaryotic phytoplankton and the smallest ones among the zooplankton are bacterioplankton because they are also recyclers.
PHYTOPLANKTON
These are the autotrophic, prokaryotic or eukaryotic algae that live near the water surface where there is sufficient light to support photosynthetic activity. Phytoplankton fixed the inorganic carbon (as carbondioxide) utilized by marine and freshwater organisms into organic compound by their photosynthetic activity. And this makes the carbon available for higher trophic levels, thereby making them (phytoplankton) the primary producers; the base of the food web in oceans, lakes, rivers, and estuaries. They also absorb mineral elements such as nitrogen, phosphorous and iron and transformed these nutrients into organic matter using the light energy received by the chlorophyll they contain via the process of photosynthesis.
There are some major environmental factor influencing phytoplankton growths. They include temperature, light, nutrients availability. Phytoplankton growth is limited to the euphotic zone- the depth to which sunlight penetrates in the water. In the presence of nutrients such as nitrogen and phosphorous at desirable temperature, phytoplankton undergo rapid population growth making more food available to organism at higher trophic levels. But when this growth becomes very high i.e. phytoplankton bloom, this becomes harmful to the overall aquatic life. This is because most of the phytoplankton die and sink to the bottom where they decompose and this process depletes the dissolved oxygen of the water which is necessary for survival of other organisms in the water.
Among the more important groups of phytoplankton are the diatoms, cyanobacteria and dinoflagellates.
Diatoms
Diatoms are microscopic one-celled algae that have cell walls of silica and pectin. They may also occur as stalklike or branching colonies. Their cell walls consist of almost identical halves which are fixed together. Their cytoplasm contains the green pigment chlorophyll, but other pigment, especially the yellowish xanthophyll, gives the organism a golden-brown appearance. The reproduction is usually by cell division but periodical sexual reproduction by means of fertilization of haploid gametes may occur.
There are more than 8000 species of diatoms exist, mostly in freshwaters or on the surface layers of the oceans, where they are a major component of the plankton on which marine life depends.
Depending upon the classification system used, diatoms either make up Bacillariophyceae in the phylum Chrysophyta, or they make up the separate phylum Bacillariophyta.
Cyanobacteria (Blue-Green algae)
Cyanobacteria make up the phylum Cyanophyta in the kingdom Prokaryotae. This phylum contains about 150 genera and 2000 species. They are the blue-green algae. They are photosynthetic prokaryotes which are single-celled and lack enclosed nucleus and other specialized cell structures like mitochondria and chloroplasts. Cyanobacteria contain only chlorophyll “a” which is located in the chromatophores (not both chlorophyll “a” and “b” in the chloroplasts like green plants), where photosynthesis is carried out. The chromatophorebs are the infoldings of the plasma membrane. Cyanobacteria are distinguished from bacteria by the presence of internal membranes, called thylakoids (chromatophores). Cyanobacteria colour varies from blue-green to red or purple and is determined by the proportions of two secondary pigments which are; c-phycocyanin (blue) and c-phycoerythrin (red), which tends to mask the green chlorophyll present in the thylakoids.
Reproduction in cyanobacteria is by simple cell division, spore production or fragmentation of the filaments; forming singular cells, colonies, filaments or gelatinous masses. Although, most lack flagella and are non-motile, filamentous form such as Oscillatoria rotate in a screw-like manner, and the gelatinous forms glide along their slimy mucus.
Dinoflagellates
Dinoflagellates are unicellular aquatic organisms of phylum Dinoflagellata and they are always being covered with stiff cellulose plates that resemble armored helmets. Many species have unusual ornamentation, such as horns, spines or wings like. The armor is encircled by a narrow groove and there is a second groove that runs perpendicular to this narrow groove. Dinoflagellates are with two uneven flagella (for locomotion) that beat within these grooves, causing the dinoflagellates to spin like tops as they move through the water.
Several thousand species of dinoflagellates are known, most of which but not all contain chlorophyll and are photosynthetic. Most of the 130 genera and 2000 species in this phylum are planktonic and live in saltwater, although many are found in freshwater. Dinoflagellates show traits of both protozoans and algae because of their ability to photosynthesize and also absorb nutrients by being parasitic, or by ingesting organic particles. They are believed to second diatoms in contributing to primary production in aquatic ecosystem. Reproduction is by cell division.
However, certain species of the genera Gymmodinium and Gonyaulax produce a strong nerve toxin and responsible for the blooms called red tides. These blooms lead to the deaths of large number of fish and contamination of clams and mussels, which may be lethal to humans who eat them. Many species of dinoflagellates produce bioluminescence (i.e. emitting a pale-blue glow seen at night).
ZOOPLANKTON
Zooplanktons are planktonic free-floating animals of fresh and marine aquatic systems and are the major consumers of the organisms in the microbial food web. These organisms possess a wide range of feeding strategies. From nematocysts (sting cells) of cnidarians (e.g. jellyfish) to the complicated mouth path of copepods. Some are carnivorous (animal-eaters), some are herbivorous (plant-eaters), and some are omnivorous (eaters of plants and animals). The smallest zooplankton can be characterized as the recyclers of aquatic systems nutrients. The larger ones are food for forage fish species. These animals (zooplankton) can move by means of cilia, flagella, jointed appendages, jet propulsion, or tailed larva (as in tunicates to larval fish). Reproduction varies from asexual, to fission and fragmentation, to sexual reproduction; where some gametes are released into the water and fertilized, yet others retained and fertilized internally.
Zooplankton include many phylum; some live their entire life cycle in the water (holoplankton), whereas, some like the larval stages of fish and other benthic organisms (such as starfish) are only planktonic for part of their lives (meroplankton). The major zooplankton phyla include:
Protozoa- This group includes ciliates, dinoflagellates, foraminifera, and radiolarian.
Coelenterate (cnidaria) - They are typically known as jellyfish, the major group are Hydrozoa, Scyphozoa, and Anthozoa. The medusae of the hydrozoa group are the prominent members in zooplankton and the most common forms are aurelia, pelagia and siphonophores. They are gelatinous and are the major consumers of smaller zooplankton and some of the microbial food web.
Ctenophore- They are otherwise referred to as comb jellies, these posses eight “combs” rows of fused cilia. These animals consume phytoplankton and zooplankton and when they are abundant they can clear the water of food for other zooplankton.
Chaetognatha- They are the arrow worms and the common member of deep-water plankton. They are predacious carnivores that grasp their prey and paralyze them before ingesting them.
Annelida- This includes many species of marine polychaetes. Many of these organisms shed their gametes on the surface of water for sexual reproduction. Their larvae are abundant in the zooplankton community.
Mollusca- This includes gastropods larvae, pteropods and cephalopods (squid and octopus). They are consumers of larger zooplankton.
Echinodermata- This includes starfish, brittle stars and sea cucumbers. They are all meroplankton. Their larvae are a major presence in the zooplankton community.
Athropoda- This includes copepods, shrimps, crabs, lobsters and crustaceans. They are considered the major consumers of most of the organisms in the microbial loop. They can consume a wide range of food particles from nanoplankton to microplankton, as they mature. Copeopods are responsible for much of the carbon energy transferred from phytoplankton to larger zooplankton.
Chordates- They are the urochordates (tunicates), they include sea squirts and salps. Tunicates are now realized to be major consumers of phytoplankton and smaller zooplankton and can contribute to the entire food web dynamics as much as or even more than copepods.
BIOGEOCHEMICAL SIGNIFICANCE OF PLANKTONS
Aside from representing the bottom few levels of a food chain that lead up to commercially importance fisheries, plankton ecosystems play a role in the biogeochemical cycles of many important chemical elements. They play a significance role in the ocean’s carbon cycle. Phytoplankton fix carbon in sunlit surface waters via photosynthesis and through zooplankton grazing, this carbon enters the planktonic food web, where it is either respired to provide metabolic energy or accumulates as biomass or detritus. This tends to sink and in open ocean ecosystem away from the coasts, these lead to the transport of carbon from surface water to the deep. This process is known as the Biological pump, and is one of the reasons that oceans constitute the largest carbon sink on earth.
Also, plankton are often used as indicator and aquatic pollution because of their high sensitivity to water quality and environmental changes. They respond to low dissolved oxygen, nutrients levels and toxin contamination.
A good picture of current condition in any water body can be derived by studying the zooplankton indicator such as biomass abundance and species diversity.
PLANKTON DISTRIBUTION
Plankton are found in oceans, seas, lakes, and ponds. However the abundance and distribution of planktons varies horizontally, vertically, and seasonally.
(a) The primary cause of this variability is the availability of light. All plankton ecosystems are driven by the input of solar energy and this confines primary production to surface waters, and to geographical regions and seasons when light is abundant.
(b) The availability of nutrients is a secondary cause of the variability in the distribution of plankton. Large areas of the tropical and sub-tropical oceans have abundant light, but they experience relatively primary production due to poor availability of nutrients such as nitrates and phosphates. This is a result of large scale ocean circulation and stratification of the water column. Some regions of the oceans are unproductive despite significant concentration of macronutrients in them. Fields studies have found that the mineral nutrients are deficient in these regions, and that adding them can lead to the formation of blooms many kind of phytoplankton.
(c) Also, physical state of the water column and abundance of other plankton affect the variability of plankton distribution and abundance. Temperature changes may cause some plankters to migrate vertically to the cooler deeper water during the day light hour and then at night they migrate back to the surface (diel vertical migration). Also, for example, plankton such as jellyfish may migrate upward to the surface to locate food (protozoan) during the day light and downward to deeper water at night for bacteria.
However, all this above factors governing distribution of plankton may also responsible for diel vertical migration of plankton.
Plankton are found in the greatest abundance in surface water, they occur throughout the water column. Even at depth where no primary production occurs, zooplankton and bacterioplankton instead make use of organic material sinking from the more productive surface waters above.
ROLE OF PLANKTON IN AQUATIC PRODUCTIVITY
Plankton are the basis of biological productivity in the aquatic ecosystem. They are the major factor that biological processes in the aquatic system depend on. As the phytoplankton increase due to their ability to source energy (solar energy) from the sun to manufacture their food through the process of photosynthesis, the primary productivity for the food web in the aquatic system increases. High photosynthetic activity also contributes to increase in the water column pH by removing carbon dioxide and hydrogen ion from the water body thereby creating quality system for biological process in the aquatic system. At the same time, there will improvement in the addition of dissolved oxygen for aquatic animals, like fish. The phytoplankton also absorbs mineral elements such as nitrogen, phosphorous and iron, and transformed them into organic matter using the light energy received via photosynthesis thereby making more food available to organisms at the higher trophic levels. Consequently, increasing aquatic productivity. However, when the growth of these phytoplankton become very high, most die and sink to the bottom thereby increasing organic matter in the water column for increased microbial activities; leading to dissolved oxygen depletion in the system for aquatic life. This may also result in release of toxin to the water body. These reduce aquatic productivity.
Phytoplankton are consumed by zooplankton which are eaten by aquatic insects and small fishes and these small organisms serve as food for the larger animals in the water. Zooplankton are the sole prey items for almost all fish larvae as they switch to external feeding for nutrition and also adult fish and other large aquatic organisms. Even, they are the major source of nutrition for some larger like some whales and shark. The zooplanktons link the primary producers (phytoplankton) with the larger or higher trophic level organisms of aquatic world. Fish species rely on the density and distribution of zooplankton to coincide with first-feeding larvae for good survival of fish larvae and improve fish productivity and aquatic productivity at large.
REFERENCES
Barnes, R.S.K., and K.H. Mann. (1980): fundamentals of aquatic ecosystems. Oxford, U.K: Blackwell Scientific Publications.
Deidre, M.G. (2007): Plankton.http:www.waterencyclopedia.com/oc-po/plankton.html.
Martin, J.H., and S.E. Fitzwater. (1988): Iron-deficiency limits phytoplankton growth in the Northeast Pacific Subartic. Nature 331:341-343.
Valiala, Ivan. (1995): Marine Ecological Processes, 2nd ed. New York: Springer Verlag.
