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Reef Food PART-1
BY: ERIC BORNEMAN
This article varies from the subject of corals, strictly, and is more concerned with coral reefs, in general. I have fielded many questions in "The Coral Forum" about feeding corals, and have provided talks to several groups on coral nutrition. I plan to discuss this in more depth in upcoming articles, but felt an overview of nutrition to a coral reef community and coral reef aquarium would be a beneficial prologue to develop a more complete view of the subject.
A coral reef supports a tremendous variety of life, all of which are dependent on energy sources for their survival, growth and reproduction. There are two basic types of organisms in terms of their method of gaining energy: heterotrophs and autotrophs. Autotrophs are the primary producers; they use sunlight, converting its energy through photosynthesis into energy rich products (reduced forms of carbon, usually in the form of simple sugars) that are used by the organism. In this way, they form the beginning of the food chain, as they are the original or primary source of dietary energy for all other organisms. Photosynthetic bacteria or cyanobacteria, may also be considered to be primary producers, and their biomass on and near coral reefs, including in the water column, is enormous. Heterotrophs are those organisms that must attain at least some nutrition from feeding or absorption to acquire a reduced source of carbon. Even primary producers need more than sunlight to survive, and this is part of a great misconception; that being, that autotrophs can "do it all." Consider the houseplant that dies without nutrients from soil or fertilizer; it obviously needs additional nutrients besides light and water. Consider, as well, that fertilizers and soils are commonly described by their nitrogen and phosphorus content; these are also among the most important nutrients required by heterotrophic organisms. The main difference between autotrophs and heterotrophs is not that one "eats" and the other "just needs sun," but that one can provide various amounts of required carbon by using light energy.
The word "nutrient" is often misunderstood. The terms "high nutrient" and "low nutrient" can be taken in many contexts. In general, nutrients are those organic and inorganic compounds necessary to sustain life. While this comprises a very large group of potential compounds, nutrients are often simplified in terms of those elements that are major "building blocks" for fats, amino acids, and carbohydrates. Furthermore, they are frequently those elements which tend to limit further growth by their availability and ability to be procured. In general, carbon, nitrogen and phosphorus are often used to describe the "nutrient" condition of reef organisms (and others, as well). Plants and animals with photosynthetic symbionts tend to be nitrogen and/or phosphorous limited under normal conditions, since photosynthesis usually provides non-limiting carbon source material. Coral reef waters are typically "nutrient poor" as they contain very low levels of nitrogen and phosphorus (they are both precious commodities and any excess is usually taken up quickly). In nearshore areas where there is significant organic loading from land runoff, waters tend to be rather nutrient rich. Both types of environments sustain their own flora and fauna with varying amounts of habitat overlap in terms of the organisms that can exploit the continuum of nutrient conditions. The nutrients available in water to coral reefs can be dissolved in the water, in the form of particulate material, or as living biomass.
The coral reef is a place of both high primary productivity and consumption of nutrients, with a great deal of nutrients being recycling within the community. For many years, coral reefs were thought to be "nutrient poor deserts." In fact, this is not the case. It would be a very poor assumption to imagine that any species-rich community was not highly dependent on nutrients. While measurement of the water column shows it to be relatively devoid of organic and inorganic dissolved nitrogen, carbon and phosphorous and, therefore, "nutrient poor," it is largely because of the efficiency of the reef community that such water conditions are attained. Waters around coral reefs are rich in nutrients in the form of various types of microplankton; these are largely removed by coral reef organisms. It should be noted that most of the plankton on coral reefs is produced by and lives within the reef or nearby communities, and is not borne into it in great quantities by the open ocean.
An adaptation that has allowed for such diversity, to a large degree, is the symbiosis of animal and plants (algae and cyanobacteria) to make efficient use of each other's limitations. Such symbioses occur commonly in sponges, corals, nudibranchs, anemones, clams, hydroids, foraminiferans, and many other invertebrates that make up a large portion of the total reef community. These organisms are not autotrophs, no matter how efficient and substantive the contribution of their symbionts, and they must be fed. So, what do they eat?
-Reef Food
Coral reef inhabitants have widely varied diets, and most aquarists are familiar with the often highly specific dietary needs of some of these animals. Motile invertebrates may be predatory, like fish. Others are scavengers of decomposing material, or they can be "filter-feeders" by any number of mechanisms. Some employ numerous methods of nutrient uptake. Both "filter-feeding" by passive means and active prey capture are used by many of the sessile invertebrates commonly maintained in aquaria. At various early stages of their life, the diet of reef organisms may require planktonic organisms, and they, themselves, may be planktonic at some part of their life. Some of these animals (and all algae) are also capable of acquiring nutrition through the absorption (or direct uptake) of dissolved organic and inorganic nutrients. Normally, the levels of these substances on a coral reef are very low, and such nutrients are often a limiting aspect of the growth of any one life form. Because of the number of species present on a coral reef, most any food source is often a source of fierce competition, even if not directly. Often, simple competition for space is enough to limit nutrient availability.
Nutrients enter a coral reef from a variety of sources. They can arrive from freshwater or terrestrial sources; rivers and rain can both wash land based nutrients out to sea. Cooler water from deep in the ocean moves upward, bringing nutrient rich water upwards to the reef. This water is nutrient-rich because of the "downfall" of organic material into ocean depths and a comparative lack of planktivory in the deep ocean compared to that which exists in the upper photic zones. Currents, tides, storms, and waves bring plankton and nutrients from various distances to wash back and forth over the reef. The production and waste material of the reef organisms also provide important nutrition to other animals on the reef, and they are part of what is know as the detrital food chain. Detritus, marine snow, particulate organic material, and suspended particulate matter are all names for the bits of "dirt" that flow around the reef; material that is composed of fecal material, borings, algae, plant material, mucus, associated bacteria, cyanobacteria and other particles. Decomposers (mainly bacteria and associated flora and fauna) break down waste material in the water, on the reef, and, primarily, in the soft sediments. The result of their presence and action is not only a food source in and of itself, but provides raw material for channeling back into the food chain, largely through the benthic algae and phytoplankton.
Phytoplankton are small unicellular algae, or protists, that drift in the water column. They may be very abundant in and around coral reefs, and they are capable of absorbing large amounts of organic and inorganic nutrients. When conditions are proper, they can reproduce very quickly, and areas of high nutrients will often have a greenish, reddish, or brownish, cast and lower water clarity, mostly resulting from high phytoplankton populations. Some of the reef animals can feed directly on phytoplankton; many soft corals, some sponges, almost all clams, feather-duster worms, and other filter feeders utilize phytoplankton directly as a food source. Small animals in the water column, termed zooplankton, also utilize phytoplankton as a food source. For the smaller zooplankton, phytoplankton and bacteria are the primary food source.
(CONT)
Reef Food PART-1
BY: ERIC BORNEMAN
This article varies from the subject of corals, strictly, and is more concerned with coral reefs, in general. I have fielded many questions in "The Coral Forum" about feeding corals, and have provided talks to several groups on coral nutrition. I plan to discuss this in more depth in upcoming articles, but felt an overview of nutrition to a coral reef community and coral reef aquarium would be a beneficial prologue to develop a more complete view of the subject.
A coral reef supports a tremendous variety of life, all of which are dependent on energy sources for their survival, growth and reproduction. There are two basic types of organisms in terms of their method of gaining energy: heterotrophs and autotrophs. Autotrophs are the primary producers; they use sunlight, converting its energy through photosynthesis into energy rich products (reduced forms of carbon, usually in the form of simple sugars) that are used by the organism. In this way, they form the beginning of the food chain, as they are the original or primary source of dietary energy for all other organisms. Photosynthetic bacteria or cyanobacteria, may also be considered to be primary producers, and their biomass on and near coral reefs, including in the water column, is enormous. Heterotrophs are those organisms that must attain at least some nutrition from feeding or absorption to acquire a reduced source of carbon. Even primary producers need more than sunlight to survive, and this is part of a great misconception; that being, that autotrophs can "do it all." Consider the houseplant that dies without nutrients from soil or fertilizer; it obviously needs additional nutrients besides light and water. Consider, as well, that fertilizers and soils are commonly described by their nitrogen and phosphorus content; these are also among the most important nutrients required by heterotrophic organisms. The main difference between autotrophs and heterotrophs is not that one "eats" and the other "just needs sun," but that one can provide various amounts of required carbon by using light energy.
The word "nutrient" is often misunderstood. The terms "high nutrient" and "low nutrient" can be taken in many contexts. In general, nutrients are those organic and inorganic compounds necessary to sustain life. While this comprises a very large group of potential compounds, nutrients are often simplified in terms of those elements that are major "building blocks" for fats, amino acids, and carbohydrates. Furthermore, they are frequently those elements which tend to limit further growth by their availability and ability to be procured. In general, carbon, nitrogen and phosphorus are often used to describe the "nutrient" condition of reef organisms (and others, as well). Plants and animals with photosynthetic symbionts tend to be nitrogen and/or phosphorous limited under normal conditions, since photosynthesis usually provides non-limiting carbon source material. Coral reef waters are typically "nutrient poor" as they contain very low levels of nitrogen and phosphorus (they are both precious commodities and any excess is usually taken up quickly). In nearshore areas where there is significant organic loading from land runoff, waters tend to be rather nutrient rich. Both types of environments sustain their own flora and fauna with varying amounts of habitat overlap in terms of the organisms that can exploit the continuum of nutrient conditions. The nutrients available in water to coral reefs can be dissolved in the water, in the form of particulate material, or as living biomass.
The coral reef is a place of both high primary productivity and consumption of nutrients, with a great deal of nutrients being recycling within the community. For many years, coral reefs were thought to be "nutrient poor deserts." In fact, this is not the case. It would be a very poor assumption to imagine that any species-rich community was not highly dependent on nutrients. While measurement of the water column shows it to be relatively devoid of organic and inorganic dissolved nitrogen, carbon and phosphorous and, therefore, "nutrient poor," it is largely because of the efficiency of the reef community that such water conditions are attained. Waters around coral reefs are rich in nutrients in the form of various types of microplankton; these are largely removed by coral reef organisms. It should be noted that most of the plankton on coral reefs is produced by and lives within the reef or nearby communities, and is not borne into it in great quantities by the open ocean.
An adaptation that has allowed for such diversity, to a large degree, is the symbiosis of animal and plants (algae and cyanobacteria) to make efficient use of each other's limitations. Such symbioses occur commonly in sponges, corals, nudibranchs, anemones, clams, hydroids, foraminiferans, and many other invertebrates that make up a large portion of the total reef community. These organisms are not autotrophs, no matter how efficient and substantive the contribution of their symbionts, and they must be fed. So, what do they eat?
-Reef Food
Coral reef inhabitants have widely varied diets, and most aquarists are familiar with the often highly specific dietary needs of some of these animals. Motile invertebrates may be predatory, like fish. Others are scavengers of decomposing material, or they can be "filter-feeders" by any number of mechanisms. Some employ numerous methods of nutrient uptake. Both "filter-feeding" by passive means and active prey capture are used by many of the sessile invertebrates commonly maintained in aquaria. At various early stages of their life, the diet of reef organisms may require planktonic organisms, and they, themselves, may be planktonic at some part of their life. Some of these animals (and all algae) are also capable of acquiring nutrition through the absorption (or direct uptake) of dissolved organic and inorganic nutrients. Normally, the levels of these substances on a coral reef are very low, and such nutrients are often a limiting aspect of the growth of any one life form. Because of the number of species present on a coral reef, most any food source is often a source of fierce competition, even if not directly. Often, simple competition for space is enough to limit nutrient availability.
Nutrients enter a coral reef from a variety of sources. They can arrive from freshwater or terrestrial sources; rivers and rain can both wash land based nutrients out to sea. Cooler water from deep in the ocean moves upward, bringing nutrient rich water upwards to the reef. This water is nutrient-rich because of the "downfall" of organic material into ocean depths and a comparative lack of planktivory in the deep ocean compared to that which exists in the upper photic zones. Currents, tides, storms, and waves bring plankton and nutrients from various distances to wash back and forth over the reef. The production and waste material of the reef organisms also provide important nutrition to other animals on the reef, and they are part of what is know as the detrital food chain. Detritus, marine snow, particulate organic material, and suspended particulate matter are all names for the bits of "dirt" that flow around the reef; material that is composed of fecal material, borings, algae, plant material, mucus, associated bacteria, cyanobacteria and other particles. Decomposers (mainly bacteria and associated flora and fauna) break down waste material in the water, on the reef, and, primarily, in the soft sediments. The result of their presence and action is not only a food source in and of itself, but provides raw material for channeling back into the food chain, largely through the benthic algae and phytoplankton.
Phytoplankton are small unicellular algae, or protists, that drift in the water column. They may be very abundant in and around coral reefs, and they are capable of absorbing large amounts of organic and inorganic nutrients. When conditions are proper, they can reproduce very quickly, and areas of high nutrients will often have a greenish, reddish, or brownish, cast and lower water clarity, mostly resulting from high phytoplankton populations. Some of the reef animals can feed directly on phytoplankton; many soft corals, some sponges, almost all clams, feather-duster worms, and other filter feeders utilize phytoplankton directly as a food source. Small animals in the water column, termed zooplankton, also utilize phytoplankton as a food source. For the smaller zooplankton, phytoplankton and bacteria are the primary food source.
(CONT)
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