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The trophic levels and ecological efficiency of aquatic ecosystems

Trophic level is a feeding positions in a web or food chain of the ecosystem or step in a nutritive series. Group of organisms classified into these levels on the basis of their feeding behavior and generally aquatic ecosystem have not more than seven trophic levels within a food chain.

These trophic levels can be illustrated by trophic pyramid and organisms are grouped according to the role they play in the food web.

The first level which is the primary producers forms the base of the trophic pyramid. Primary producers are able to make their own food or described as autotrophic and they convert energy from the sun into food energy through the process of photosynthesis. Examples of aquatic primary producers are phytoplankton and seaweed.

The second trophic levels of aquatic ecosystems are the herbivorous consumers such as zooplankton and cockles that consume the primary producers. First level carnivorous consumers form the third trophic level which includes juvenile stages of larger animal such as fish and jellyfish as well as small fish, crustaceans and sea stars.

This third trophic level consume the zooplankton and cockles.

Next is the fourth trophic level which is the second level carnivorous consumers include the larger fish that consume the small fish, jellyfish, crustaceans and sea stars.

Larger fish eaten by the the fifth trophic level, the third level of carnivorous consumers and the examples of this level is squid and octopus.

Lastly the sixth trophic level on the top of the trophic pyramid are shark, dolphin and albatross that is the top carnivorous or apex predators. In the aquatic ecosystem trophic levels, not all the top predators live in the water. Decomposers, mainly bacteria that break down dead organisms exist on every trophic level. They play important role in the process of releasing nutrients to support producers and consumers that feed through absorbing organic material in the water column as they break down waste or dead tissue.

Trophic Level

Where It Gets Food

1st Trophic Level : Primary producers. Makes its own food (autotrophic) Phytoplankton, seaweed.

2nd Trophic Level : Herbivorous Consumers (primary consumers) Consumes producers Zooplankton, cockles.

3rd Trophic Level : First level carnivorous consumers (secondary consumers) Consumes primary consumers. Juvenile stages of fish and jellfish, small fish, crustaceans and sea stars.

4th Trophic Level : Second level carnivorous consumers (tertiary consumers) Consumes secondary consumers. Larger fish.

5th Trophic Level : third level carnivorous consumers (quaternary consumers) Consumes tertiary consumers. Squid, octopus, larger fish.

6th Trophic level : Top carnivorous (quinary consumers) Consumes quaternary consumers. Shark, dolphin, albatross.

Ecological efficiency of aquatic ecosystems

Ecological efficiency defined as the transfer of energy between trophic levels or in other words is the efficiency with which energy transferred from one trophic level to the next trophic levels. There are two important physics laws in the study of energy flow through the ecosystem. First thermodynamics law states that energy cannot be created or destroyed and it can only changed from one form to another. Sun is the source of energy that absorbed by the primary producers where in it is converted to stored chemical energy for the functioning of an ecosystem. The second law of thermodynamics states that whenever energy is transformed, there is a loss energy through the release of heat and loss of energy also occurs during respiration and movement. Energy loss occurs when energy is transferred between trophic levels where more and more energy lost as one moves up through trophic levels when one animal feeds off another animal. In aquatic ecosystems, phytoplankton carry out most of the photosynthesis that occurs. Most of phytoplankton primary production is consume and used for energy because phytoplankton are small organisms with simple structures. Eventhough phytoplankton are small but they grow very rapidly and they support large populations of herbivores. This is the reason why aquatic ecosystems can support more trophic levels than the land ecosystems. Aquatic ecosystems usually have higher efficiency compare to land ecosystems because higher proportion of ectotherms and producers like the algae are lack in ligin.

Above diagram on the right side shows the ecological efficiency of aquatic trophic level as average 10 percent of the energy transferred from one trophic level to another trophic level. Phytoplankton, the primary producers with 1000 units energy decreased 10 percent to 100 units energy for the herbivorous. Continuously decreased of 10 percent energy to 10 units, 1 unit and 0.1 unit energy for carnivorous zooplankton, carnivorous fish and tuna. Left side diagram also shows the same but with four trophic levels.

The primary producers with 1000 units energy transferred 10 percent to 100 units energy for herbivorous. Continuously decreased of 10 percent energy to 10 units and 1 unit energy for first order carnivores and up to second order carnivores. All of the consumers as it move up from one trophic level to another trophic level differs in its nutritional relationship with the producer. Energy is passed up a food chain or web from lower to higher trophic levels varies from 5 to 20 percent, average 10 percent of the energy at one level available to next trophic level. The other 90 percent of energy used for metabolic processes or given off as heat to the environment. Every trophic level loses energy so it often illustrated as a triangle with primary producers forming the base. The more trophic levels present, the less energy conserved at higher trophic levels. Each trophic level supports a smaller number of organisms as the result of only average 10 percent of the energy from an organisms transferred to its consumer. For example in the aquatic trophic level, a top level consumer like shark or tuna is supported by millions of primary producers such as phytoplankton from the base of trophic pyramid or the food web.

Measurement of energy transfer efficiency between two successive trophic levels is termed as the trophic level transfer efficiency (TLTE) and is defined by the formula: Trophic level transfer efficiency is a measure of how much energy occurs at one level divided by the energy at the level below it times hundred. Ecological efficiency describes efficiency with which energy transferred from one trophic level to the next trophic levels and determined by a combination of efficiencies relating to organismic resource acquisition and assimilation in an ecosystem. Trophic efficiency incorporates with three types of efficiency. First is the proportion of available energy that is consumed known as consumption efficiency. Second, the proportion of ingested food that is assimilated or assimilation efficiency and thirdly is the proportion of assimilated food that goes into new consumer biomass known as production efficiency.

Ecological efficiency is the transfer of energy between trophic levels or the energy flows within a food web. Large amounts of energy are lost from one trophic level to the next level as energy flows from primary producers to the various trophic levels of consumers and decomposers. Percentage of energy transferred varies from 5 to 20 percent, average 10 percent from one trophic level to the next levels and the rest loss as a heat. The major factor that limits the length in a food chain or food web is the low efficiency of energy transfer between trophic levels. There is not enough energy to support another trophic level after four up to six energy transfers. Aquatic ecosystems can support more trophic levels compare to the land ecosystems because of the higher efficiency.

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