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Topic 1

Pond ecosystem and productivity

In a pond of any kind there exists a dynamic system of material/energy cycle, broadly between all the living organisms and the non-living environment which are, in nature, inseparably interrelated and interact upon each other.

The non-living substances of pond ecosystem include the inorganic and organic compounds. These compounds remain in solution in pond water, in reserve in bottom deposits as well in the living organisms - both plants and animals.

The living organisms (biotic community) in a pond ecosystem, consisting of all sorts of plants and animals, are broadly categorized into two main groups:

The autotrophic community viz. the producers include all the green plants which produce complex organic substances or cell materials from inorganic soluble nutrients in presence of sun light (photosynthesis). The producers are of two types -- phytoplankton and macrophytes.

The minute floating algae or phytoplankton distributed in the pond water column mainly throughout the light-limited limnetic zone. Abundance of phytoplankton population in water gives greenish colour. Production of phytoplankton is thus dependent on sun light and available soluble nutrients in water. Tropical fish ponds are generally found rich in natural food organisms as the sun light duration is longer.

The macrovegetation, though fall under producer group, in reality, their contribution is negligible in primary productivity of well managed fish pond. They are, however, found inhibitor in fish production system if not utilized properly as fish feed.

The heterotrophic community in a fish pond ecosystem comprising of two major groups of organisms namely:

The consumers - the organisms depending on other organisms living or dead for food such as zooplankton, insect larvae, fish etc. The consumers group (benthic invertebrates and bottom feeding fish) subsisting on organic detritus are called detritivores.

The decomposers - the group of heterotrophic organisms such as bacteria and fungi break down the dead plants and animals and release partially decomposed materials and inorganic nutrients into pond water for utilization by the producers viz. phytoplankton and macrophytes. The decomposers are most abundant at the pond bottom where the dead plants and animal bodies are most abundant.

The food chains or energy pathways in a pond ecosystem are of two types: the autotrophic food chain and the heterotrophic food chain.

Autotrophic food chain - It is the solar energy dependent food chain which begins with the plants, basically phytoplankton, at the pond surface and then sequentially pass through primary consumers (zooplankton and planktivours fishes) to secondary and tertiary consumers (carnivorous insects and fishes).

The pathways in autotrophic food chain are:

Phytoplankton ⇒ fish
Phytoplankton ⇒ zooplankton ⇒ fish
Phytoplankton ⇒ zooplankton ⇒ aquatic insects ⇒ fish
⇒ predator fish

Heterotrophic food chain - This energy pathway starts from the decomposition of dead organic matters by the decomposing bacteria and fungi at the pond bottom and then pass through zooplankton - aquatic insects to fishes (secondary and tertiary consumers) as shown below :

Bacteria ⇒ fish
Bacteria ⇒ protozoa ⇒ fish
Bacteria ⇒ Protozoa ⇒ zooplankton ⇒ Aquatic insect larvae ⇒ fish

The productivity of a fish pond depends on the availability of dissolved nutrients in pond water which can be utilized for production of organic materials by the producers (mainly phytoplankton) using solar energy.

The sources of pond nutrients may be of organic and/or inorganic origin. Organic manure mainly supports the heterotrophic food chain while the inorganic fertilizers supports the autotrophic food chain. Both the pathways are closely inter-linked to each other. However, in most fish ponds, production largely depends on the solar energy dependent autotrophic food chain, but the ponds receiving high quantity of organic manures on regular basis have significant heterotrophic use of wastes by bacteria and protozoa.

Fish production not only depends on the rich food chains but also largely on the stocking density of fish as well as the species combination having different feeding and spatial niches - so that total fish food organisms of a pond originated from both the food chains are properly utilized by the fish.

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