Aquatic ecology and Biodiversity

Unit 2- Ecosystem ecology

2.1.6. Pond ecosystem- production and decomposition
In the pond ecosystem solar energy is utilized for primary production by chlorophyll-bearing plants such as phytoplnkton and macrophytes. This conversion of solar energy into chemical energy is due to photosynthetic and chemosynthetic activities in the aquatic plant community and the rate at which organic matter produced is called primary productivity. Primary production is cycled through different trophic levels resulting in fish production. Different forms producers and consumers are linked together through predator-prey relationship (Fig.1). This chain of food production, involves general pattern, primary producers - herbivores-carnivores - appears too simple and straight. But, in fact, it is a complex food web with various cross linkages.
Fish populations may be classified into several trophic levels, depending upon their position in this food chain. Phytoplankton, zooplankton, detritus and benthic organisms serve as food for the stocked fishes. Thus, as much of the available solar energy as possible is utilized for fish production by proper pond management.
Pond productivity is dependent on light, carbon dioxide, temperature and essential nutrients. Of these factors affecting in ponds, nitrogenous, phosphatic and potassic fertilizers determine the pond production. Besides, production and decomposition of minute plant and animal organisms in ponds also modify the properties of the pond bottom to a great extent. The nature and quantity of fertilizers determines the species composition to be used in a culture system. At low phosphate concentration, diatoms are common, but with increasing concentrations green algae become more frequent, eventually giving way to blue-green algae. In addition, excessive phosphate gives rise to phytoplankton blooms which check the light penetration and thus lower the pond productivity through ‘autoshading’ (Prowse, 1968).

Fig: Pond Ecosystem

Fig: Food Pyramid
Aerobic decomposition of organic matter by bacteria in pond is also an important drain on the oxygen supply in ponds. Aerobic decomposition requires a continuous supply of oxygen and proceeds more rapidly when DO concentrations are near saturation. However, decomposition also occurs under anaerobic conditions, but the rate of degradation of organic matter is not as rapid and complete as under aerobic conditions. Under aerobic condition, the end product of decomposition is primarily carbon dioxide. At times high rate of bacterial decomposition of dead organisms and other organic bottom deposits lead to a condition favouring the increase of the level of carbon dioxide and other abnoxious gases, with a simultaneous depletion of DO, resulting in fish kills and planktonic collapses (Radheyshyam et al., 1986).

Microbial Decomposition Process at the Sediment - Water Interface