In intensive aquaculture ponds, natural carrying capacities are greatly exceeded, and heavily laden artificial ecology is established among the various organisms and the environment they live.
Due to the high densities of fish or prawn stock for profitable commercial aquaculture, ponds receive large nutrient inputs from uneaten fish feeds, fish excretion, and sediment mineralization/resuspension.
Nutrient budget in aquaculture ponds revealed that fish could only assimilate 20 – 27% nitrogen and 8 – 24% phosphorus of the total inputs. It was reported that fish feed accounted for 90 – 98% nitrogen and 97 – 98% phosphorus of the total inputs in aquaculture ponds.
The major nutrients sank into the sediment, which accounted for 54 – 77% nitrogen and 72 – 89% phosphorus of the total inputs.
Growth and proliferation of blue-green algae in aquaculture ponds:
A large portion of nutrients from fish feed is chemically or biologically transformed and then released into the water and taken up by the algae (including blue-green algae), resulting in the excessive production of algae.
Because of “static” pond systems and high amounts of nutrients added daily, algal blooms are encouraged to grow and proliferate. Blue-green algae are advantageous over other algae because of their ability to control buoyancy to access areas of increased nutrients and light. Therefore, their growth rate is much higher than other non-harmful algae in aquaculture ponds. Although phytoplankton/algae are important to the aquatic food chains because they are primary producers, blue-green algae are generally not eaten by other aquatic organisms, because they produce secondary metabolites (e.g. toxins).
Therefore, blue-green algae are not an important part of the aquatic food chain and food web. Uneaten excessive blue-green algae can produce blooms in aquaculture ponds (Figure 2).
Figure 2. Food chain and food web in aquatic ecosystems. A food chain is the flow of energy from one organism to the next and a food web extends the food chain concept from a simple linear pathway to a complex network of interactions. Algae are the base of aquatic food chain and food web. Blue-green algae are not eaten by zooplankton or fish, because they produce toxins. Uneaten blue-green algae produce blooms in aquaculture ponds.
Blue-green algal blooms in aquaculture ponds are also promoted by zooplankton’s predatory activity. The relationship between algae and their zooplanktonic predators typically involves consumption of nutrients by algae, grazing of the algae by zooplankton which in turn enhances predator biomass, controls algal growth and regenerates nutrients.
However, the daily feed supply for aquaculture raises nutrient levels, but does not simply increase normal predator–prey activity; rather, harmful algal bloom events develop often with serious ecological and aesthetic implications. In the absence of the predator (zooplankton or fish), the non-harmful algal species outgrow the harmful algal species and inhibit its growth.
However, in the presence of the predator (e.g. zooplankton, fish or prawns in aquaculture ponds) it completely overturns by grazing out the non-harmful species, for which it demonstrates a preference (Figure 3). The dynamics of regeneration of the limiting nutrients versus nutrient consumption by algae is critical for bloom formation, as is the role of predators that exhibit prey selectivity (eat harmless algae only). When the supply of limiting nutrients exceeds the demand, the harmful algal biomass increases but nutrient status and palatability remain depressed (Figure 3).
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