The definition of a limiting nutrient is: one that is necessary for algal growth, but available in a concentration insufficient to support continued growth. Once supply of this nutrient is exhausted, algal growth ceases independently of other remaining nutrient quantities.
Any nutrients, for example, nitrogen, phosphorus or certain metals can become limiting nutrients for phytoplankton growth. Limitation of nitrogen is an expensive process, which required high energy and chemical costs and specialized equipment.
Certain microorganisms, including algae, are able to fix atmospheric nitrogen opportunistically. Removal of metals may disrupt the local ecology, especially that of aquatic plants.
Therefore, phosphorus limitation is the cheapest and most practical means of preventing the growth of phytoplankton, particularly toxic blue-green algae. Phosphorus is an essential requirement of living, functional algae.
Phosphorus is a component of nucleic acids governing protein synthesis and of the adenosine phosphate transformations that power intracellular transport.
How phosphorus limitation control blue-green algae in aquaculture ponds:
In the events of phosphorus availability, phytoplankton intake excess phosphorus then their requirements which is called ‘luxury uptake’. As a result, the cell may contain 8 – 16 times more phosphorus than the minimum required quota.
As a consequence, it is theoretically able to sustain three or possibly four cell doublings without taking up any more phosphorus form the environment. It has been suggested that phosphorus storage in blue-green algae may be larger than in other algae providing them with a competitive advantage.
The phytoplankton biomass and the cyanobacterial component responded to phosphorus remedial action in four stages:
- No biomass reduction if phosphorus in excess to requirements: This stage occurrs in nutrient rich water bodies where nutrients are never growth limiting and there is an unused fraction of the total phosphorus. In these cases there is no immediate effect on phytoplankton biomass or species composition.
- Declining amount of unused phosphorus, small reduction in biomass: This stage of recovery depends upon the behaviour of the phytoplankton in those water bodies where phytoplankton community is dominated by motile algae such as dinoflagellates and buoyant cyanobacteria. As a consequence of the reduced nutrient load, these phytoplankton move to greater depths as they seek additional nutrients.
- Phytoplankton biomass falls, minimal unused phosphorus: The phosphorus concentration continues to decline as a consequence of both the internal and external reduction of the phosphorus-loading. The overall result is a significant decrease in the phytoplankton biomass as p-limitation begins to take effect.
- Further decline in biomass and changes in composition of the phytoplankton: The fourth stage of recovery occurs once the water body reaches its new equilibrium state with a change in species composition. In this stage, the N:P ratio increase and the algal speciation shifts from toxic harmful blue-green algae to harmless beneficiary green algae because green algae required N:P ~30 whereas red algae required ~10, diatom required ~10, Dinophyceae required ~12 and blue-green algae required less than 29.
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