Question

How does algal bloom destroy the quality of a fresh water body? Explain.

Answer 1

Hint: Algae are a very important part of aquatic ecosystems. As primary producers, they provide nutrition to consumers, as well as to decomposers maintaining water quality. Too dense a growth can however be detrimental. The so-called Red Tide is an example of one such.

Complete answer:
When the algal population undergoes a rapid increase in a marine or freshwater ecosystem, it is called an algal bloom. These can occur as a result of a sudden change in nutrient quality. On marine ecosystems, it is often after heavy storms when the ocean bottom is affected and nutrients are released. In fresh water bodies this can happen due to fertilizer run-off which increases the concentrations of phosphorus and nitrogen.

As the concentration of alga increases, more of other plants die, and there are more naturally dying algal cells. These then provide nutrients for the decomposers. The concentration of decomposing bacteria also shoots up. As this continues, dissolved oxygen in the water is used up, and the aquatic organisms start to die, including fish and insects, till the water is no longer able to support animal life.
Some algal also produce neurotoxins, so the effect of a bloom will also harm wildlife that drink the water. These are known as Harmful Algal Blooms, or HABs. If humans consume sea food that has come from algal bloom areas, they can get several neurological symptoms as well.

Note: Algal blooms can occur in several colours like bright green, red, and yellow-brown. Blue-green algae or cyanobacteria can also cause algal blooms. In a bloom there may be as many several million cells per millilitre.