Question

Why does the increased level of nutrients (or fertilisers) in the water affect the survival of aquatic organisms (like fish)?

Answer 1

Excessive quantities of chemicals (fertilizers, weedicides,insecticides, pesticides etc.) get washed away from the fields entering into water bodies like ponds,rivers,sea.

These act as nutrients for algae to flourish.

Once these algae die, they serve as food for decomposers like bacteria.

In this process a lot of oxygen in water gets used up.

This results in a decrease in the oxygen level for the aquatic organisms.

Thereby reducing the survival chances of aquatic organisms.