The answer is The increased nitrates from the fertilizers leech into the water and cause algal blooms, which take oxygen out of the water and this suffocates fish..
Algae blooms are the consequence of large amounts of farm
fertilizers in the rivers, which increases nitrates level in the water. Algae use the presence of increased nutrients
and enhance their own growth. It is known that they produce
an oxygen in the photosynthesis, so it is not a problem while they are
still alive. But when these organisms die, their decomposition begins. For the
process of decomposition, the oxygen is needed and ultimately it will be depleted
from a body of water. This could lead to the death of aquatic animals, such as fish, that
depend on oxygen.
