Answer:
Oxygen
Step-by-step explanation:
When excessive minerals(N.P.K) and nutrient are released into the body of water from leaching and other sources,(detergents,fertilizers application,sewage etc )which leads to corresponding increase in the population of algae(algae boom)the condition is called Eutrophication.
This disrupts the oxygen cycle because,the increased algae population(algae boom) makes use of available oxygen in the habitat for respiration.With continue supply of the nutrients,the population continue to rise.The competition for oxygen with other fauna leads to shortage of available oxygen,and therefore death of these fauna,and later of the algae.
The decomposing bacteria,also used the fragment pf oxygen to decompose the algae and other dead fauna,further reducing the oxygen levels of the water,with increase in water pollution.