Question

How does the presence of algea in water reduce the oxygen content?

Answer 1

The algae forms a layer over water which cuts off the contact of water with atmospheric oxygen and then it absorbs the oxygen already absorbed into the water which reduces the oxygen content of water. This is also called eutrophication. This leads to the death of marine organisms and makes the water unfit.

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Answer 2

if algae covers the water surface it causes depletion of oxygen in water. the primary source of oxygen for water bodies are phytoplankton and submerged plant which secrete oxygen in presence of sunlight. and when algae covers water surface they cannot get enough sunlight moreover algae uses the dissolved oxygen in water.