An Increased growth in microorganisms results in oxygen depletion
The growth of algal bloom is caused due to excessive runoff rich in the nutrients like nitrogen and phosphorous. This run off comes from the agricultural field.
These nutrients aids in the growth of the algae(microorganisms) in large number. This growth forms a layer on the water body and use the oxygen present in the water and makes the water oxygen deficient.
Harmful algal blooms, or red tides, can occur naturally, but new research indicates that after an oil spill, the application of dispersant may increase the chance of red tides.