Why can't plants and animals use the nitrogen directly from the atmosphere?
answer
a.there is not enough of it in the air.
b.nitrogen gas contains the element in an inert, unusable state.
c.they do use it directly from the atmosphere.
d.plants and animals do not need nitrogen for their life processes.

Hypoxia or low levels of oxygen would be the result during the eutrophication of nitrogen.

Option (b);

EXPLANATION:

Eutrophication means a process by which lakes and streams become excessively loaded nutrient rich water.  Because of this huge number of algae or aquatic plants grow.

When Eutrophication of Nitrogen happens, growth of algae results in low levels of oxygen in water and light also doesn't reach down the water. Many animals die because of Hypoxia and as a result ecosystem is affected.