Question

When microbial action during decomposition in aquatic ecosystems removes too much oxygen, the body of water experiences

Answer 1

N and P limitation is strongest in aquatic ecosystems (because light and water isn't) governed by same principles as terrestrial ecosystems radically different environmental controls streams have been treated as "sewers of terrestrial ecosystems" yet stream ecosystems have their own characteristics generated by the interaction of biota with the physical and chemical properties of the system unlike terrestrial ecosystems, nutrients "spiral" rather than cycle. The more limiting the nutrient, the tighter the spiral. Detritus/heterotrophs <---> autotrophs. 

Answer 2

The right answer is Eutrophication.

Eutrophication occurs mainly in ecosystems with slowly changing waters, especially in deep lakes.

In the depths of the lake, where dead algae are deposited, the aerobic bacteria that feed on them proliferate in turn, consuming more and more oxygen. However, in the absence of sufficient water circulation, which is often the case in a deep lake, the lake bottom is poorly oxygenated and the bacteria eventually deplete the oxygen in the deep water layers. They can no longer degrade all the dead organic matter and it accumulates in the sediments. The lake is said to be aging. Such a situation, when it occurs, worsens when it is hot because the solubility of oxygen in water (like that of all gases) decreases as the temperature increases.