Consider the Following Representation of the Marine Nitrogen Cycle: ‬

Multiple Choice

Consider the following representation of the marine nitrogen cycle: ‬ NH₄⁺ (ammonium ion) plays an important role in the nitrogen cycle, and is excreted by many aquatic organisms. Why is the fixation of atmospheric nitrogen necessary if there is a readily available source of biologically usable nitrogen already present in aquatic biomes?

A) Atmospheric nitrogen serves as a reservoir to be converted when other forms of nitrogen have been depleted from an area.
B) Ammonium ions will eventually be oxidized to nitrogen gas (N₂) that will be returned to the atmosphere.
C) Nitrogen still must be fixed from the atmosphere so that global climate doesn't change.
D) Nitrogen must be fixed so that terrestrial plants can utilize it for growth.