Final answer:
Nitrates from the ocean floor are returned to the ocean surface through upwellings, which are caused by prevailing winds pushing offshore and bringing up nutrient-rich waters from the deep ocean.
Step-by-step explanation:
The nitrates from the ocean floor are returned to the ocean surface through upwellings. Ocean upwelling is the process in which deep ocean waters are brought to the surface due to the movement of prevailing winds along the surface waters near a coastline. As the wind pushes the surface waters offshore, water from the bottom of the ocean moves up to replace it. This brings up nutrients, including nitrates, from the ocean depths and makes them available for living organisms.
For example, upwellings are responsible for the high productivity of marine ecosystems along the western coast of South America, where the nutrient-rich waters support a diverse array of marine life.