Most of the nutrients in the open ocean are in the bottom of the ocean. This is because nutrients are really plant and animals that die and sink. 

In the open seas, water in the sunlit zone receives lots of sun and has a higher temperature than the water below. Under normal condition the process of "upwelling" brings cooler water from down below towards the top. Cold sea water is rich in nutrients. 

The supply of nutrient rich cold water to the sunlit zone is critical in maintaining healthy oceans. Most marine life lives in the sunlit zone, and these creatures need a steady supply of food and nutrients. 

Upwelling brings nutrients to where they are needed most, at the top of the ocean where most animals live.