Waters in the coastal ocean tend to be more productive than waters of similar depth in the open ocean. What is the main reason for the productivity difference?

The coastal ocean seems to be more productive than the waters of the similar height in the open ocean due to ample availability of water, sunlight, and nutrients. The coastal ocean has more nutrients due to the process of upwelling, that is, movement of water offshore by the currents.

Also, water situated on the continental shelves is comparatively shallower and thus it is turbulent. This mixing or turbulence keeps settling the nutrients stirred up and accessible, and eventually, ample of nutrients erode off the continents themselves and so water nearby to the shore seems to be greater in nutrients.  

On the other hand, in the surface water away from the coastlines, usually, there is a lot of sunlight but no availability of adequate nutrients. Therefore, the majority of the ocean surface is not much productive.

Explanation:

b. read the phase at the intersection of any given temperature and pressure.