Primary production is high in regions where water drains into the ocean because of the

The answer is eutrophication. It happens when the nutrient levels get high in areas with growing plants and algae. It eventually would result in an increase of living organisms.  Thus, the outcome is a gradual loss of oxygen and decline of primary production. 

This explanation supports the idea that high primary production is present in high places where water gets to drain. Water draining prevented the overpopulation of aquatic organisms.