In the ocean, why are the greatest amount of primary producers found in the Epipelagic zone and how does that affect biodiversity?

Because the Epipelagic Zone (or Euphotic Zone) receives enough sunlight for photosynthesis to occur. Since those tiny organisms who live there such as cyanobacteria and planktons are photosinthetic ones, they make up the base of the food chain and therefore are called primary producers. If they ceased to exist we would face several problems from loss of breathable air to food shortage.