Why do organisms living in the pelagic zone below 200 meters depend on organisms living above that depth to supply their energy?

Plant photosynthesis is the primary source of nutrients in every ecosystem. Deep in the ocean, where little or no sunlight is present, photosynthesis is impossible. Thus, organisms in deep water ecosystems depend on detritus (discarded waste) and other organic matter that drifts down from (and is generated) nearer to the surface, or on species that migrate back and forth.