Final answer:
The middle layer of the ocean is the pycnocline, where water density changes rapidly with depth. Photosynthetic organisms are primarily found in the photic zone of the ocean, which includes the intertidal and neritic zones, where sunlight is sufficient for photosynthesis.
Step-by-step explanation:
The middle layer of the ocean is known as the pycnocline. This zone is characterized by a rapid change in water density with depth, which often correlates with changes in temperature and salinity.
When considering oceanic zones based on depth and light penetration, the photic zone, extending down to approximately 200 meters, is where enough sunlight penetrates to support photosynthesis. On the contrary, the aphotic zone lies beyond this depth, where sunlight is insufficient for photosynthesis. The neritic zone encompasses the coastal waters over the continental shelf, while the oceanic zone refers to the vast, open ocean beyond the continental shelf. Among these, photosynthetic organisms are predominately found in the photic zone, including the intertidal and neritic zones.