Final answer:
The aphotic zone is the part of the ocean below 200 meters deep where there is insufficient sunlight for photosynthesis. Organisms here must use chemosynthesis or rely on organic matter from above, unlike in the photic zone where sunlight fosters photosynthesis.
Step-by-step explanation:
The ocean's dark lower layer where photosynthesis cannot occur is known as the aphotic zone. This zone begins at depths greater than 200 meters, where sunlight is too weak to support the process of photosynthesis. Due to the lack of light, primary producers in the aphotic zone must rely on chemosynthesis to generate energy, or on organic matter drifting down from the layers above where photosynthesis can occur. While the photic zone supports a diverse range of photosynthetic organisms such as algae, the aphotic zone houses species that have adapted to the dark conditions, often relying on food produced in the well-lit upper layers of the ocean.