Which oceanic layer is the twilight zone where there is some light but not enough for photosynthesis?

• A. Photic Zone
• B. Disphotic Zone
• C. Aphotic Zone
• D. Chlorophyll

Answer: (B)

Light penetration with depth and its effect on photosynthesis is being tested here. The ocean has layered light environments: the upper photic zone is bright enough for photosynthesis, while below it lies a twilight (disphotic) zone where there is some light but not enough to support photosynthesis. In the deepest parts there is no usable light, the aphotic zone. The twilight zone name matches the description of “some light but not enough for photosynthesis,” making it the best fit. In clear water, the photic zone is roughly 0–200 m, the disphotic about 200–1000 m, and the aphotic below that; this helps explain why photosynthesis can’t occur in the twilight layer. The other options don’t fit because the photic zone is bright enough for photosynthesis, the aphotic zone has no light, and chlorophyll is a pigment, not a depth layer.