There are several places where diatoms might not receive enough sunlight even at the sea surface.

1. In polar oceans areas where continuous darkness marks the winter, there is little daylight for months.
2. In areas where large quantities of suspended sediments cloud the ocean, such as near the outflow of large rivers, sediments can shade the sea surface reducing the photic zone to much less than the ideal vertical extent of 200 m.
3. In areas with high productivity, the abundance of small organisms can also cloud the ocean resulting in a reduced photic zone.