Fewer algae exist in deep ocean water because algae require to thrive.

Sunlight and carbon dioxide.

Explanation:

Fewer algae exist in deep ocean water because algae needs sunlight and carbon dioxide to perform the process of photosynthesis.

The algae are producers and needs carbon dioxide and sunlight so that they can make food. The consumers feed on the producers for their energy needs.

Hence, the fungi needs sunlight which is trapped being on the surface. If they will be deep inside ocean then their number will decrease.