Final answer:
The availability of sunlight decreases with depth, affecting organism distribution in tube C.
Step-by-step explanation:
The reason there are more organisms at the top of tube C than at the bottom is due to the availability of sunlight. Light plays a crucial role in the distribution of photosynthetic organisms in aquatic environments. Surface water receives more sunlight, allowing photosynthetic organisms to thrive in the upper levels of the water. As you go deeper in the water, the availability of sunlight decreases, which affects organism distribution.