Ocean waters have a varying carrying capacity for phytoplankton and other organisms that perform photosynthesis. The carrying capacity is very high near the surface, and then decreases with depth. It is zero in the deepest ocean waters. Which statement about the water is most useful for explaining this variation?

A. Temperature decreases with depth.
B. Brightness decreases with depth.
C. Salinity increases with depth.
D. Dissolved carbon dioxide decreases with depth.
E. Dissolved carbon dioxide increases with depth.