Question

dissolved oxygen levels are highest in surface waters because of:
respiration
decomposition
photosynthesis
exchange with the atmosphere

Explanation:

1. Respiration: Organisms respire to consume oxygen, so it would decrease dissolved oxygen, not increase.
2. Decomposition: Decomposers break down organic matter and consume oxygen, leading to lower oxygen levels.
3. Photosynthesis: Aquatic plants (like phytoplankton) in surface waters perform photosynthesis, which produces oxygen. Surface waters have more light for photosynthesis, so this process increases dissolved oxygen here.
4. Exchange with the atmosphere: While gas exchange with the atmosphere does contribute to dissolved oxygen, the primary reason for the highest levels in surface waters is photosynthesis by aquatic plants (since surface waters have more light for photosynthesis compared to deeper waters, and photosynthesis produces significant oxygen).

Answer:

C. photosynthesis