Given a scientific passage and an answer, generate a question for the given answer.
Q: Passage: In addition to sunlight, aquatic producers also need dissolved oxygen and nutrients. Water near the surface generally contains more dissolved oxygen than deeper water. Many nutrients enter the water from the land. Therefore, water nearer shore usually contains more dissolved nutrients than water farther from shore. Answer: oxygen
A:
Aquatic producers need nutrients and what dissolved gas, which is more abundant near the surface of water?