Final answer:
Oxygen in water is less than in the atmosphere because it is less soluble in water than in air, and the factors like temperature and reduced mixing in aquatic environments affect oxygen concentration.
Step-by-step explanation:
The availability (concentration) of oxygen in water is far less than that in the atmosphere primarily because oxygen is less soluble in water than in air. Oxygen's solubility in water is influenced by various factors such as temperature, with higher temperatures decreasing the amount of dissolved oxygen water can hold. Furthermore, while aquatic environments may have localized areas with varying oxygen concentrations due to slower movement and mixing compared to the atmosphere, terrestrial environments maintain more constant oxygen levels due to convectional mixing (winds).
Additionally, aquatic organisms like fish utilize gills to take up the dissolved oxygen from water, allowing them to live in environments with lower oxygen concentrations. The concentration of dissolved oxygen is also dependent on the proximity to the water surface and the presence of photosynthetic activity, both of which usually increase oxygen levels.