Sea
The amount of oxygen found in seawater depends mostly on the plants growing in it. These are mainly algae, including phytoplankton, but also include some vascular plants such as seagrasses. In daylight the photosynthetic activity of these plants produces oxygen which dissolves in the seawater where it is used by marine animals. At night, photosynthesis stops, and the amount of dissolved oxygen declines. In the deep sea where not enough light penetrates for plants to grow, there is very little dissolved oxygen.