Water doesn't hold much oxygen, so lakes and the oceans are relatively low in oxygen, especially if the water is warm.
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In ocean and freshwater environments, the term hypoxia refers to low or depleted oxygen in a water body. Hypoxia is often associated with the overgrowth of certain species of algae, which can lead to oxygen depletion when they die, sink to the bottom, and decompose.
During rainy seasons, oxygen concentrations tend to rise in most surface waters because rain saturates with oxygen as it falls. More sunlight and warmer temperatures also increase plant growth and animal activity, which may increase or decrease DO concentrations and increase diurnal fluctuation.
The colder water is, the more oxygen it can hold. As the water becomes warmer, less oxygen can be dissolved in the water. Salinity is also an important factor in determining the amount of oxygen a body of water can hold; fresh water can absorb more oxygen than salt water.
During this cycle, the dissolved oxygen from the top layer of the lake moves to the bottom of the lake until ice forms across the surface. After the lake freezes, the only new dissolved oxygen is produced by underwater plants, so it has to be used sparingly.
The most important chemicals in a lake are nitrogen and phosphorus. These chemicals allow nutrient-rich plants and algae to grow. Other organisms feed off these plants and algae, creating a complex, healthy ecosystem.