Dissolved oxygen is supplied to a lake from two main sources: plant and algae photosynthesis and diffusion from the atmosphere.
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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.
Oxygen enters water through diffusion from air, wind and wave action and plant photosynthesis. Dissolved oxygen levels are increased by supplementing wind and wave action, adding plants to water and exposing water to purified oxygen.
Water near the surface of the lake– the epilimnion– is too warm for them, while water near the bottom–the hypolimnion– has too little oxygen. Conditions may become especially serious during a period of hot, calm weather, resulting in the loss of many fish.
In all lakes, oxygen is generally low right at the bottom where water meets the lake sediment or mud. This is because there are many bacteria and animals that live and breathe in the sediment. These bacteria and animals decompose dead material that sinks to the bottom and use up oxygen.
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.
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.