Like land organisms, aquatic animals and plants need oxygen to live.
Oxygen enters water in two ways:
Physically, when air mixes with water; this is usually the primary source of dissolved oxygen (DO) in streams.
Biologically, when aquatic plants release oxygen during photosynthesis; this is usually the primary source of DO in lakes and oceans.
Oxygen from the atmosphere enters water more readily through the action of wind and waves, or when water passes over riffles or waterfalls. DO is naturally at least 10,000 times more concentrated in air than in water! Organic matter, both natural and from pollution, can create high biological oxygen demands (BOD) and remove oxygen from water. This may cause “fish kills” and otherwise alter aquatic organism communities. A DO value of at least 5.0 ppm is desirable for most aquatic organisms, and is required for streams classified as “Fish and Wildlife” or higher. Dissolved oxygen decreases with increasing temperature, so DO values are expected to be higher during the winter and lower during the summer. Dissolved oxygen usually decreases with increasing depth, so DO values are expected to be higher at the surface of a lake and lower toward the bottom.
In lakes and ponds with high nutrient concentration DO can change dramatically throughout the day because of photosynthesis by aquatic plants. Dissolved oxygen in water is higher in winter and lower in summer (opposite of temperature) because the solubility of oxygen is greater in colder water.