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Temperature Effects
Water molecules respond to heat by absorbing energy, moving apart and vibrating faster. As the temperature continues to rise, the water boils and its molecules absorb enough energy to escape from the liquid surface into the air as vapor. Oxygen dissolves more easily in cold water because the molecules have less energy and are moving more slowly in the liquid. As water heats up, its molecules move and vibrate faster, making it easier for oxygen molecules to escape from the solution.
Dissolved Oxygen
Oxygen levels in water are essential to aquatic life, but the variability of dissolved oxygen levels primarily depends on the water temperature and green plants. During the winter, when the water is cooler, it holds higher levels of dissolved oxygen than during the summer when the sun heats up the surface water and drives off oxygen molecules. Fish dive into the cooler depths of lakes and ponds seeking sufficient oxygen. If the oxygen levels fall off, fish kills can result.
Sources of Oxygen
Oxygen can be absorbed by water through several different mechanisms: diffusion, aeration, and photosynthesis. Oxygen makes up about 21 percent of the atmosphere. At the surface of lakes, ponds, and rivers, it diffuses into the water. Aeration occurs when water rapidly flowing over rocks, as in a waterfall, captures oxygen as its turbulence exposes more water surface area to the atmosphere, allowing oxygen diffusion into the water spray.
Algae and aquatic plants produce oxygen as a waste product of photosynthesis. On warm, sunny days, aquatic plant life produce oxygen, but as the water warms up, some of the oxygen escapes the water's surface, lowering the dissolved oxygen levels. At night, photosynthesis ceases and the dissolved oxygen levels drop off although the water cools during the dark hours. Oxygen depletion hits its lowest levels at dawn.
Human Activities
Many human activities can damage the water levels in streams and rivers by changing the temperature of the water. Release of hot water and chemical waste from industries into natural water sources as well as agricultural runoff, sewers, power plants and storm drains can all raise water temperatures and lower dissolved oxygen levels. Oxygen is a chemically active element which can react with waste compounds dumped into the water. As aquatic life dies, it decomposes. Decomposition is a chemical process which requires oxygen and sometimes releases heat. Erosion is another culprit because the sediment load absorbs more heat energy from the sun and blocks off sunlight for photosynthesis .