Hi, I’m Emerald Robinson, and in this “What is” video, we’ll examine the question, “What is water (http://www.redorbit.com/topics/water/) circulation?” In nature, water circulation is the mixing of layers found in a body of water. This mixing helps to equalize the distribution of oxygen concentration, nutrient levels, salt concentration, the amount of waste and pollution, and temperature. While water circulation occurs to some degree in many bodies of water, it’s especially important in lakes, estuaries, and oceans. Since lakes are largely still and calm, water circulation is vital to their ecosystems’ health. Lake water tends to form three layers: top, middle, and bottom. When these layers mix, the lake experiences what is called “turnover.” Turnover is driven by differences in water density. Cold water is more dense than warm water and sinks; warm water rises. When air temperatures at a lake’s surface get colder, so does the temperature of the top layer. This layer becomes more dense and sinks, forcing the other layers to rise and mix. Estuaries form when fresh water from a river’s mouth mixes with seawater. Salty water is less dense than freshwater, and forms a layer at the surface of an estuary. Since different kind of organisms can live in different amounts of salt water, water circulation is crucial in estuary ecosystems. Winds, tides, rainfall, and even storms play vital roles in the rate of an estuary’s water circulation. Water circulation in oceans is called thermohaline circulation – circulation driven by both temperature and salt concentration. Thermohaline circulation occurs when water carried by currents flowing from the equator to the poles becomes cool, and sinks, flowing deep below the surface to refill the ocean basins, then rising back towards the surface as it becomes warmer and saltier. This type of water circulation carries heat to different parts of the globe, affecting land masses and, in turn climate and weather patterns – even the weather where you are!