Context: The chemical elements and water that are needed by living things keep recycling on Earth. They pass back and forth through biotic and abiotic components of ecosystems. Thats why their cycles are called biogeochemical cycles. For example, a chemical element or water might move from organisms (bio) to the atmosphere or ocean (geo) and back to organisms again. Elements or water may be held for various periods of time in different parts of a biogeochemical cycle. An exchange pool is part of a cycle that holds a substance for a short period of time. For example, the atmosphere is an exchange pool for water. It usually holds water (as water vapor) for just a few days. A reservoir is part of a cycle that holds a substance for a long period of time. For example, the ocean is a reservoir for water. It may hold water for thousands of years. The rest of this lesson describes three biogeochemical cycles: water cycle, carbon cycle, and nitrogen cycle. Water is an extremely important aspect of every ecosystem. Life cant exist without water. Most organisms contain a large amount of water, and many live in water. Therefore, the water cycle is essential to life on Earth. Water on Earth is billions of years old. However, individual water molecules keep moving through the water cycle. The water cycle is a global cycle. It takes place on, above, and below Earths surface, as shown in Figure 24.7. During the water cycle, water occurs in three different states: gas (water vapor), liquid (water), and solid (ice). Many processes are involved as water changes state to move through the cycle. Watch this video for an excellent visual introduction to the water cycle: . MEDIA Click image to the left or use the URL below. URL: Water changes to a gas by three different processes called evaporation, sublimation, and transpiration. Evaporation takes place when water on Earths surface changes to water vapor. The sun heats the water and gives water molecules enough energy to escape into the atmosphere. Most evaporation occurs from the surface of the ocean. Sublimation takes place when snow and ice on Earths surface change directly to water vapor without first melting to form liquid water. This also happens because of heat from the sun. Transpiration takes place when plants release water vapor through pores in their leaves called stomata. Rising air currents carry water vapor into the atmosphere. As the water vapor rises in the atmosphere, it cools and condenses. Condensation is the process in which water vapor changes to tiny droplets of liquid water. The water droplets may form clouds. If the droplets get big enough, they fall as precipitation. Precipitation is any form of water that falls from the atmosphere. It includes rain, snow, sleet, hail, and freezing rain. Most precipitation falls into the ocean. Eventually, this water evaporates again and repeats the water cycle. Some frozen precipitation becomes part of ice caps and glaciers. These masses of ice can store frozen water for hundreds of years or even longer. Condensation may also form fog or dew. Some living things, like the lizard in Figure 24.8, depend directly on these sources of liquid water. Precipitation that falls on land may flow over the surface of the ground. This water is called runoff. It may eventually flow into a body of water. Some precipitation that falls on land soaks into the ground. This water becomes groundwater. Groundwater may seep out of the ground at a spring or into a body of water such as the ocean. Some groundwater is taken up by plant roots. Some may flow deeper underground to an aquifer. An aquifer is an underground layer of rock that stores water. Water may be stored in an aquifer for thousands of years. The element carbon is the basis of all life on Earth. Biochemical compounds consist of chains of carbon atoms and just a few other elements. Like water, carbon is constantly recycled through the biotic and abiotic factors of ecosystems. The carbon cycle includes carbon in sedimentary rocks and fossil fuels under the ground, the ocean, the atmosphere, and living things 
 Question: __part of a biogeochemical cycle that holds a substance for a short period of time
Output: exchange pool