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A water column is a hypothetical section of water, from the surface all the way down to the bottom, including muds and sediments. This concept is heavily used in the environmental sciences, where people may be concerned with topics like which organisms live at various depths of the water, as well as pollution and characteristics seen at different depths. People can use a variety of tools to study aquatic environments, including observations, sampling, and experiments to learn more in the field, as well as in laboratory settings where more controls are available.
Conditions in the water column change, depending on the depth. Towards the top, more light is available. Organisms that need light to survive can be found in the upper reaches and may seek out specific zones in search of prey or optimal living conditions. Lower down, it is darker and colder. There is also more pressure, created by the weight of all the water above. Organisms adapted for survival in the lower regions may be resistant to pressure, or have bodies that can easily compress and expand. These include burrowing organisms living in sediment at the very bottom.
Characteristics of the water column can change in response to climate conditions, like storms leading to muddy runoff and corresponding darkness and debris distributed through the water, as well as winds whipping up the surface of the water and disrupting oxygen levels. Concentrations of dissolved oxygen, nutrients, and other compounds may shift in different areas and at various heights within the water column. People can take water samples and lower sampling probes to take a series of readings as the probes fall through the water.
Problems with the water column can include high nutrient levels leading to a proliferation of invasive organisms, pollution causing death of delicate animals and plants, or turbidity, impairing visibility and making it harder for some animals to survive. Oceanographers study the water column extensively in their work, as do people interested in inland science, like freshwater biologists.
Textbooks on water and bodies of water often include an overview of the water column with information about what lives at various depths and what to expect depending on the overall depth of the water column. It may be stratified into layers reflecting different zones where organisms can survive. Divers also study this information so they can dive safely and determine the depths where the most interesting sights can be found.