The Limnology is concerned with both natural and man-made lakes, their physical characteristics, ecology, chemical characteristics, internal energy fluxes, and exchanges with the environment. It often includes the ecology and biogeochemistry of flowing freshwaters. The study of former lakes is known as paleolimnology. It involves inferring the history of a former lake basin on the basis of the evidence contained in the sediments of the lake bed.
Lakes may be formed as a result of tectonic activity, glacial activity, volcanism, and by solution of the underlying rock. Man-made lakes or reservoirs may result from the building of a dam within a natural catchment area or as a complete artificial impoundment. In the former case the reservoir may be filled by natural flow from upstream; in the latter the supply of water must be piped or pumped from a surface or subsurface source. The use of reservoir water for water supply, river regulation, or hydroelectric power generation may cause rapid changes in water levels that would not normally occur in a natural lake. In addition, water is usually drawn from a reservoir at some depth, resulting in a shorter residence time relative to an equivalent natural lake.
The most important physical characteristic of the majority of lakes is their pattern of temperatures, in particular the changes of temperature with depth. The vertical profile of temperature may be measured using arrays of temperature probes deployed either from a boat or from a stationary platform. Remote-sensing techniques are being used increasingly to observe patterns of temperature in space and, in particular, to identify the thermal plumes associated with thermal pollution.
In summer the water of many lakes becomes stratified into a warmer upper layer, called the epilimnion, and a cooler lower layer, called the hypolimnion. The stratification plays a major role in the movement of nutrients and dissolved oxygen and has an important control effect on lake ecology.
Between the layers there usually exists a zone of very rapid temperature change known as the thermocline. When the lake begins to cool at the end of summer, the cooler surface water tends to sink because it has greater density. Eventually this results in an overturn of the stratification and a mixing of the layers. Temperature change with depth is generally much smaller in winter. Some lakes, called dimictic lakes, can also exhibit a spring overturn following the melting of ice cover, since water has a maximum density at 4 °C.
