An important characteristic of lakes is the way that the availability of light changes with depth. Light decreases exponentially (as described by Beer’s law) depending on the turbidity of the water. At the compensation depth the light available for Photosynthetic production is just matched by the energy lost in respiration. Above this depth is the euphotic zone, but below it in the aphotic zone phytoplankton—the lowest level in the ecological system of a lake.
Patterns of sediment deposition in lakes depend on the rates of supply in inflowing waters and on subsurface currents and topography. Repetitive sounding of the lake bed may be used to investigate patterns of sedimentation. Remote sensing of the turbidity of the surface waters also has been used to infer rates of sedimentation, as in the artificial Lake Nasser in Egypt. In some parts of the world where erosion rates are high, the operational life of reservoirs may be reduced dramatically by infilling with sediment.
The water balance of a lake may be evaluated by considering an extended form of the catchment water balance equation outlined above with additional terms for any natural or artificial inflows. An energy balance equation may be defined in a similar way, including terms for the exchange of long-wave and shortwave radiation with the Sun and atmosphere and for the transport of sensible and latent heat associated with convection and evaporation. Heat also is gained and lost with any inflows and discharges from the lake. The energy balance equation controls the thermal regime of the lake and consequently has an important effect on the ecology of the lake.
An important role in controlling the distribution of Temprature in a lake is played by currents due to either the action of the wind blowing across the surface of the lake or the effect of the inflows and outflows, especially where, for example, a lake receives the cooling water from a power-generation plant. In large lakes, Earth's rotation has an important effect on the flow of water within the lake. The action of the wind can also result in the formation of waves and, when surface water is blown toward a shore, in an accumulation of water that causes a rise in water level called wind setup. In lake Erie in North America, increases in water level of more than one metre have been observed following severe storms. After a storm the water raised in this way causes a seiche to travel across the lake and back.
