Lakes
Lakes (see Figures 1 through 4) are usually defined as large bodies of standing fresh water but there are no clear distinctions in size between lakes and ponds, especially since some of the smallest bodies are ephemeral. In the UK, the minimum area for water bodies to be included on maps of 1:250,000 scale is approximately 4 hectares.
Rain is fundamental in providing water to form lakes through runoff within catchments, runoff being the net result of rainfall less evaporation which in turn is influenced by winds and solar radiation. Catchment size, topography, geology, soils, vegetation and land use are also influential factors. Lakes with stable water levels tend to lie in porous catchments where much of the inflow comes from groundwater storage. Lakes on impervious catchments respond more quickly to rain events and generally experience a greater range and frequency in water level.
The ecological status of lakes is partially characterized by lake level stability, i.e. lakes with a narrow range will have a greater proportion of silty substrates in the shoreline margins than those with pronounced changes which then generally have stone/gravel shore areas. These contrasts affect plant and animal communities.
Solar radiation and thermal conductivity at a lake surface are the main sources of heat exchange governing water temperature. Thermal stratification is an important physical feature in lakes deep enough to support it. Summer heating and wind mixing of surface water creates a relatively buoyant warm layer (the epilimnion) which is effectively cut off from the cooler, lower water (the hypolimnion) by the thermocline. This thermal/hydraulic structure is important ecologically in that the epilimnion zone promotes biological activity in contrast to the cooler, darker and possibly poorly oxygenated hypolimnion.
Mixing of waters in lakes is driven largely by wind stress at the surface, with the creation of waves, circulation patterns and oscillations (seiches) being dependent on wind speed, direction and duration, and the morphometric characteristics of the lake (shape, fetch and bathymetry). Currents and turbulence thus created also influence the chemical and biological status. In shallow waters, strong wind-induced turbulence which reaches the bed will increase turbidity and release and mix nutrients from the sediments; increased turbidity reduces light penetration and affects photosynthesis while released nutrients may promote productivity.
The rate of water flow through lakes thus clearly affects the hydrological dynamics and also their chemical and biological status. Throughflow is therefore a useful parameter in lake classification; it can range from only a few hours during floods to several days for years in large waters.[ref]
Figure 1. Lake Saiful-Mulik, Pakistan (Source:Artsonearth.com)
Figure 2. Lake Titicaca in Bolivia and Peru (Source: Artsonearth.com)
Figure 3. Laguna Colorada, Near the Border Chile (Source:Artsonearth.com)
Figure 4. Plitvice Lakes, Croatia (Source: Smashinglist.com)
References
T.Bailey-Watts, A.Lyle, R. Batterbee, R. Harriman and Biggs, J. Lakes and ponds. In: The hydrology of the UK: A study of change (Ed. M Acreman). Routledge. 2000. ISBN 9780415187619
Further Reading
3. National Lakes Assessment (U.S. EPA)
4. Assimilative Capacity of Rivers, Lakes and Estuaries (Powerpoint Presentation)
