Lakes
-most lakes of the world are of glacial origin, therefore mostly located in glaciated landscapes
Types
Tectonic - formed by movement of earth’s crust
Pothole or Kettle - Formed when ice left from retreating glacier is buried in till (solid material deposited by glacier) and then melts
Glacial – formed by glacial ice scour, cirque lakes, morainal dams, and other
Landslide – formed by movement of earth dams a stream or river
Volcanic – formed by volcanic explosion which causes a hole that is filled with water.
Dissolution – caused by dissolution of Limestone
Oxbow – caused by floodplain River bend pinches off, leaves lake behind; tends to have areas of seasonal inundation
Light and Heating of Water
-Photosynthesis is driven by light
-Water is heated by light
-Light attenuates with depth
-Hence Ps is greatest and water temperatures warmest in surface layers
-water behaves in funny ways. As it gets warmer it becomes a gas. As it gets colder it becomes more dense. It gets less dense below 4 and then becomes ice. If this was not true, lakes would freeze from the bottom up. Because it starts to get lighter below 4, it rises to the top and freezes on the top of the water. This is because of the properties of water and its chemical bonding.
-mixing force mostly comes from wind on the surface as it cools the water. 4 degree water stays in the lower layer
Thermal Stratification
-Lakes stratify vertically into layers of roughly the same temperature
-Lake with same temperature from top to bottom is called isothermal
-A common pattern during summer is to develop three layers
-Cause is differences in water density due to temperature or salinity
-Stratification alters biogeochemistry and ecology
Depth of Thermocline is determined by clarity, depth, latitude, wind, warmth of sun, shape of the basin.
Lake Typology – Stratification
-Monomictic- mixes once a year
-Amictic- never mixes (e.g. saline lakes)
-Polymictic- mixes several times a year
-Dimictic- mixes twice a year
Lake Productivity
-Primary production (algal photosynthesis) controls productivity in most lakes
-Ps depends on light, temperature, nutrients, as well as water movements and other chemical factors
-These limiting factors vary with season, lake depth, lake type, and surrounding landscape