The amount of light within the water column is a major factor controlling temperature and photosynthesis. It also affects:
Light varies seasonally and with cloud cover and depth. The deeper light infiltrates a lake, the deeper photosynthesis can occur.
Photosynthesis provides the food that supports a large portion of the food web. Since photosynthesis depends greatly on light, changes in light have the potential to result in biological and chemical impacts.
Water is less dense as a solid than as a liquid. Water is most dense at 4°C. Therefore, ice floats because it is less dense than its liquid form, water.
Because of this unique relationship between density and temperature, many lakes in temperate climates tend to separate into distinct layers (stratify). In the summer, lakes will stratify, creating a cold layer near the bottom with a warmer layer near the surface. In general, the larger the lake the deeper it will stratify.
A pattern evolves relating to the temperature changes in a lake.
In late summer or early fall:
The exact opposite happens in spring after ice-off as the surface water warms to 4°C. A lake can re-stratify in a few days to a couple weeks.
Winterkill is the dying of fish in late winter because of a lack of oxygen at the bottom of lakes (anoxia).
When ice covers a lakes surface, water cannot obtain oxygen through the mixing with air because of the ice cover. Limited sunlight penetration reduces photosynthesis, causing vegetation to die and decompose, which uses up more oxygen. Winterkill occurs when these conditions reduce dissolved oxygen concentrations below threshold levels and the lake is unable to support fish.
Summerkill is the dying of fish in the late summer because of a lack of oxygen at the bottom of lakes (anoxia).
When warmer weather occurs, water cannot obtain oxygen through mixing with air because the lake has stratified. This warmer water is able to hold less oxygen, but encourages the growth of algae. large algae blooms can occur, and as algae blooms die and decompose they consume large amounts of oxygen. Summerkill occurs when these conditions reduce dissolved oxygen concentrations below threshold levels and the lake is unable to support fish.