Generally, whether in lakes, rivers, or ground water, the earth's freshwater is unevenly distributed, leading to wars and major political battles. Texas, e.g., has the greatest surface area of water of any of the States, but all its lakes are artificial. None are natural. The water is temperature-stratified lakes does not mix because it takes too much energy. When the lake is stratified, the warm surface water is more or less isolated from cold, deep water.
Mixing patterns are very important in lake ecology.
In many temperate lakes, hypolimion often about 4 C in both winter and summer. Varies with depth, season, time of day, productivity. Low oxygen often results in fish kills. Biota Oligotrophic lakes tend to have higher diversity of phytoplankton, fish that require high oxygen. Little is known of tropical lakes, because most limnologists live in temperate zones.