Reservoir ecology is a bit different from that in lakes, e.g., outflow in reservoirs is usually from surface, while in a lake outflow is from a greater range of depths. Many of the most famous ecologists of the early and mid-20 th century were limnologists. A handful of big lakes contain most of the freshwater on the planet. 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.