The circulation pattern begins to change. It takes more and more wind energy to mix the reservoir from top to bottom because of differences in water temperature and density. At some point in early summer, the reservoir stratifies with warmer and lighter, less dense water, on the surface, and heavier, colder, denser water on the bottom. Biologists call the upper, mixing layer the epilimnion and the deep, non-mixing bottom layer the hypolimnion. The warm, epilimnion is also where most production in a reservoir takes place.
Rivers feed the reservoir a constant supply of raw nutrients.
The sun provides the energy to drive the photosynthetic process, where microscopic plants convert nutrients to living carbon. Small invertebrates, zooplankton and insects, feed on the plants, which in turn are fed on by larger animals. This process is the basis of all life. In the upper productive zone, this wondrous life process continues through the year. As new plants and animals are produced, old ones die, sink to the bottom, where they decompose, releasing nutrients back to the water.