Molles equates lakes with small seas, a viewpoint I don't share. Lakes differ radically from seas, most notably in their isolation and oxygenation. Lakes are a surprisingly rare phenomenon in nature. Most were created by glacial activity. Some by major tectonic events.
Reservoirs are man-made lakes.
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.