Without aquatic plankton, there would be few living organisms in the world, and certainly no humans. A variety of freshwater fish also occupy this zone. This zone is much colder and denser than the other two. Little light penetrates all the way through the limnetic zone into the profundal zone. The fauna are heterotrophs, meaning that they eat dead organisms and use oxygen for cellular respiration.
Temperature varies in ponds and lakes seasonally.
In between the two layers, there is a narrow zone called the thermocline where the temperature of the water changes rapidly. This mixing also circulates oxygen throughout the lake. These are bodies of flowing water moving in one direction. The characteristics of a river or stream change during the journey from the source to the mouth. The temperature is cooler at the source than it is at the mouth.