What is a thermocline, and where can they be found? How does this affect the ocean productivity and biodiversity?

Thermoclines are typically found in bodies of water such as lakes or rivers that serve as a "transition layer" between the mixed layer of the upper part of the ocean, the epipelagic zone, and the colder layer of the ocean located at the bottom, the mesopelagic zone. It is important to the organisms living in the ocean.