At low latitudes, the is a zone of rapid temperature change over a relatively short depth. it is associated with a similar depth zone marked by a change in density, called the

"At low latitudes the thermocline is a zone of frequent temperature change in a relatively short depth, it is associated with a similar depth zone identified by a change in density called pycnocline''.  

Explanation:

The thermocline is known as ''metalimnion'' in lakes or referred to as the ''thermal layer''. In water temperature of the thermocline changes immediately with the depth. The pycnocline is present in between the mixed layer and deep layer where the density of the water changes frequently due to the changes in the temperature or due to salinity of  the water.