The layer of ocean water between the depth zone of 300 m- 1000 m characterized by sharp change of temperature in the vertical section of sea water is called:

Thermocline, oceanic water layer in which water temperature decreases rapidly with increasing depth. A widespread permanent thermocline exists beneath the relatively warm, well-mixed surface layer, from depths of about 200 m (660 feet) to about 1,000 m (3,000 feet), in which interval temperatures diminish steadily.