The layer of water where temperature drops sharply is called the thermocline.

Here's a breakdown:

* Thermocline: This is a thin, transitional layer of water in lakes and oceans where temperature changes rapidly with depth.

* Above the thermocline: The water is typically warmer due to sunlight absorption.

* Below the thermocline: The water is much colder, as sunlight cannot penetrate to those depths.

The thermocline is an important feature of aquatic ecosystems because it influences the distribution of organisms, nutrient cycles, and water mixing.