In the aquatic biome, the light-penetrated layer is warm, the thermocline experiences rapid temperature change and the deep water is uniformly cold. The rapid temperature change in the thermocline is because it serves as a separator for the uniformly cold deep water and the warm light-penetrated layer.
The temperature of the water in the different layers of the aquatic biome is due to light penetration. In the light-penetrated layer, plenty of light gets through and the energy from this light warms the water. In the thermocline layer, only a small amount of light gets through. In deep waters, light is unable to penetrate to provide warming.