Final answer:
The pycnocline is a layer in the ocean where there is a rapid change in density due to variations in temperature and salinity.
Step-by-step explanation:
The pycnocline is a layer within the ocean characterized by a rapid change in density with depth. This change in density can be attributed to variations in both temperature and salinity, with temperature generally decreasing as depth increases, which contributes to a high-nutrient environment in deeper oceanic regions such as the benthic realm. Organisms such as fungi, sponges, and sea stars thrive in this nutrient-rich part of the ocean, contributing to the diverse marine ecosystem.