Meromictic lakes stratify because they have denser (usually saline) water in the bottom and less dense (usually freshwater) nearer the surface.

Why is the inside of the orange denser than water?

Hypoxia is caused when surface waters expand and become lighter in the heat, separating from the colder, denser bottom waters.

One is the formation of sea ice, which releases the salt in seawater as it forms, making underlying waters saltier and, thus, denser.

This concentration of salt contributes to the salinated water's density, and this cold, denser water sinks to the bottom of the ocean.

The interface, called a pycnocline, between the brackish near-surface water and the denser water below supports internal wave motion.

Varying degrees of salinity and temperature act to modify the density of water, and the denser water always lies below the less dense water.

In the winter, inverse stratification occurs as water near the surface cools freezes, while warmer, but denser water remains near the bottom.

When Ekman transport is occurring away from the coast, surface waters moving away are replaced by deeper, colder, and denser water.

It is the densest water in the free ocean, and underlies other bottom and intermediate waters throughout most of the southern hemisphere.