Why can high sills cause oxygen deficiency in some gulfs?

A high sill—a submarine ridge or rise located at the mouth of a gulf—hampers the exchange of water between the gulf and the adjacent ocean or sea. By restricting this circulation, the sill can lead to stagnation and oxygen deficiency within the gulf's deeper waters.1

In such environments, the deep water may become trapped in a basin behind the sill, remaining largely undisturbed by the surface water flow. Because the sill prevents the regular replenishment of oxygenated water from the open ocean, the water can become stagnant, a condition observed in various fjords, the Red Sea, and the Black Sea. While local winds and river runoff can generate independent circulation, they are often insufficient to prevent these oxygen-depleted conditions when the gulf is highly isolated by a high sill.21