Britannica Sources used in this answer

Why can deep water in a stratified lake become low in dissolved oxygen during summer?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

lake Lake Ann in North Cascades National Park, Washington, U.S., viewed from the park's Maple Loop Trail. The North Cascades National Park is a large wilderness area that preserves majestic mountain scenery, snowfields, glaciers, and other unique natural features.

In a stratified lake during summer, the water column separates into two distinct layers: the warmer, well-mixed upper layer known as the epilimnion and the colder, deeper layer called the hypolimnion.12

This thermal stratification acts as a physical barrier, preventing the two layers from mixing. While the epilimnion remains in contact with the atmosphere and can be replenished with oxygen through wind-driven mixing and photosynthesis, the hypolimnion is effectively isolated.134

Deep water becomes low in dissolved oxygen because organic matter—such as dead plankton—sinks from the upper layer into the hypolimnion. Bacteria and other microorganisms decompose this organic material, a process that consumes oxygen. Because the stratification prevents the exchange of gases with the atmosphere, the oxygen consumed by this decay cannot be replaced, leading to a depletion of dissolved oxygen in the deeper waters.345