Some lakes have water quality problems related to the layering of the lake’s waters, which occurs in the summer months. During the summer, the warm surface layer of water does not mix with the colder bottom layer of water. If a lot of algae has grown in the lake, decay of the dead algae on the lake bottom may use up all of the oxygen in the cold bottom water layer. In the fall, the surface water cools and mixes with the bottom water, resupplying the bottom water with oxygen needed for life. Learn how stratification of lake waters influences water quality and phosphorus affects oxygen levels in lakes.