In a stratified pond the cooler, denser water on the bottom does not mix with the surface water where the oxygen is. It is also too dark to allow photosynthesis. Most of the oxygen-consuming organics have settled to the bottom. A short time after stratification forms, the cooler water below the thermocline becomes devoid of oxygen. To make things worse, the loss of oxygen starts a series of detrimental events: carbon dioxide builds up, making the water more acidic. The lack of oxygen changes the decomposers from aerobic to anaerobic. The resulting byproducts are black muck and hydrogen sulfide, which is toxic to fish and smells like rotten eggs. Anaerobic conditions on the bottom also cause the release of phosphorus from the sediments, feeding more algae in the surface waters.