Water holds the most oxygen at 5 degrees Celsius. This is fortunate. During daylight hours, green plants, including algae, can keep pond water near the warmer surface rich with oxygen. Even though the warmer water holds less oxygen, the plants are replacing the oxygen as quickly as it is being used. The cooler water below the surface holds more oxygen even though there are fewer plants.
Plants use oxygen at night instead of producing it.
This means there is less oxygen in the water. Therefore, some surface animals can find pond water a difficult place to breathe. During the cold months, the surface of a pond cools off rapidly and even freezes. Temperatures are now reversed. The coldest water is near the surface of the pond or just under the ice.