Turnover

Area ponds have been frozen for the last several weeks due to the extremely cold temperatures we have experienced.  But, thawing started last weekend which initiates turnover, the mixing of pond water as surface  temperatures change with the progression of the seasons.  This process happens several times each year, and has profound effects on the insects, amphibians, fish and other wildlife that live in these waters year-round.

Water can exist as a gas, a liquid, or a solid depending on its temperature.  Water density and its capacity for carrying oxygen also changes with temperature.   Like most other liquids and gases, cold water is heavier, denser and can hold more oxygen than warm water.  However, water has a unique property in that it reaches maximum density at 39˚F (4˚C).  As water gets colder, it expands, becoming lighter weight and less dense.  At 32˚F (0˚C), water freezes into a solid, crystalline structure called ice, which floats on top of the denser water below.  This property makes it possible for wildlife to survive below the ice.

During summer, deeper bodies of water will have layers differentiated by temperature.  The top layer is warmed by the sun and atmosphere and is the warmest, with layers getting progressively cooler as the pond gets deeper.  Plant matter that falls in the pond will start the process of decomposition, using light and oxygen.  As decomposing material sinks to the bottom where there is less light, this process can rapidly use all of the oxygen in the lower layers.  Most animal life is found in the upper layers at this time of year.

The top layer of water is constantly picking up oxygen from wind and wave action.  As atmospheric temperatures start to drop in autumn, the top layer becomes colder, denser and able to carry more oxygen.  As winter progresses and temperatures continue to drop, the denser, heavier water accumulates on the bottom of the pond.  Aquatic species will move to this highly-oxygenated layer to spend the winter.  In spring, as ice melts, the top layers of the pond pick up more oxygen from wind and wave action.  As water temperatures at the surface start to climb, approaching 39˚F (4˚C), the layering effect may disappear for a short period as the entire pond mixes to a uniform temperature and density.  Wildlife will start to move toward surface areas, finding well-oxygenated water everywhere in the pond.    Layering will return as top layers warm more rapidly than underlying water.

In Dupage County, Northern Leopard Frogs, Lithobates pipiens, American Bullfrogs, Lithobates catesbeianus, and adult Painted Turtles, Chrysemys picta, all spend the winter underwater.  Painted Turtles burrow into the mud at the bottom.  They slow their metabolism down to where they only need a small amount of oxygen each day.  The blood vessels in their butts are very close to the skin, and they are able to absorb oxygen from the water directly into their bloodstream.  They can remain in this state for 3-4 months.  Northern Leopard Frogs and American Bullfrogs also slow down their metabolisms and have a similar ability to absorb oxygen through the skin into the bloodstream.  They rest on the bottom, not immersing themselves in the mud because this would prevent the oxygen rich water from being in contact with their skin.

There are many factors that affect turnover and the availability of oxygen-rich water for the animals that overwinter under the ice.  A mild winter may not freeze over the ponds, leaving the water well mixed and oxygenated throughout the season.  Topography of the land surrounding a pond is very important.  Open ponds that receive a lot of sunlight tend to turn over rapidly and often, and will have a shorter period under ice with lots of oxygen-rich water available.  Ponds closed in by land or forest canopy will experience much less wind and wave action resulting in little turnover and smaller amounts of oxygen in the water.  As spring starts to arrive, be on the lookout for changing conditions as wildlife starts to emerge.