Hypolimnion

 

Whether a true pond contains a hypolimnion is a point of conjecture, as is the very definition as to whether a water is a pond or a lake. But we include a description so that the individual can determine for themselves if one is likely to be found within their water.

The cold lower layers of a body of water are called the hypolimnion. They are prevented from warming by the thermocline. Life is scarce within this zone as food for most creatures is in short supply, and the reduced temperatures and light penetration prevent plants from growing. Dying plankton from the epilimnion falls through the hypolimnion where the nutrients released by bacterial activity become trapped. This slowly reduces the available resources in the epilimnion until winter when the waters once again mix. This bacterial activity can consume the available oxygen within the hypolimnion thereby adding to the hostility of the zone for colonisation. Chemical analysis has shown that the waters within the hypolimnion are well mixed. How mixing occurs is open to debate, but it is restricted to this zone, confining the available resources of the water throughout the summer.

These conditions are normally recognised as only being found in waters that are deep, normally beyond 15 metres (48 feet) deep. Also temperatures at this depth would be below 8°C., dropping to a maximum of 4°C. This is not to say that similar conditions might not be created within a pond of less depth if certain criteria were met with regards to circulation and temperature.