Thermocline

 

The thermocline plays an important part in the behaviour of the pond habitat. The thermocline is an area of water within the water column in which the warmer upper waters are prevented from mixing with those at a deeper level. This barrier prevents the interchange of nutrients between the two and so in a way produces two separate environments for pond creatures.

We suggest a visit to the epilimnion and hypolimnion pages at this time. These are the two area's within the water column that converge upon the thermocline and are unable to mix because of its presence.

As has been stated earlier in these pages, wind action causes water within the pond to circulate. This effect is not total within the whole water column and the zone of separation is termed the thermocline. This prevents the warmer upper layers from mixing with those beyond it, in effect trapping the nutrients below. When plankton or creatures living within the epilimnion die, there bodies sink through the thermocline and into the hypolimnion below. The nutrients produced, due to their bacterial decay, are unable to circulate beyond the thermocline and so that resource becomes trapped as long as the thermocline exists.

This is all very well, but the thermocline is not a constant presence, and as water temperatures change the ability to mix increases as the winter approaches. The thermocline disappears at this point, allowing these lower waters to once again mix with those of the epilimnion, releasing all this trapped fertility back into the system.

The presence and depth at which a thermocline is likely to occur very much depends on the ability of the circulation within the pond to allow mixing. It is normally accepted that thermoclines only occur in deeper waters, but I do not believe that this is necessarily the case. It's ability to occur is dependent on water mixing, and therefore would be able to occur in waters in which wind action and circulation are not excessive.