If you have ever swam in a pond/lake and felt the warm surface water yet your feet were reaching cold water, that area of different temperature is the thermocline.
The desity of water is dependent upon water temperature. Ponds and lakes may stratify thermally, beacuse heat is absorbed more rapidly near the surface of a water body and the warm upper waters are less dense than cool lower waters. Stratification occurs when differences in desity of upper and lower strata become so great thaty the two strata cannot be mixed by wind.
To summarize, at the spring thaw, or at the end of winter in a lake or pond without ice cover, the water column has a relatively uniform temperature. Although heat is absorbed at the surface on sunny days, there is little resistance to mixing by wind and the entire volume of the water circulates and warms.
As spring progresses, the upper stratum heats more rapidly than heat is distributed from the upper stratum to the lower stratum by mixing. Finally, waters of the upper stratum become considerably warmer that those of the lower stratum.
In lakes, a thermocline is defined as a layer across which the temperature drops at a rate of at least 1 deg. C/m of depth. The depth of the thermocline below the surface may fluctuate depending upon weather conditions, but most large lakes do not destratify until autumn.
The reason the fish do not pass the thermocline is that the denser colder water contains less oxygen than the upper waters. It does affect the bass too.
I think someone on this site mentioned a cheap way of checking the temperature other than the fancy products on the market.
Maybe someone will remember that.
