Thermal stratification in some New Zealand lakes

[First paragraph(s)...]
The effect of thermal stratification is so great that it controls the structure on which the biological framework rests. Once a lake becomes thermally stratified the upper stratum, the Epilimnion, and the lower stratum, the Hypolimnion, form two distinct entities separated by the Thermocline. The vertical temperature variation within the upper and lower strata is small, but the difference be- tween the two may be great. The thermocline provides the transition zone, and here the fall in temperature may be several ¡C per metre. A drop of 1¡C per metre is considered by Birge to produce sufficient differential water density to segregate the epilimnion from the hypolimnion.
Once .a thermocline becomes stabilized it may last several months. During this period the utilisation of the nutrients in the trophic zone of the epilimnion alters the chemical composition of the upper stratum, whilst metabolic processes in the hypolimnion reduce the oxygen in the lower stratum. Thermal stratification therefore implies chemical stratification to a greater or lesser degree depending on the depth of a lake, and the standard of its productivity.