Q.Explain van't Hoff's rule and describe cyclomorphosis in Daphnia as an example of temperature-driven seasonal change.

Summer stratification is the layering of a lake in summer into a warm upper epilimnion, a middle thermocline (metalimnion) of sharp temperature change, and a cold lower hypolimnion; the layers do not mix.

Explanation:

• In deep lakes of temperate regions, during SUMMER the surface water gets heated by the sun while the deeper water remains cold. Because warm water is lighter (less dense) and cold water is heavier (denser), the water separates into distinct layers that do not mix. This layering is called SUMMER STRATIFICATION.

The three layers formed (from top to bottom) are:

1. Epilimnion:

• The upper layer of WARM, light, well-mixed and oxygen-rich water, exposed to sunlight and wind.

2. Thermocline (Metalimnion):

• The MIDDLE transition layer in which the temperature falls RAPIDLY with depth (a sharp temperature gradient). It acts as a barrier separating the upper and lower layers.

3. Hypolimnion:

• The bottom layer of COLD, dense, still water that is poorly lit and often low in oxygen.

Consequence: …

Summer stratification is the thermal layering that develops in deep temperate lakes during summer, dividing the water column into three distinct zones that do not mix.

What is summer stratification?

In many lakes/ponds located in temperate regions, during summer the surface water is warmed by sunlight while the deeper water stays cool. Since warm water is less dense than cold water, the lake becomes stratified (layered) into three zones, from top to bottom:

1. Epilimnion — the upper, warm, less dense layer, well-mixed by wind and rich in dissolved oxygen (due to sunlight-driven photosynthesis and atmospheric exchange).
2. Thermocline (metalimnion) — a middle transition zone in which temperature drops sharply with depth over a short vertical distance, acting as a barrier that prevents mixing between the layers above and below.
3. Hypolimnion — the lower, cold, denser layer, which is cut off from atmospheric oxygen and sunlight; oxygen here is progressively depleted by the decomposition of organic matter settling from above, and it can become low in dissolved oxygen and nutrient-rich by the end of the season. …