Question

Read the sources given and answer any four from the questions given below:

In temperate cold regions thermal stratification occurs in lakes and deepwater bodies during summer and winter. During summer, the top layer of water becomes warmer than the bottom layer. The warm top layer does not mix with the bottom cold layer of water. As a result, a distinct zone forms between the top and the bottom layer of water called thermocline and the stratification is called summer stratification. Upper, warm circulating water is called epilimnion and the bottom, non-circulating colder water is called hypolimnion. In autumn, the temperature of upper layer lowers and becomes same as of hypolimnion. It is called autumn turnover. During winter, the surface water cools below 4°C, it expands, becomes lighter and freezes. The temperature in the lower layers remains at about 4°C and so water remains in liquid state. This is called winter stratification. In spring, when ice melts, water becomes warmer and heavier and sinks to the bottom. It leads to mixing of water in the whole lake and results in uniform mixing of oxygen and nutrients in the lake. This is called spring turnover. During autumn and spring turnover, there is redistribution and mixing of oxygen and nutrients and this results in the bloom of phytoplankton growth. During summer and winter stratification, the oxygen availability and nutrient supply is greatly reduced and so is phytoplankton growth.

1. Where is thermocline present?
   1. upper layer of water in summers
   2. bottom layer of water in summers
   3. a layer of water between upper and bottom layer of water in autumn
   4. a layer of water between upper and bottom layer of water in summers
2. Which statement is true for summer stratification?
   1. no mixing of warm and cold water
   2. epilimnion is at 4°C
   3. hypolimnion is very warm
   4. thermocline zone forms above epilimnion
3. The phytoplankton bloom occurs due to the
   1. mixing of oxygen and nutrients during summer
   2. mixing of oxygen and nutrients during autumn
   3. presence of thermocline layer
   4. highoxygen content in hypolimnion
4. The lower layers of water in winter are
   1. at less than 4°C
   2. frozen as ice
   3. lighter and remain as liquid water
   4. at 4°C and in liquid state of water
5. Thermocline zone is present in the following season
   1. winter
   2. summer
   3. autumn
   4. both (a) and (b)

Answer 1

1. a layer of water between upper and bottom layer of water in autumn
2. no mixing of warm and cold water
3. mixing of oxygen and nutrients during autumn
4. at 4°C and in liquid state of water
5. summer