Given below are two statements: one is labelled as Assertion (A) and the other as Reason (R):
Assertion (A): Marl deposits are common in lakes/ponds having high amounts of Ca2+ and excessive algal growth.
Reason (R): Removal of carbon dioxide through algal action tends to cause a shift in the form of alkalinity from bicarbonate to carbonate and from carbonate to hydroxide.
Choose the correct answer from the options given below:

Correct option is A

1. Assertion (A):
· Marl deposits are primarily composed of calcium carbonate (CaCO3) and are common in water bodies rich in calcium ions (Ca2+) and subject to photosynthetic activity.
· Algal growth enhances photosynthesis, which removes CO2 from the water. This leads to precipitation of CaCO3​, forming marl deposits.
· Thus, Assertion (A) is correct.
2. Reason (R):
· The removal of CO2​ by algae during photosynthesis reduces the concentration of bicarbonate (HCO3−​) ions and shifts the equilibrium toward the formation of carbonate (CO32−​) and hydroxide (OH−).
· This process increases pH and promotes the precipitation of calcium carbonate.
· Therefore, Reason (R) is also correct.
3. Relation Between (A) and (R):
· The shift in alkalinity caused by CO2 removal (Reason) directly explains the formation of marl deposits in lakes/ponds with high calcium levels and algal growth (Assertion).
Hence, both the assertion and reason are correct, and the reason explains the assertion.