CBSE Class 10 Science 2022 · Paper 31/2/1 · Question 13

(a) Ponds and lakes are natural, self-sustaining ecosystems: their decomposers (bacteria and fungi) break down dead plants and animals and other wastes into simple substances, which are recycled into the water and soil. An aquarium is an artificial ecosystem that does not have enough decomposers (or a complete food web), so uneaten food, excreta and dead matter pile up in a small volume of water and pollute it; it must therefore be cleaned regularly. (b) The ozone layer at the higher levels of the atmosphere is being depleted chiefly by synthetic chemicals such as chlorofluorocarbons (CFCs), used as refrigerants and in fire extinguishers. Released CFCs reach the upper atmosphere, where UV radiation breaks them down and the chlorine they release destroys ozone. Harmful effect: more ultraviolet radiation reaches the Earth's surface, which can cause skin cancer in human beings.