Question

The second trophic level in a lake is
a. Phytoplankton
b. Zooplankton
c. Benthos
d. Fishes

Answer 1

Hint: Food chain is a sequence of living organisms which involves transfer of food energy from producers, through a series of organisms with repeated eating and being eaten is called food chain. Each level or step in a food chain where transfer of energy takes place is called trophic level.

Complete answer:
All living organisms i.e., plants, animals and microorganisms that are present in the environment constitute the biotic components of the ecosystem. On the basis of their role in the ecosystem, these can be classified into three main groups:
(1) Producers : They are green photosynthetic plants that entrap solar energy through chlorophyll to synthesise organic food from inorganic raw materials. So, they are called autotrophs (self-nourishing).
In the aquatic ecosystem, chief producers are phytoplankton, algae and the floating, submerged and marginal plants found at the edges.
(ii) Consumers : They are the animals that are not capable of synthesizing the food materials. They are dependent on producers directly or indirectly for their survival. Thus, they are called heterotrophs.
Consumers are of following types:
Primary Consumers (PC) or First Order Consumers : These animals directly feed on producers. They are also called herbivores or key industry animals. (Convert plant matter into animal matter)
Aquatic ecosystem : Common herbivores are molluscs, tadpole and mosquito larvae.
Secondary Consumers (SC) or Second Order Consumers or Primary Carnivores : They are animals which feed on herbivores.
Aquatic ecosystem : Hydra, frog , and zooplanktons.
Tertiary Consumers (TC) or Third Order Consumers or Secondary Carnivores : Carnivores
which feed upon secondary consumers e.g., large fishes (aquatic ecosystem), snake (terrestrial ecosystem). There may be quaternary or fourth order consumers which prey upon secondary carnivores.
Now let us match this with given options :-
Phytoplankton :- they are the chief producers, that is they prepare food.
Zooplankton :- they are secondary consumers in aquatic food chains which feed on molluscs.
Benthos :- they are top level carnivores and are kind of fishes.
Fishes :- these are tertiary consumers.

Our required answer is b) Zooplankton.

Note: The energy transfer takes place from one trophic level to another and only 10% energy is transferred . The law was proposed by Lindeman in 1942. The transfer of energy from one trophic level to another trophic level is accompanied by loss of energy at each level or step. When the plants are eaten by herbivores, about 10% of energy in the food is fixed into animal flesh while 90% is consumed in ingestion, respiration, maintenance of body heat and other activities. Similarly, when a carnivore consumes that herbivore, again about 10% of energy is fixed. Therefore, at each transfer only 10% of the total energy is actually available to the next trophic level. It is called 10% law.