What is the primary succession in freshwater called?

Which of the following can be identified as secondary consumers?

Match the following columns:

Column-$I$	Column-$II$
$(a)$ Earthworm	$(i)$ Pioneer species
$(b)$ Succession	$(ii)$ Detritivores
$(c)$ Ecosystem service	$(iii)$ Birth rate
$(d)$ Population growth	$(iv)$ Pollination

Consider the following statements about food chain:
$I.$ The transfer of energy from producers to top consumers through a series of organisms is called a food chain.
$II.$ $A$ food chain is always straight and proceeds in a progressive straight line.
$III.$ In a food chain,there is a unidirectional flow of energy from the sun to producers and subsequently to a series of different types of consumers.
Which of the statements given above are correct?

Which of the following are not primary producers in an aquatic ecosystem?

Distinguish between:
$(a)$ Grazing food chain and detritus food chain
$(b)$ Production and decomposition