What role do small crustaceans forming plankton play in fresh water bodies and seas?

1) They are a link in the power circuit,
2) provide purification of reservoirs.
3) The formation of minerals.

Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.