Question

how do the aquatic plants fulfil the need of nitrogen?

Answer 1

Aquatic plants take nitrogen in the form of ammonium ions, nitrates or nitrites from water or by symbiotic association with nitrogen fixing organisms.
Nitrogen enters water by rain or from atmosphere. Water fern Azolla has Anabaena in its leaf which fixes nitrogen for the plant. It is a symbiotic association. Anabaena fixes nitrogen in its heterocysts (cells having enzyme nitogenase).