Zostera marina vs aquatic plant

Zostera marina and aquatic plant both are nouns.

Word	Noun	Adjective	Verb	Adverb
Zostera marina	Yes	No	No	No
aquatic plant	Yes	No	No	No

As nouns, aquatic plant is a hypernym of Zostera marina; that is, aquatic plant is a word with a broader meaning than Zostera marina:

Zostera marina: submerged marine plant with very long narrow leaves found in abundance along North Atlantic coasts
aquatic plant: a plant that grows partly or wholly in water whether rooted in the mud, as a lotus, or floating without anchorage, as the water hyacinth

Other hypernyms of Zostera marina include hydrophyte, hydrophytic plant, water plant.

Zostera marina (noun)	aquatic plant (noun)
submerged marine plant with very long narrow leaves found in abundance along North Atlantic coasts	a plant that grows partly or wholly in water whether rooted in the mud, as a lotus, or floating without anchorage, as the water hyacinth