Zuiderzee vs Zuider Zee

Zuiderzee and Zuider Zee both are nouns.

Word	Noun	Adjective	Verb	Adverb
Zuiderzee	Yes	No	No	No
Zuider Zee	Yes	No	No	No

As nouns, Zuiderzee and Zuider Zee are synonyms defined as:

Zuiderzee and Zuider Zee: a former inlet of the North Sea in the northern coast of the Netherlands; sealed off from the sea in 1932 by a dam that created the IJsselmeer

Zuiderzee (noun)	Zuider Zee (noun)
a former inlet of the North Sea in the northern coast of the Netherlands; sealed off from the sea in 1932 by a dam that created the IJsselmeer	a former inlet of the North Sea in the northern coast of the Netherlands; sealed off from the sea in 1932 by a dam that created the IJsselmeer