1. Isthmian: This refers specifically to a narrow strip of land connecting two larger landmasses, the classic definition of a land bridge.
2. Continental: This emphasizes the connection between continents that a land bridge provides.
3. Ephemeral: This describes a land bridge that exists for a limited time, highlighting its temporary nature.
You could also use adjectives like narrow, fragile, historic, geological, or ancient, depending on the specific context.