The Spelling of the Spanish language indicates that the nouns and adjectives that are part of the names of antonomastic nature (appellatives) that, as a stylistic alternative, have some toponyms, are written with an initial capital letter.

This rule also includes capitalization and therefore “Main Port” must be capitalized; also the name “Pearl of the Pacific”, another nickname used for the place name Guayaquil.

Articles and prepositions can be used in your writing; but these elements should be in lower case unless they appear at the beginning of the statement. In addition, they must agree with the grammatical gender of the expression they precede or determine. Examples: The Main Port… The Pearl of the Pacific… I visited the Main Port. In the Pearl of the Pacific there are many tourist places.

These components are lowercase when functioning as common or generic nouns; Consequently, they do not replace the proper name, but are used to emphasize the importance of the city or to emphasize the characteristic that most identifies the city. Examples: Guayaquil is Ecuador’s main port. The city of the river and estuary is the brightest pearl of the Pacific Ocean. In this last example, “Pacific” is capitalized because it is the name of an ocean.

Let us not forget that expressions of an appellative or antonomastic nature are capitalized; but they are in lowercase when in common use. (F)

SOURCES:

Spanish dictionary And Spelling of the Spanish language (electronic versions), from the Royal Spanish Academy and the Association of Academies of the Spanish Language.