Changes effected by environmental factors on the biological colonization of an historic marine limestone quay were considered, as a baseline study for the monitoring and test plan definition. Environmental factors play a fundamental role in the deterioration of stone monuments, particularly those located in a marine environment. This eighteenth century quay, situated at the Royal Square of the Portuguese capital, is subject to tidal movement, marine aerosol, high levels of pollution and vibration and is partly submerged in polluted water. Part of the quay was dismantled in 1997 and returned to the river water in 2008; since then, progressive (re)colonization by biological organisms has occurred. By means of periodic photographic registry, from 2010 to 2014, the main alterations and deposits on its surfaces were registered. The present paper discusses and presents a vulnerability assessment and monitoring plan for monuments in a marine environment with a view to establishing a preventive conservation plan.