Because salinity (in concert with temperature) affects the density of seawater, it can play a role in its vertical stratification. Essentially, lower salinity water (= lower density) floats on top of higher salinity water (= higher density).
Alternative definition, a choice limited to one of two or more possibilities, as of things, propositions, or courses of action, the selection of which precludes any other possibility: You have the alternative of riding or walking. See more.
This page in a nutshell: Alternative text allows the content and function of an image to be understood by text-only readers. Alternative text is text associated with an image that serves the same purpose and conveys the same essential information as the image.