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.
Usage Note: A traditional view holds that alternative should be used only when the number of choices involved is exactly two. This reasoning is based on the word's historical relation to Latin alter, the other of two..