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.
Opposite, contrary, reverse imply that two things differ from each other in such a way as to indicate a definite kind of relationship. Opposite suggests symmetrical antithesis in position, action, or character: opposite ends of a pole, sides of a road, views. Contrary sometimes adds to opposite the idea of conflict or antagonism: contrary statements, beliefs. Reverse suggests something that faces or moves in the opposite direction: the reverse side of a coin; a reverse gear.