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.
explanation, account - a statement that makes something comprehensible by describing the relevant structure or operation or circumstances etc.; the explanation was very simple; I expected a brief account. explanandum, explicandum - (logic) a statement of something (a fact or thing or expression) to be explained.