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.
17. An alternative obligation is, where a person engages to do, or to give several things in such a manner that the payment of one will acquit him of all; as if A agrees to give B, upon a sufficient consideration, a horse, or one hundred dollars. Poth. Obl.