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.
A researcher is challenged by the null hypothesis and usually wants to disprove it, to demonstrate that there IS a statistically-significant relationship between the two variables in the hypothesis. An alternative hypothesis, simply, is the inverse, or opposite, of the null hypothesis.