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.
An alternative hypothesis simply is the inverse, or opposite, of the null hypothesis. So, if we continue with the above example, the alternative hypothesis would be that there IS indeed a statistically-significant relationship between what type of water the flower plant is fed and growth.