Final answer:
An increase in the salinity of seawater results in an increase in its density. This is because more dissolved salts add to the mass of the water without substantially increasing its volume.
Step-by-step explanation:
When the salinity of seawater increases, the density of the water also increases. Salinity refers to the amount of salts (principally, but not solely, sodium chloride) dissolved in water, and greater amounts of these dissolved salts add to the mass of the water without significantly increasing its volume, thus resulting in a higher density. Therefore, when seawater becomes more saline, its density goes up, which can affect ocean circulation and marine life.