Which term describes a body of seawater with relatively uniform properties that moves as a unit?

• A. A body of seawater with relatively uniform properties that moves as a unit.
• B. A narrow strip of land that separates lagoons from the open sea.
• C. A layer where salinity changes strongly with depth.
• D. A region of upwelling.

Answer: (A)

Water masses are bodies of seawater that have relatively uniform properties, such as temperature and salinity, and thus move together as a single unit. They form when water acquires characteristic temperature and salinity at a location—through heating or cooling, evaporation or precipitation, river input, or sea-ice processes—and then spreads horizontally with the ocean currents. Because mixing across density surfaces is limited, the mass retains its properties as it travels, which is why you can trace it by its distinctive temperature–salinity signature and density. This idea contrasts with features like a halocline, a layer where salinity changes rapidly with depth, or regions of upwelling where deep water rises to the surface.