The principle difference between where salt marshes and mangrove forests grow is

Mangrove forests cannot withstand freezing temperature that is why they can only grow at tropical and subtropical latitudes whereas salt marshes grow in middle to high latitudes. Mangrove forests can be recognized by their tangle of roots that allow them to stand still on stilts above water. The roots of the mangrove trees slow the movement of tidal waters causing the sediments to settle out of water and build up the muddy bottom. For salt marshes, they are coastal wetlands that are flooded and drained by salt water brought by tidal waves. The difference between the two is that mangrove forests are woodlands and salt marshes are coastal wetlands. 

Hydrosphere is the correct answer

The answer is constant.