The decay of the plant matter in peat absorbs oxygen, so the peat itself often has extremely low oxygen levels. The bacteria that absorb the oxygen as they break down the plant matter release gases with sulfur in them, often causing salt marshes to smell like rotten eggs. Salt marshes are rich with both plant and animal life, and they serve as an important buffer against erosion on coastlines. They also play an important role in delivering nutrients to coastal waters.
Salt marshes usually form where rivers flow into marine estuaries or other areas that are protected from waves and other powerful forms of erosion. The rivers deposit sediment, which forms the mud and sand of the marsh. As the sediment builds up over time, the area is flooded more slowly and for shorter periods, which allows plants to colonize the exposed mud. The higher the level of the marsh is, the less often it is flooded, and the lower the salt levels, allowing for different species in different places.