Zonation Within a Salt Marsh

The tides cause a gradient of conditions in a salt marsh. Aquatic, wetland and upland habitats are present along this gradient. The gradient of tidal conditions and their environmental consequences sorts plant and animal species into zones within the salt marsh. The low marsh is the zone between normal low tide and normal high tide. The high marsh is only inundated by the tides twice each month during the spring tides, and also during storm tides. The transition zone between the upper reaches of the salt marsh and the upland is known as the upper border.

            The alternation of tidal flooding and exposure results in a gradient of salinity in both the water and the sediment/soil. In general, the salinity decreases from the shoreline to the upland. The salinity of the open ocean is about 35 ppt with a pH of about 8.2. Water in Long Island Sound has a lower salinity of 27-32 ppt due to the freshwater runoff from terrestrial surfaces that enters the Sound. The low marsh has a salinity of 20-30 ppt. The high marsh has salinities that are usually less than 20 ppt. However, where salt water evaporates on the high marsh, during intervals between the spring tides, the salinity can increase substantially. This often happens in shallow depressions in the high marsh known as salt pannes. Here the salinity can be in the range of 40-60 ppt. The salinity of the upper border is the lowest in the marsh. It is usually much less than 10 ppt. In upland areas adjacent to the high marsh the salinity is less than 0.5 ppt.

           The salty environment is difficult for both plants and animals. For example, it makes the acquisition of fresh water more difficult for plants. During low tide, and for much of the time on the high marsh, organisms are not covered by water. They have to be able to survive the alternation of wet and dry conditions. These environmental parameters, plus others, dictate which types of organisms can survive in what parts of the marsh. This establishes zones of plants and animals in the marsh. Usually these zones gradually fade into one another. Occasionally, the transitions can be fairly abrupt, especially when the topography changes rapidly.