Although plants are present in the impoundments, their decay and the resultant oxygen demand by decomposing bacteria can contribute to critically low dissolved oxygen levels. When dissolved oxygen levels become too low, fishes, crabs, shrimps and other bottom dwellers die or become so stressed that they are subject to easy predation by birds and alligators. Sediments in the marsh play an important role in determining plant and animal communities, as well as influencing water movement between ground water and surface water. Sediments in impoundments generally consist of a layer of organic material over an impermeable clay-silt layer. The organic material is primarily derived from dead plants.
Impoundments receive little input from groundwater because of the impermeable clay-silt layer.
Most water exchange in impoundments occurs through the water control structures. Thus, the amount of nutrients and organisms that are exchanged between the adjacent tidal creek and the impoundment is controlled by human manipulation of the water control structure. Diked marshes are generally managed by owners to enhance the growth of plants consumed by waterfowl, mainly migratory ducks. The intensity of management can have a significant effect on the plants and animals inhabiting an impoundment. An Ongoing Controversy During the past two decades, many objections have been raised to the diking and impounding of salt marshes.