A ditch produced when earthen material was excavated to build the dikes generally borders the impoundment on three sides. Water quality in estuarine impoundments varies. Dissolved oxygen, which is essential for respiration by aquatic organisms, also varies greatly. When temperatures and salinities are high, water holds less dissolved oxygen than when the water is cold and the salinity is low. Organisms that live in the water also decrease levels of dissolved oxygen as part of their breathing process.
Plants produce oxygen during photosynthesis and can actually enhance oxygen levels during the day.
In summer, when temperature and salinity are high, oxygen levels can be severely depleted in impoundments. 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.