Ecological effects of coastal marsh impoundments: A review

Many coastal resource managers believe estuarine marshes are critically important to estuarine fish and shellfish, not only because of the habitat present for juvenile stages, but also because of the export of detritus and plant nutrients that are consumed in the estuary. Concern has been widely expressed that diking and flooding marshes (impounding) for mosquito control and waterfowl management interferes with these values of marshes. Major changes caused by impoundment include an increase in water level, a decrease in salinity, and a decrease in the exchange of marsh water with estuarine water. Alteration of species composition is dramatic after impoundment. Changes in overall production and transport phenomena, however—and the consequences of these changes— may not be as great in some cases as the concern about these has implied. Although few data are available, a more important concern may be the reduction of access by estuarine fish and shellfish to the abundant foods and cover available in many natural, as well as impounded, marshes. Perhaps even more important is the occasional removal of free access to open water when conditions become unfavorable in impounded marsh that is periodically opened and closed. Collection of comparative data on the estuarine animal use of various configurations of natural and impounded marshes by estuarine animals should lead to improved management of both impounded and unimpounded marshes.